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I once had an Orgo TA refer to a diazo compound as "diazo-boom-boom" (the technical term). I have always been curious as to the reason behind the instability and reactivity.
According to [wikipedia](http://en.wikipedia.org/wiki/Diazo)
> Some of the most stable diazo compounds are α-diazoketones and α-diazoesters since the negative charge is delocalized into the carbonyl. In contrast, most alkyldiazo compounds are explosive
What is it about the alkyldiazos that makes them so much more unstable? There doesn't appear to be any bond strain or other factors. Naively, I can say that having a resonance structure should make it marginally more stable.
![enter image description here][1]
[1]: https://i.stack.imgur.com/8Dc3w.png |
What makes diazo compounds so unstable and explosive? |
[Neil Bartlett](http://en.wikipedia.org/wiki/Neil_Bartlett_(chemist)) (1932–2008) first synthesized $\ce{XeF{_4}}$ (and $\ce{XeF{_6}}$) in 1962. In the [synthesis](http://en.wikipedia.org/wiki/Xenon_tetrafluoride#Synthesis), a nickel chamber is used, and heated to 400°C, causing the formation of $\ce{NiF{_4}}$, which Wikipedia claims is not part of the reaction.
Due to the nature of the noble gas, xenon, one might imagine that the fluorines would need to could be "forced" onto the $\ce{Xe}$ atom at the same time, otherwise with each addition, the molecule would be even less energetically likely. I was trying to ascertain in what geometric form the four fluorine atoms would configure themselves to make this addition possible.
I had first assumed that the nickel was being used as a catalyst, and the fluorines could be plucked from its surface accordingly, but this does not jive with the description of the synthesis above. According to the response below, one synthesis of the molecule does rely on forming fluoride radicals on a catalyst, but it's mysterious that the synthesis above does not mention this.
Assuming there is no catalyst, how are the fluorines at 400°C oriented to allow the addition of 2 molecules at presumably the same time?
F - F
. .
. .
F - F
(and is that even possible?) |
Well, your question is equivalent to *“what is it about α-diazoketones that makes them so much more stable?”*, which is easier to see. Compared to an alkyldiazo, the α-diazoketone has a resonance structure in which the negative charge goes to the ketone’s oxygen (and far away from the positively-charged nitrogen atom):
![resonance structures][1]
Because the oxygen is a quite electronegative element, the resonance form is quite stable and explains the extra stability of α-diazoketones. It is for the same reason that the protons in position α to the ketones are always more acidic than alky chain protons.
Coming back to alkyldiazo compounds, you have to realize that merely being able to write a resonance structure does not intrinsically imply stabilization: the resonance structure has to have some intrinsic stability factor. In the alkyldiazo, the resonance form you wrote is a carbanion, which is considered quite unfavorable unless it has a further stabilizing factor.
[1]: https://i.stack.imgur.com/op5nu.png |
Well, your question is equivalent to *“what is it about α-diazoketones that makes them so much more stable?”*, which is easier to see. Compared to an alkyldiazo, the α-diazoketone has a resonance structure in which the negative charge goes to the ketone’s oxygen (and far away from the positively-charged nitrogen atom):
![resonance structures][1]
Because the oxygen is a quite electronegative element, the resonance form is quite stable and explains the extra stability of α-diazoketones. It is for the same reason that the protons in position α to the ketones are always more acidic than alky chain protons.
Coming back to alkyldiazo compounds, you have to realize that merely being able to write a resonance structure does not intrinsically imply stabilization: the resonance structure has to have some intrinsic stability factor. In the alkyldiazo, the resonance form you wrote is a carbanion, which is considered quite unfavourable unless it has a further stabilizing factor. Moreover, the most common reactions gives N<sub>2</sub>, which is a very stable compound… the reaction is thermodynamically very favourable.
[1]: https://i.stack.imgur.com/op5nu.png |
Besides elucidating or verifying a chemical structure, NMR can also be used e.g. for quantifying a mixture of different chemicals.
Depending on the quality of the spectrum and the specific substance, integrating the NMR signals for the same molecule can result in significant variations from the actual ratio of nuclei, which is known from the chemical structure.
What factors do affect the quantification of NMR signals in a proton 1D spectrum? What precautions should one take during acquisition and processing of NMR spectra to make sure that the spectra can be analyzed quantitatively? |
What factors are important for quantitative analysis of a proton 1D-NMR spectrum? |
It seems that a rinsing agent helps the dishes in the dishwasher dry much better than they do without it (which I recently noted on my new dishwasher). What molecules are used in the composition of these rinsing agents? How do they help the plates dry? |
What's the chemical nature of a dishwasher's rinsing agent? |
I've always seen benzyne (benzene with a triple bond) classified as an "intermediate". I really don't see why it needs to be an intermediate, though. The possible reasons I can come up with are flimsy:
- It has an unstable <em>sp</em><sup>2</sup>–<em>sp</em><sup>2</sup> $\pi$ bent bond: we have bent bonds with more angle strain in cyclopropane, which is not an intermediate. Though those bonds are $\sigma$ bonds.
- It immediately reacts with itself to form a dimer (can't recall the name) in the absence of other reagents: there are many other molecules which spontaneously dimerize (eg $\ce{NO2}$ at certain temperatures), yet they are not classified as "intermediates".
So, what makes benzyne an "intermediate"?
More generally (but possibly too broad), what makes a molecule an "intermediate"? |
One of the drugs I work with is a beta-lactam fused to a sulfone ring, tazobactam.
![enter image description here][1]
When it's beta-lactam carbonyl is attacked by an enzyme, it undergoes rearrangement that results in the sulfur-bridgehead carbon bond being broken. The scheme provided in [the literature by Kuzin](http://pubs.acs.org/doi/abs/10.1021/bi0022745) glosses over how this works (the arrow in 3 is where attack can occur leading to other intermediates):
![enter image description here][2]
Other fused bicyclic beta-lactam drugs don't open the other ring, where in lieu of the sulfone there can be a thioether (penicillins) or just a carbon (carbapenems). I'm not sure if an ether there (as in [clavulanate](http://en.wikipedia.org/wiki/Clavulanic_acid)) encourages the same ring opening. The sulfone group seems more electron hungry, but I don't see how the electrons would dance around.
I've heard some people suggest that the lone pair on the nitrogen is able to make this happen, but why doesn't it then happen in the intact drug?
[1]: https://i.stack.imgur.com/colHO.png
[2]: https://i.stack.imgur.com/WLbOi.png |
Why does the sulfone ring in tazobactam open when the lactam is hydrolyzed? |
Very short answer:
$\Delta G = \Delta H - T\Delta S$
As this is an org-chem question, no need to solve the equation numerically. As you might know, N<sub>2(g)</sub>'s standard formation enthalpy ($\Delta H_f^0$) is 0 - at standard T and P, N<sub>2(g)</sub> is gaseous. Therefore, its enthalpic contribution is 0. Its $S^0$ is 191.32 $\frac{J}{mol K}$, which indicates that, at 273.15K, $\Delta G^0_f$ is not only negative, but also large.
Thermodynamically, N<sub>2(g)</sub> is favored. Take a look at [nitrogen's phase diagram](http://www.wolframalpha.com/input/?i=nitrogen+phase+diagram), generated by Wolfram Alpha. |
Very short answer:
$\Delta G = \Delta H - T\Delta S$
As this is an org-chem question, no need to solve the equation numerically. As you might know, N<sub>2(g)</sub>'s standard formation enthalpy ($\Delta H_f^0$) is 0 - at standard T and P, N<sub>2(g)</sub> is gaseous. Therefore, its enthalpic contribution is 0. Its $S^0$ is 191.32 $\frac{J}{mol K}$, which indicates that, at 273.15K, $\Delta G^0_f$ is not only negative, but also large.
Thermodynamically, N<sub>2(g)</sub> is favored. Take a look at [nitrogen's phase diagram](http://www.wolframalpha.com/input/?i=nitrogen+phase+diagram), generated by Wolfram Alpha.
**EDIT**
I've just noticed you're considering _diazonium ions_, not _azo_ compounds. For the reasons F'x explained, the ions are more prompt to explosion than are the azo compounds. I think azo compounds are used in the fabrication of CDs, or so my teacher told us... They're supposedly light sensitive. |
During nitration of phenol, both para- and ortho-nitrophenols will be formed.
![enter image description here][1]
Is there any way which we can synthesize para-nitrophenol only?
[1]: https://i.stack.imgur.com/R5OXE.png
|
Oscillating reactions are a funny aspect of chemistry. I have tried to find various simplified kinetic models of oscillating reactions such as the Belouzov-Zhabotinsky, the Briggs–Rauscher or the Bray–Liebhafsky reactions in order to be able to study them. However, the models I have found so far are either too complicated (8 or more species considered), or loose chemical significance. For an example of the second case, Ball's 1994 model<sup>\[1]</sup> corresponds to:
![enter image description here][1]
![enter image description here][2]
While this is useful and introduces a nice circularity in the model, creating the possible feedback, it has lost all chemical sense — by which I mean that no correspondance can be established between the species of this model and a real oscillating system.
So, my question is: ***what's the simplest known chemically-meaningful model of an oscillating reaction?***
---
\[1] Ball, P. 1994 *Designing the molecular world: Chemistry at the frontier*. Princeton, NJ: Princeton University Press.
[1]: https://i.stack.imgur.com/AYhm1.png
[2]: https://i.stack.imgur.com/byTzq.png |
What's a minimal yet chemically-meaningful kinetic system for an oscillating reaction? |
One of the drugs I work with is a beta-lactam (4-membered ring with an amide bond) fused to a sulfone ring, tazobactam.
It's relatively stable in water; the lactam is not significantly hydrolyzed without being catalyzed. When the lactam is hydrolyzed, however, typically by an enzyme, and the lactam ring opened, the sulfone ring also opens cleaving the bond between the sulfur and bridgehead carbon. A [paper by Kuzin](http://pubs.acs.org/doi/abs/10.1021/bi0022745) gives the following reaction scheme (this is part of figure 2, click for the whole thing)
[
](https://i.stack.imgur.com/ABosK.png)
and says of it (emphasis mine):
> The Ser70-bound moiety (2) is thought to be the initial intermediate [...] *Ring opening after departure of the sulfinate from C5* produces a reactive imine (3) and the more inert tautomeric enamine forms (4, 5).
**But why is the sulfuryl** (sulfone, sulfonyl(?)...) **leaving only when the lactam is opened and not spontaneously beforehand?**
Other fused bicyclic beta-lactam drugs don't open the other ring, where in lieu of the sulfone there is commonly a thioether (as in penicillin). The sulfone group seems more electron hungry, but I don't see how the broken lactam bond affects this.
I've heard some people suggest that the lone pair on the nitrogen is able to make this happen, but why doesn't it then happen in the intact drug? |
Periodic acid (per-iodic acid, $\ce{HIO4}$ ) cleaves vicinal diols(to carbonyls) via a cyclic intermediate.
Due to the cyclic intermediate, it needs a syn configuration for the $\ce{-OH}$ groups for the reaction.
Exactly which configurations are allowed on cyclohexane derivatives?
Let's take 1,2-dihydroxycyclohexane(boat conformer). If the two hydroxy groups are both on the axial positions (_trans-_ ), then obviously the reaction will not take place.
What about when the two are _cis-_ (one axial, one equatorial), and when they are both equatorial (_trans-_ )?
(My original question had to do with diastereomers of cyclic glucose--aldohexoses--but I realised that the extra hydroxy groups make the reaction more complicated)
|
The [partition coefficient][1] is a very useful metric in characterizing compounds particularly drugs. What chemical properties influence that partitioning? Are chemical moieties additive in their LogP? What if the compound has both hydrophobic and hydrophilic ends?
[1]: http://en.wikipedia.org/wiki/Partition_coefficient |
I have come across many reactions that are zero order reactions but at one glance I'm unable to tell if they are zero order or not. Is there any criteria that can be used to identify these reactions or are they purely experimentally determined? |
Historically, thiols $\ce{-SH}$ were known as "mercapto-" due to their strong propensity to bond with mercury. Certain drugs still carry this designation, though [mercaptopurine](http://en.wikipedia.org/wiki/Mercaptopurine) has an $\ce{=S}$ group instead of a thiol.
What makes the thiol group bond so strongly with mercury in particular? Why don't alcohols make such a strong bond with $\cf{Hg}$?
If it's known, was there any historical reason why it was important to be able to "capture" mercury? |
Why do thiols have such a propensity for bonding with mercury? |
In a thesis I am reading, it is said that one of the reasons for using plane-wave basis sets for first-principles molecular dynamics (aka *ab initio* MD) is that the Pulay forces that arise from an MD using atomic basis sets are computationally expensive to calculate.
While I understand that having additional terms means more code to write, i.e. they make writing the software harder to write, is it true that they are CPU-intensive to compute? In particular, do they necessitate the calculation of integrals between basis functions that would not otherwise be calculated? |
Are Pulay force expensive to compute? |
> Related (very similar, but here I want a mechanism) http://physics.stackexchange.com/q/21827/7433
By the [Kohlrausch law][1], all ions contribute to the conductivity of an electrolyte.
Now, as I understand it, the mechanism of conduction in an electrolyte is thus:
- Ions migrate in solution
- These ions get reduced or oxidized at the electrodes and converted to electrons
- These electrons continue down the wire, leading to an increased/maintained conductivity/current
But, this mechanism doesn't work for ions which do not get redoxed--movement of ions in the solution cannot be translated to movement of electrons in the wire and thus it seems (to me) that conductivity should _not_ increase.
But ions like $\ce{NO3-}$ have a comparable $\lambda$ (ionic molar conductivity)--so they experimentally do increase conductivity.
What is the mechanism for conduction via these ions?
[1]: http://en.wikipedia.org/wiki/Molar_conductivity#History |
How does conductivity work for non-redoxed ions? |
I mixed 6.0mL of .01M Pb(NO3)2 and 6.0mL of .02M KI and 8mL of water and a precipitate (barely) formed. I calculated the Ktrial with [Pb2+][I]^2 to get 1.1e-7
The problem is that the accepted value is 9.8×10-9 which is off by a factor of 10.
I'm pretty sure I had clean equipment and measured carefully and accurately, where did I go wrong here?
I calculated [PB2+] as 0.0030M and [I-] as 0.0060M
I'm assuming my math is wrong here, is it just more likely that I had some inaccuracies in my experiment?
Yes this was done for school, but my question is not a question in my homework.
|
I mixed $6.0~\rm mL$ of $.01\:\rmM$ $\ce{Pb(NO3)2}$ and $6.0~\rm mL$ of $0.02 ~\rm mL$ $\ce{KI}$ and $8.0~\rm mL$ of water and a precipitate (barely) formed. I calculated the $K_{sp}$ with $[\ce{Pb(2+)}][{I}]^2$ to get $1.1\times 10^{-7}$
The problem is that the accepted value is $9.8\times 10^{-9}$ which is off by a factor of 10.
I'm pretty sure I had clean equipment and measured carefully and accurately, where did I go wrong here?
I calculated $[\ce{Pb(2+)}]$ as $0.0036 ~\rm M$ and $[I-]$ as $0.0060~\rm M$
I'm assuming my math is wrong here, is it just more likely that I had some inaccuracies in my experiment?
Yes this was done for school, but my question is not a question in my homework.
|
I mixed $6.0~\rm mL$ of $.01\:\rm M$ $\ce{Pb(NO3)2}$ and $6.0~\rm mL$ of $0.02 ~\rm mL$ $\ce{KI}$ and $8.0~\rm mL$ of water and a precipitate (barely) formed. I calculated the $K_{sp}$ with $[\ce{Pb(2+)}][{I}]^2$ to get $1.1\times 10^{-7}$
The problem is that the accepted value is $9.8\times 10^{-9}$ which is off by a factor of 10.
I'm pretty sure I had clean equipment and measured carefully and accurately, where did I go wrong here?
I calculated $[\ce{Pb2+}]$ as $0.0036 ~\rm M$ and $[I-]$ as $0.0060~\rm M$
I'm assuming my math is wrong here, is it just more likely that I had some inaccuracies in my experiment?
Yes this was done for school, but my question is not a question in my homework.
|
I mixed $6.0~\rm mL$ of $.01\:\rm M$ $\ce{Pb(NO3)2}$ and $6.0~\rm mL$ of $0.02 ~\rm mL$ $\ce{KI}$ and $8.0~\rm mL$ of water and a precipitate (barely) formed. I calculated the $K_{sp}$ with $[\ce{Pb^{2+}}][{I^-}]^2$ to get $1.1\times 10^{-7}$
The problem is that the accepted value is $9.8\times 10^{-9}$ which is off by a factor of 10.
I'm pretty sure I had clean equipment and measured carefully and accurately, where did I go wrong here?
I calculated $[\ce{Pb^{2+}}]$ as $0.0036 ~\rm M$ and $[I^-]$ as $0.0060~\rm M$
I'm assuming my math is wrong here, is it just more likely that I had some inaccuracies in my experiment?
Yes this was done for school, but my question is not a question in my homework.
|
It's probably an experimental error. The 'barely' may be important here.
For one, you may not be sure how accurate your initial molarities are. A mistake in one of them ruins it all. Titrate them properly and check. Titration usually gives two significant digits.
Also, since its a cube overall, a small mistake can get much larger. You may want more accuracy in your readings (one significant digit isn't good enoigh), and you need to improve on the 'barely'. Though a factor of 11 needs a variation by a factor of 2.3 in both readings. So I don't know.
Have you redone the experiment? Doing an experimemt more than once os integral yo getting good values.
Or, the answer is wrong. |
It's probably an experimental error. The 'barely' may be important here.
For one, you may not be sure how accurate your initial molarities are. A mistake in one of them ruins it all. Titrate them properly and check. Titration usually gives two significant digits.
Also, since its a cube overall, a small mistake can get much larger. You may want more accuracy in your readings (one significant digit isn't good enough), and you need to improve on the 'barely'. Though a factor of 11 needs a variation by a factor of 2.3 in both readings. So I don't know.
Have you redone the experiment? Doing an experimemt more than once is integral to getting good values.
Or, the answer is wrong. |
Are Pulay forces expensive to compute? |
I mixed $6.0~\rm mL$ of $.01\:\rm M$ $\ce{Pb(NO3)2}$ and $6.0~\rm mL$ of $0.02\:\rm M$ $\ce{KI}$ and $8.0~\rm mL$ of water and a precipitate (barely) formed. I calculated the $K_{sp}$ with $[\ce{Pb^{2+}}][{I^-}]^2$ to get $1.1\times 10^{-7}$
The problem is that the accepted value is $9.8\times 10^{-9}$ which is off by a factor of 10.
I'm pretty sure I had clean equipment and measured carefully and accurately, where did I go wrong here?
I calculated $[\ce{Pb^{2+}}]$ as $0.0036 ~\rm M$ and $[\ce{I-}]$ as $0.0060~\rm M$
I'm assuming my math is wrong here, is it just more likely that I had some inaccuracies in my experiment?
Yes this was done for school, but my question is not a question in my homework.
|
How long you observed your mixture? It is possible that you had not reached the equilibrium required for the solubility product constant to be valid. Allowing time for the system to reach equilibrium might have resulted in the appearance of more precipitate that would indicate that you had added too much material. |
My understanding is that the NBO methodology transforms the molecular orbital picture of a molecule into a collection of pairwise interactions (bond orbitals). This is obviously handy for didactic and comprehension purposes as it distils what might be a very complex and non-local MO picture into manageable lobes conforming to the introductory theories of chemical bonding (VB, orbital hybridisation) taught in high-school and early university courses.
Does natural bond orbital theory have utility outside of this role? |
What is natural bond orbital theory used for? |
When I was reading through the ADF-BAND tutorials, one of the toy systems presented was a 1-D periodic structure involving 3 collinear hydrogen atoms. The tutorial pointed out that, topologically speaking, this is cylindrically symmetrical (more specifically, it's ring symmetrical).
In the instance of a 2-D structure, can the calculation be considered a model for the surface of a torus (this seems logical), a sphere (I doubt this because if you fit a rectilinear grid to a sphere you end up with two poles and dissimilar meridians and parallels), or other?
Bonus questions: Has anyone used periodic calculations to model electronic structure/chemistry on the surface of a sphere or torus? Can you introduce a curvature term to account for these structures having a finite size? |
Is a 2-D periodic structure isomorphic with the surface of a torus, a sphere, neither or both? |
In a thesis I am reading, it is said that one of the reasons for using plane-wave basis sets for first-principles molecular dynamics (aka *ab initio* MD) is that the Pulay forces<sup>\[1,2]</sup> that arise from an MD using atomic basis sets are computationally expensive to calculate.
While I understand that having additional terms means more code to write, i.e. they make writing the software harder to write, is it true that they are CPU-intensive to compute? In particular, do they necessitate the calculation of integrals between basis functions that would not otherwise be calculated?
---
\[1] P. Pulay, *Molec. Phys.* 19, 197 (1969)<br>
\[2] See also slide 6 of [this](http://tfy.tkk.fi/~asf/physics/lectures/PDF/lect11.pdf) |
Yes, a 2D-periodic space can be mapped to a torus, but [that's more a question for the math.SE](http://math.stackexchange.com/questions/66789/if-a-rectangular-grid-where-left-right-and-top-bottom-wrap-can-be-mapped-o)…
Regarding your bonus question, why would there be? What would you do with it? Molecular structures are intrinsically 3D, so I don't see what you would do in a 2D (periodic or not) space? Even when we talk about planar or pseudo-2D structures (buckyball, nanotube, etc.) they are 3D objects with 3D electronic densities and wavefunctions. |
I don't have time to do the calculations manually, but inputing your problem data into a [chemical equilibrium software](http://chemistry.stackexchange.com/q/24/30) (here Dozzaqueux) reveals that **the precipitate you observe is Pb(OH)<sub>2</sub>**, while no PbI<sub>2</sub> should form. The former is white, while the later is yellow; can you tell us what color was your precipitate? |
Can derive the quantity of a substance from a [GC/MS](http://en.wikipedia.org/wiki/Gas_chromatography%E2%80%93mass_spectrometry) report if I know:
* the ratio to another substance in the data,
* the quantity of that second substance, and
* the molecular mass of both substances.
The site was very dense and vague regarding how GC/MS test work, but fairly upfront regarding how they derive their ratios.
Here are the numbers (slightly tweaked, but more or less in proportion):
Substance A:
* Quantity: 300mg (Know already, not part of GC/MS report)
* Mol. Mass: 500g/mol (found in external source)
* Peak Proportion: 3
Substance X:
* Quantity: Unknown
* Mol. Mass: 425g/mol (again, external source)
* Peak Proportion: 1
To be clear, the ratio of 1:3 means that substance X has a horizontal peak 1/3 of substance A. I'm not clear on what the x-axis is actually a measurement of, which is part of why I'm at a loss. The site does make it very explicit that the ratios are not directly proportional to mass ratio (so if it found a ratio of 3:1 of glucose to arsenic, this doesn't mean the substance is 75% glucose and 25% arsenic, only that the glucose "peaks" 3 times higher, which I've taken to mean "3 times the oomph", but that may be incorrect, as well).
So, if I know that the molecular mass of substance X is .85 of substance Y, can I derive the actual mass of substance X using the formula:
\begin{equation}qty_X=\frac{qty_A\times(molmass_X/molmass_A)}{peak_A/peak_X}\end{equation}
with data being:
\begin{equation}qty_X=\frac{300mg\times(425/500)}{3/1}\end{equation}
Which simplifies to:
\begin{equation}qty_X=(300mg\times .85)/3\end{equation}
and finally the result of 85mg.
So I guess in the end there are 3 questions:
1. Is this even how GC/MS results work?
2. If so, is my assumption to derive the mystery quantity using the molecular mass correct?
3. Is the math itself in order? (I'm specifically worried that I should invert either the mass ratio or the peak ratio or both).
Of course, if the answer to the first questions is no, then my true question is: can I derive the quantity of substance X with the given data, and if so, what would be the right approach?
If anyone is curious for some context, I need to know the actual quantity of substance X as I know that, by mass, it has a threshold between harmless and toxic, so just knowing substance X is 1/3 "peak" of substance A doesn't let me know if I should let my dog/child/self ingest it.
Thanks for your patience and help.
[1]: http://xkcd.com/435/ |
Gas chromatography/mass spectrometry: Can I use peak ratios and molecular mass to derive quantity? |
Can derive the quantity of a substance from a [GC/MS](http://en.wikipedia.org/wiki/Gas_chromatography%E2%80%93mass_spectrometry) report if I know:
* the ratio to another substance in the data,
* the quantity of that second substance, and
* the molecular mass of both substances.
The site was very dense and vague regarding how GC/MS test work, but fairly upfront regarding how they derive their ratios.
Here are the numbers (slightly tweaked, but more or less in proportion):
Substance A:
* Quantity: 300 mg (Know already, not part of GC/MS report)
* Mol. Mass: 500 g/mol (found in external source)
* Peak Proportion: 3
Substance X:
* Quantity: Unknown
* Mol. Mass: 425 g/mol (again, external source)
* Peak Proportion: 1
To be clear, the ratio of 1:3 means that substance X has a horizontal peak 1/3 of substance A. I'm not clear on what the x-axis is actually a measurement of, which is part of why I'm at a loss. The site does make it very explicit that the ratios are not directly proportional to mass ratio (so if it found a ratio of 3:1 of glucose to arsenic, this doesn't mean the substance is 75% glucose and 25% arsenic, only that the glucose "peaks" 3 times higher, which I've taken to mean "3 times the oomph", but that may be incorrect, as well).
So, if I know that the molecular mass of substance X is 0.85 of substance Y, can I derive the actual mass of substance X using the formula:
\begin{equation}qty_X=\frac{qty_A\times(molmass_X/molmass_A)}{peak_A/peak_X}\end{equation}
with data being:
\begin{equation}qty_X=\frac{300\text{ mg}\times(425/500)}{3/1}\end{equation}
Which simplifies to:
\begin{equation}qty_X=(300\text{ mg}\times 0.85)/3\end{equation}
and finally the result of 85 mg.
So I guess in the end there are 3 questions:
1. Is this even how GC/MS results work?
2. If so, is my assumption to derive the mystery quantity using the molecular mass correct?
3. Is the math itself in order? (I'm specifically worried that I should invert either the mass ratio or the peak ratio or both).
Of course, if the answer to the first questions is no, then my true question is: can I derive the quantity of substance X with the given data, and if so, what would be the right approach?
If anyone is curious for some context, I need to know the actual quantity of substance X as I know that, by mass, it has a threshold between harmless and toxic, so just knowing substance X is 1/3 "peak" of substance A doesn't let me know if I should let my dog/child/self ingest it.
[1]: http://xkcd.com/435/ |
Yes, a 2D-periodic space can be mapped to a torus, but [that's more a question for the math.SE](http://math.stackexchange.com/questions/66789/if-a-rectangular-grid-where-left-right-and-top-bottom-wrap-can-be-mapped-o)…
Regarding your bonus question, why would there be? What would you do with it? Molecular structures are intrinsically 3D, so I don't see what you would do in a 2D (periodic or not) space? Even when we talk about planar or pseudo-2D structures (buckyball, nanotube, etc.) they are 3D objects with 3D electronic densities and wavefunctions.
---
*Edit:* 3D structures that are periodic in two dimensions and finite in the other one can be studied by many computational chemistry codes. They are often referred to as *slab calculations* or *surface calculations*. The most common issue is that of Coulombic interaction (or Poisson equation solver), which typically requires special treatment in the 2D case. |
Can derive the quantity of a substance from a [GC/MS](http://en.wikipedia.org/wiki/Gas_chromatography%E2%80%93mass_spectrometry) report if I know:
* the ratio to another substance in the data,
* the quantity of that second substance, and
* the molecular mass of both substances.
The site was very dense and vague regarding how GC/MS test work, but fairly upfront regarding how they derive their ratios.
Here are the numbers (slightly tweaked, but more or less in proportion):
Substance A:
* Quantity: 300 mg (Know already, not part of GC/MS report)
* Mol. Mass: 500 g/mol (found in external source)
* Peak Proportion: 3
Substance X:
* Quantity: Unknown
* Mol. Mass: 425 g/mol (again, external source)
* Peak Proportion: 1
To be clear, the ratio of 1:3 means that substance X has a horizontal peak 1/3 of substance A. I'm not clear on what the x-axis is actually a measurement of, which is part of why I'm at a loss. The site does make it very explicit that the ratios are not directly proportional to mass ratio (so if it found a ratio of 3:1 of glucose to arsenic, this doesn't mean the substance is 75% glucose and 25% arsenic, only that the glucose "peaks" 3 times higher, which I've taken to mean "3 times the oomph", but that may be incorrect, as well).
So, if I know that the molecular mass of substance X is 0.85 of substance Y, can I derive the actual mass of substance X using the formula:
\begin{equation}qty_X=\frac{qty_A\times(molmass_X/molmass_A)}{peak_A/peak_X}\end{equation}
with data being:
\begin{equation}qty_X=\frac{300\text{ mg}\times(425/500)}{3/1}\end{equation}
Which simplifies to:
\begin{equation}qty_X=(300\text{ mg}\times 0.85)/3\end{equation}
and finally the result of 85 mg.
So I guess in the end there are 3 questions:
1. Is this even how GC/MS results work?
2. If so, is my assumption to derive the mystery quantity using the molecular mass correct?
3. Is the math itself in order? (I'm specifically worried that I should invert either the mass ratio or the peak ratio or both).
Of course, if the answer to the first questions is no, then my true question is: can I derive the quantity of substance X with the given data, and if so, what would be the right approach?
If anyone is curious for some context, I need to know the actual quantity of substance X as I know that, by mass, it has a threshold between harmless and toxic, so just knowing substance X is 1/3 "peak" of substance A doesn't let me know if I should let my dog/child/self ingest it.
|
Is there a formal definition or algorithm that can take a list of reactions and tell me whether it contains a set of species that can produce itself autocatalytically?
It is clear that the following reaction is autocatalytic:
$$
A + B \to 2 B.
$$
$B$ appears on both sides of the equation, so it's a catalysis reaction, and there is an extra $B$ produced on the right, so it catalyses its own production from $A$.
But the following reaction scheme can also be considered to contain autocatalysis:
$$
B + C \to D+ E\\
A + D \to 2C.
$$
If this system starts out with plenty of $A$ and $B$, together with one molecule of $C$, then the $C$ molecule can react with a $B$ to produce a $D$, which can then react with an $A$ to produce two $C$'s - whereas, without the initial "seed" molecule, no reactions will take place. (I'm neglecting reverse reactions here of course, but hopefully it gets the point across.) The autocataltic cycle can easily be seen in the following diagram, where circles represent species and boxes represent reactions:
![autocatalysis diagram][1]
Another non-obvious example of autocatalysis is a reaction scheme of this form:
$$
A + C \to C + D\\
B + D \to D + C + E
$$
Here, $C$ and $D$ each catalyse the production of one another from the precursors $A$ and $B$. I'm aware of Kauffman's definition of reflexively autocatalytic sets, which applies to schemes of this form, but would miss an autocatalytic cycle of the kind shown above, since it doesn't consist of single-step catalysis reactions. So I'm looking for a definition of autocatalysis that would include both cases, as well as (hopefully) other, more complex schemes that I haven't yet thought of.
[1]: https://i.stack.imgur.com/zQJMA.png |
Looking at something like $\cf{ YBa{_2}Cu{_3}O{_7}}$ which was one of the [first cuprate superconductors](http://en.wikipedia.org/wiki/Superconductivity) to be discovered, I'm always curious how the selection of these substances as likely superconductors comes about?
Does it have to do with optimal crystalline structure, unpaired electrons, or is it similar to the way semiconductors are designed with doping to create "holes"? Or are these superconductors simply determined experimentally based on small changes of what has worked in the past?
As a side question: I've always been curious as to how they stumbled upon Yttrium for this particular case. It's not something I think of as being carried in ready supply. Would this have been after YAG lasers were already around? |
Is there a theory behind selecting elements that may be successful in potential superconductors? |
How can I write a Z-matrix for the following complex?
O==C==O H--O
\
H
I have managed to write:
O
C 1 a
O 2 a 1 180
H 3 d 2 180 1 ?
O 4 b 3 180 2 ?
H 5 b 4 t 3 ?
but I don't know how to specify the dihedral angles indicated with `?`. Are they all zero? Or completely unspecified? If so, will I be able to do a coordinate optimization with these variables or will it be a problem? |
Where do I find Datasheets of Zeoliths? |
[Solvated electrons](http://en.wikipedia.org/wiki/Solvated_electron) have a long lifetime in ammonia solutions, but their counterparts in water (called hydrated electrons) have a much smaller lifetime, of the order of microseconds in very pure water.
![enter image description here][1]<br>
<sup>(image from: Boero et al, *Phys Rev Lett* **2003**, 90, 226403)</sup>
What properties of the solvent account for these very different lifetimes? Both are polar protic solvents, so what other(s) factor(s) could be involved here?
[1]: https://i.stack.imgur.com/7veZ6.png |
What dictates the lifetime of a solvated electron in a given solvent? |
I've always been a bit uncomfortable with the concept of more than two electrons in a single orbital-like region(probability-wise) which occurs in resonance.
This seems to disobey Pauli's exclusion principle.
Let's take benzene for example. It has a delocalized cloud of electrons. These all had the "original" (as in, if resonance didn't occur) quantum numbers $n=2,l=1,m=\text{(1,0,or -1; but it must be the same for all)},m_s=\pm\frac12$. But, due to delocalization, they're now all in one extra-large orbital-like region (by orbital-like, I mean that the pi cloud is similar to an orbital in the sense that it is also a probability cloud which defines the probabilities of finding one or more electrons at different coordinates).
Now, it's obvious that the original quantum numbers are insufficient here due to Pauli's exclusion principle. They may not even exist for these electrons.
The easiest thing to do would be to "propose" (not exactly propose, since I _guess_ the Dirac equation can _prove_ their existence) an extra quantum number, which can take twelve values. Or a combination of a few extra quantum numbers that have the same effect.
But this raises the question: do the electrons belonging to the pi cloud "retain" their "original" quantum numbers as well?
Basically I want to know how benzene and other resonating compounds are described quantum mechanically, with focus on quantum numbers. (links to papers are fine, probably necessary here--but please explain the gist of the link) |
I'm looking into uses of zeolites as water absorbers for heat storage and absorbtion cooling. However, I don't find hard numbers on the likes of absorption enthalpies at different temperatures, saturation moistures, etc.
I'm not an institution, chemical suppliers are traditionally reluctant to sell to private individuals. I found zeolitic absorbers in few catalogues, but only with a descriptor of the type, no further data. One company I asked (that sell zeolites in amounts of kg to private customers) was unwilling to share data on their product (They're an engineering company doing product developement with zeolitic absorbers for heating/cooling etc.).
I only found (sketchy) data on single zeolites in working papers, but no comprehensive source.
So: seeing that vendors don't share the info, where do I find datasheets of zeolites? |
Where do I find datasheets of zeolites? |
I've always been a bit uncomfortable with the concept of more than two electrons in a single orbital-like region(probability-wise) which occurs in resonance.
This seems to disobey Pauli's exclusion principle.
Let's take benzene for example. It has a delocalized cloud of electrons. These all had the "original" (as in, if resonance didn't occur) quantum numbers $n=2,l=1,m=\text{(1,0,or -1; but it must be the same for all)},m_s=\pm\frac12$. But, due to delocalization, they're now all in one extra-large orbital-like region (by orbital-like, I mean that the pi cloud is similar to an orbital in the sense that it is also a probability cloud which defines the probabilities of finding one or more electrons at different coordinates).
Now, it's obvious that the original quantum numbers are insufficient here due to Pauli's exclusion principle. They may not even exist for these electrons.
The easiest thing to do would be to "propose" (not exactly propose, since I _guess_ the Dirac equation can _prove_ their existence) an extra quantum number, which can take twelve values. Or a combination of a few extra quantum numbers that have the same effect.
But this raises the question: do the electrons belonging to the pi cloud "retain" their "original" quantum numbers as well?
Basically I want to know how benzene and other resonating compounds are described quantum mechanically, with focus on quantum numbers. (links to papers are fine, probably necessary here--but please explain the gist of the link)
I'd prefer the answer in terms of simpler quantum mechanics, though I'm not averse to the idea of some extra reading. |
In many places I've seen the "extra" bond in benzyne being labelled as $sp^2-sp^2$ overlap or distorted(not parallel) $p\pi-p\pi$ overlap. But I've failed to see _why_ we can't have a normal, parallel $p\pi-p\pi$ overlap.
Wikipedia [says][1]:
> In benzyne, however, the p orbitals are distorted to accommodate the triple bond within the ring system, reducing their effective overlap.
That makes no sense to me--why can't a normal, parallel $p\pi-p\pi$ overlap be "accomodated" within the system.
So, what is the reason for the distorted $\pi$ bond in benzyne?
I'm fine with answers involving a bit of QM/MO theory.
[1]: http://en.wikipedia.org/wiki/Benzyne |
Why is the benzyne triple bond distorted? |
In many places I've seen the "extra" bond in benzyne being labelled as $sp^2-sp^2$ overlap or distorted (not parallel) $p\pi-p\pi$ overlap. But I've failed to see _why_ we can't have a normal, parallel $p\pi-p\pi$ overlap.
Wikipedia [says][1]:
> In benzyne, however, the p orbitals are distorted to accommodate the triple bond within the ring system, reducing their effective overlap.
That makes no sense to me — why can't a normal, parallel $p\pi-p\pi$ overlap be "accomodated" within the system.
So, what is the reason for the distorted $\pi$ bond in benzyne?
I'm fine with answers involving a bit of QM/MO theory.
[1]: http://en.wikipedia.org/wiki/Benzyne |
Why does benzene bend in this reaction? |
I very much like the first answer, especially the superb visuals!
However, I will nonetheless add a simpler approach to answering your question, which is this: Pauli exclusion holds as long as there is _something_ in space or in spin (spin being the pairs of electrons) that firmly distinguishes two "stable" states.
What that means is that your initial intuition is essentially correct if _all_ the electron pairs shared exactly the same molecular orbital.
So the answer to your question in general terms is that electron pairs settle into molecule-sized orbitals (molecular orbitals) that are physically spread out over the entire ring, but which nonetheless have unique geometric features that keep them distinguished in real space. I would add that they are even more unique when represented in an abstract space called momentum space, where the geometric differences translate into actual distances between the electron pairs. These abstract distances in momentum space satisfy the Pauli exclusion principle just as well as physical separation in real space, so that even very similar looking orbitals can remain distinct under Pauli exclusion.
For more details and some easy visual analogies, you can read more in the addendum below.
----------
The same general problem of how to distinguish electron pairs to meet Pauli exclusion exists in metals. In that case, the conduction electrons are free to drift over the entire macroscopic metal. The result is an even more extreme need than in benzene for the pairs to distinguish themselves firmly from each other, since for conduction electrons _all_ of these gazillions [technical term] of pairs must find "unique places" in which they can maintain their unique identity. That's tough!
Now it turns out that these two problems of benzene electrons and metallic conduction electron pairs are related in a rather deep fashion, since a benzene ring can be interpreted (quite correctly) as a special tiny case of a one-dimensional metal. That means that if you understand what happens conceptually in metals, you are well on the way to understanding all forms of hybridization in organic molecules. So, let's take a quick look at how that works.
The situation for electron pairs in metals is most easily understood by imagining the pairs moving back and forth along the $x$ length of a very narrow, highly conductive, and unusually regular metallic wire. Next, borrow the $y$ and $z$ plane to represent the complex plane (complex phase of a wave function). The amplitude for finding an electron from the pair becomes the radius of the complex plain at each point along $x$. For stable electron pair states, these radii and their phases will exhibit a lot of continuity as you move their as along the $x$ length of the wire. They will also rotate at the same speed in the complex plane of $y$ and $z$.
In fact... if you calculate the stable or "stationary" states possible in such a configuration, you will find that they bear a striking resemblance both visually and mathematically to skip rope solutions, that is, to the ways that you can twirl a skip rope so that it maintains stable, unmoving (the "stationary" part) loops.
So let's look at that situation as a good analog solution to the quantum equations. Anyone can make one large loop, and that in fact represents the electron pair state with the lowest possible energy along the length of the wire, or the base state. Incidentally, the volume enclosed by the skip rope is proportional to the square of the radius, which as I mentioned earlier is the "amplitude" of the wave function. So... that means that if you simply look at the volume enclosed by the skip rope as it turns, you pretty much instantly know what the probability of finding an electron from the pair is. In the case of the base state, it means that the electron pair will tend to be found in the center of the wire, and almost never at the ends.
In chemistry such varying probabilities of finding electrons are usually represented as densities in ordinary space, without the benefit of the dynamics of the complex plane. And for good reason, since for anything except wires, trying to show the complex plane would result in five-dimensional spaces. Those are tough to visualize! Instead, the idea of a _phase_ is attached to tell the relative positions of the rotating points in the unseen complex plane.
The skip rope in the one-dimensional wire makes the idea of phase a lot easier to visualized. For example, good skip-ropers can easily create double loop skip ropes. These correspond to the first excited state of the electron pair, one with a bit more energy added (you have to twirl the rope faster) and two distinct loops. Phase just means this for such a double loop: When one loop is towards you, the other is opposite of you. You can label one such loop positive and one negative, but of course it's all arbitrary, and really just a technique for roughly describing those invisible but rope-like complex plane dynamics.
Notice that the volume-equals-probability rule still applies to the double loop. That means that in the first excited state of the electron pair, the odd are good and roughly equal for finding it in the left loop or right loop, but close to zero at the ends _and_ at the center. So, this really is a unique state for the excited electron pair, one that is quite distinct from the first single-loop or base state.
Now there's an interesting bit of mathematics, Fourier transforms, in which _any_ very long sinusoidal shape can always be distinguished very sharply from any other sinusoidal, even if they only differ slightly in wavelength. People use this idea all the time, since in the case of radio waves (which are sinusoidal waves, mostly) it's called tuning a radio!
The uniqueness of long sinusoidal waves is relevant here because it turns out that Pauli exclusion is met equally well if two pairs of electrons are separated from each other in _frequency_ space.
So, another way of saying all of this is that when electrons are "delocalized" either in bonds or in metals -- it really is the same idea -- they can achieve uniqueness by bumping around _either_ in ordinary space _or_ in a more abstract frequency space called momentum space. Both work equally well. The "extra quantum number" you speculated about in your question could be equated to this idea of additional separation in momentum space for example.
So, the benzene orbitals maintain Pauli exclusion uniqueness by finding specific stable solutions that are similar but geometrically distinct, and which are even more unique when interpreted in terms of frequencies or loop counts. Those equate to a quite different-looking, distance-like separations in the abstract space called momentum space.
So to finish up, what about that extreme case I mentioned of metals that have an electron or two for _every_ atom in what may be a very large crystal?
That's a truly fascinating case, a sort of "benzene gone bad" scenario in which the need for Pauli exclusion uniqueness pushes the electron pairs into absolutely bizarre behaviors, ones very important to modern (and ancient) technologies: Some of the electrons are forced to get very hot. Now that's weird!
What happens is easy to see with the above background. The first pair of electrons in a silver wire is very, very cold, near absolute zero, moving so slowly that it forms only a single loop in the center of the wire. To maintain Pauli exclusion -- the momentum space version of it to be precise -- the next pair must look like a skip rope with two loops, and the third three, and so forth. You get the picture.
But each such loop has more energy, even if only by a little bit! So what happens when you have a gazillion such electron pairs all vying for uniqueness?
Someone gets pushed very, _very_ high up in frequency, and thus also in energy, while other lurk in the deeper and colder regions of this pile of moving electrons. The result is almost like a sea in momentum space, and that is in fact what it is called: A Fermi sea, with cold electron pairs at the bottom, and some truly _hot_ ones at the surface. Fermi seas are also referred to by the more common phrase of "conduction bands," meaning the range of energies that the electrons occupy in order to maintain Pauli uniqueness.
You see Fermi surfaces all the time, since it is _only_ the hottest electrons in a metal that interact with photons and bounce them back at you. You are in effect looking directly at a hot Fermi surface every time you look into a silver or aluminum backed mirror.
How hot? Well, believe it or not, X-ray hot. A typical piece of ordinary shiny metal, say a fork or spoon, would kill you form X-ray exposure if all of its hot Fermi surface and near-surface electrons could suddenly fall back into a "ground state" of just one loop. They are tamed and prevented from doing so by all those other cooler electrons that refuse to let them fall to the bottom of the Fermi sea.
And with that I'll close. I realize that parts about metals are a bit remote from the original chemistry of benzene rings. But then again, as you get into longer and longer carbon chains such as the ones that carry off electrons from chlorophyll molecules, you may find that recognizing the "almost metallic" nature of multiple pairs and the way they interact in momentum space can be a good shorthand for understanding how some pretty complicated forms of large-molecule dynamics play out. |
For those who have never had the pleasure, see [this][1] video. It has been known since 1807 that dissolving sodium in liquid ammonia results in a beautiful color. It was originally thought the color was due to some familiar complex instead of a solvated electron. A similar phenomenon happens with other alkali metals in ammonia.
Research in the field says that it takes, at least, forty some ammonia molecules to solvate a given electron. Metstable cavities form and their stability depends highly on electrostatic interactions to solvate our electron. Ammonia will slowly react by evolving hydrogen gas,
$$ 2NH_3 + 2e^- \rightarrow H_2 + 2NH_2 ^-$$
Differences in solvent can play a huge role in the stability of our solvated electron. An analogous decomposition occurs in water,
$$ 2H_2O + 2e^- \rightarrow H_2 + 2 HO^- $$
Famously the latter occurs faster than the former, see [here][2] in comparison to my earlier link. We could say that these differences in rate reflect different stabilities of our solvated electron. Although, the addition of an appropriate catalyst to our ammonia will result in rapid evolution of hydrogen.
A more quantitative description for the difference in energies is obtained by measuring the UV-vis spectra for a solvated electron in both water and ammonia. One will quickly notice that the band appears at higher energies in water than it does in ammonia and there are considerable differences in the band shapes (the band is wider in ammonia).
**So Why Do They Differ**
So here I have to make the disclaimer that no current theories quantitatively reproduce the observed phenomena, such as the absorption spectrum for a solvated electron. This question has no established/accepted answer as it is an ongoing area of research.
The self-ionization of water has an equilibrium constant on the order of K = $10^{-14}$ and that of ammonia is on the order of $10^{-30}$. Hydronium formation occurs in the case of water and ammonium in the case of ammonia. Hydronium has a considerably lower pKa than ammonium and so it's reasonable to see why a reaction in water would be more likely with an electron. (Ammonia rarely produces a weakly acidic species, but water often produces a strongly acidic complex.)
A slightly deeper reason may be that water forms more ordered local domains/structures in solution than ammonia and this influences the rate by resulting in a larger entropy of activation when the hydrolyzed electron breaks up these larger structures, recall that $k \propto \exp (\Delta S ^\ddagger /R) $.
Just speculation though.
[1]: http://www.youtube.com/watch?v=o9KT7lM1UHU
[2]: http://www.youtube.com/watch?v=HY7mTCMvpEM |
For those who have never had the pleasure, see [this][1] video. It has been known since 1807 that dissolving sodium in liquid ammonia results in a beautiful color. It was originally thought the color was due to some familiar complex instead of a solvated electron. A similar phenomenon happens with other alkali metals in ammonia.
Research in the field says that it takes, at least, forty some ammonia molecules to solvate a given electron. Metstable cavities form and their stability could depend highly on electrostatic interactions to solvate our electron. Ammonia will slowly react by evolving hydrogen gas,
$$ 2NH_3 + 2e^- \rightarrow H_2 + 2NH_2 ^-$$
Differences in solvent can play a huge role in the stability of our solvated electron. An analogous decomposition occurs in water,
$$ 2H_2O + 2e^- \rightarrow H_2 + 2 HO^- $$
Famously the latter occurs faster than the former, see [here][2] in comparison to my earlier link. We could say that these differences in rate reflect different stabilities of our solvated electron. Although, the addition of an appropriate catalyst to our ammonia will result in rapid evolution of hydrogen.
A more quantitative description for the difference in energies is obtained by measuring the UV-vis spectra for a solvated electron in both water and ammonia. One will quickly notice that the band appears at higher energies in water than it does in ammonia and there are considerable differences in the band shapes (the band is wider in ammonia).
**So Why Do They Differ**
So here I have to make the disclaimer that no current theories quantitatively reproduce the observed phenomena, such as the absorption spectrum for a solvated electron. This question has no established/accepted answer as it is an ongoing area of research.
The self-ionization of water has an equilibrium constant on the order of K = $10^{-14}$ and that of ammonia is on the order of $10^{-30}$. Hydronium formation occurs in the case of water and ammonium in the case of ammonia. Hydronium has a considerably lower pKa than ammonium and so it's reasonable to see why a reaction in water would be more likely with an electron. (Ammonia rarely produces a weakly acidic species, but water often produces a strongly acidic complex.)
A slightly deeper reason may be that water forms more ordered local domains/structures in solution than ammonia and this influences the rate by resulting in a larger entropy of activation when the hydrolyzed electron breaks up these larger structures, recall that $k \propto \exp (\Delta S ^\ddagger /R) $.
Just speculation though.
[1]: http://www.youtube.com/watch?v=o9KT7lM1UHU
[2]: http://www.youtube.com/watch?v=HY7mTCMvpEM |
In “periodic table”, the adjective is related to the noun *period*, and comes from Ancient Greek *περίοδος* through French *périodique*. In “periodic acid”, it is formed from the prefix *per-* and *iodic* (like *peroxide* and *permanganate*).
[Wiktionary][1] lists their respective UK pronounciations of as `/pɪə(ɹ).iˈɒdɪk/` and `/ˌpɜːraɪˈɒdɪk/`, markedly different: `pɪə` (as in *piece*) vs. `pɜː` (as in *perfume*); then `i` (as in *it*) vs. `aɪ` (as in *eye*).
However, is that distinction really made in practice? Would a native US/UK/Aussie speaker make the difference when talking in the lab?
[1]: http://en.wiktionary.org/wiki/periodic#Etymology_1 |
Should one pronounce “periodic” the same in “periodic acid” and “periodic table”? |
Turbostratic graphite is graphite in which there is quenched rotational disalignment between adjacent graphene sheets, i.e. one sheet is rotated with respect to its neighbor. I suppose this could be considered a crystallographic defect of sorts. How do these turbostratic layers form, and can they possess long range order, such as glide or helical symmetry (similar, for instance, to a twisted nematic phase)? |
How are turbostratic graphite phases formed? |
What is the difference between melting and dissolving? I am looking some general features.
The answer should be adaptable to the melting/dissolving of ice cube (water) in a class of pure alcohol (ethanol) just below(or at the) melting point of ice, or similar phenomena.
I am now assuming that the ice is dissolving and melting at the same time.
|
What is the difference between melting and dissolving? |
I have a degree in chemical engineering and remember having seen a inductively coupled plasma mass spectrometer (ICP-MS) during the laboratory part of a chemistry course I took while at university, but it is many years ago. Now I would like to know what a typical output from a ICP-MS looks like and how I interpret the results.
There is no picture at the wikipedia page but some frequency domain like plots can be found with a Google search and I understand the Fourier transform and how it is used in number theory.
Are the ICP-MS results anything like the spectrum from the Fourier transform of some signal? |
What do typical results from inductively coupled plasma mass spectrometry (ICP-MS) look like, and how do I interpret them? |
For those who have never had the pleasure of personally doing this, see [this][1] video. It has been known since 1807 that dissolving sodium in liquid ammonia results in a beautiful color. It was originally thought the color was due to some familiar complex instead of a solvated electron. A similar phenomenon happens with other alkali metals in ammonia.
Research in the field says that it takes, at least, forty some ammonia molecules to solvate a given electron. Metstable cavities form and their stability could depend highly on electrostatic interactions to solvate our electron. Ammonia will slowly react by evolving hydrogen gas,
$$ 2NH_3 + 2e^- \rightarrow H_2 + 2NH_2 ^-$$
Differences in solvent can play a huge role in the stability of our solvated electron. An analogous decomposition occurs in water,
$$ 2H_2O + 2e^- \rightarrow H_2 + 2 HO^- $$
Famously the latter occurs faster than the former, see [here][2] in comparison to my earlier link. We could say that these differences in rate reflect different stabilities of our solvated electron. Although, the addition of an appropriate catalyst to our ammonia will result in rapid evolution of hydrogen.
A more quantitative description for the difference in energies is obtained by measuring the UV-vis spectrum for a solvated electron in both water and ammonia. One will quickly notice that the band appears at higher energies in water than it does in ammonia and there are considerable differences in the band shapes (the band is wider in ammonia).
**So Why Do They Differ**
So here I have to make the disclaimer that no current theories quantitatively reproduce the observed phenomena, such as the absorption spectrum for a solvated electron. This question has no established/accepted answer as it is an ongoing area of research.
The self-ionization of water has an equilibrium constant on the order of K = $10^{-14}$ and that of ammonia is on the order of $10^{-30}$. Hydronium formation occurs in the case of water and ammonium in the case of ammonia. Hydronium has a considerably lower pKa than ammonium and so it's reasonable to see why a reaction in water would be more likely with an electron. (Ammonia rarely produces a weakly acidic species, but water often produces a strongly acidic complex.)
A slightly deeper reason may be that water forms more ordered local domains/structures in solution than ammonia and this influences the rate by resulting in a larger entropy of activation when the hydrolyzed electron breaks up these larger structures, recall that $k \propto \exp (\Delta S ^\ddagger /R) $.
Just speculation though.
[1]: http://www.youtube.com/watch?v=o9KT7lM1UHU
[2]: http://www.youtube.com/watch?v=HY7mTCMvpEM |
For those who have never had the pleasure of personally doing this, see [this][1] video. It has been known since 1807 that dissolving sodium in liquid ammonia results in a beautiful color. It was originally thought the color was due to some familiar complex instead of a solvated electron. A similar phenomenon happens with other alkali metals in ammonia.
Research in the field says that it takes, at least, forty some ammonia molecules to solvate a given electron. Metstable cavities form and their stability could depend highly on electrostatic interactions to solvate our electron. Ammonia will slowly react by evolving hydrogen gas,
$$\ce{ 2NH_3 + 2e^- \rightarrow H_2 + 2NH_2 ^-}$$
Differences in solvent can play a huge role in the stability of our solvated electron. An analogous decomposition occurs in water,
$$\ce{ 2H_2O + 2e^- \rightarrow H_2 + 2 HO^- }$$
Famously the latter occurs faster than the former, see [here][2] in comparison to my earlier link. We could say that these differences in rate reflect different stabilities of our solvated electron. Although, the addition of an appropriate catalyst to our ammonia will result in rapid evolution of hydrogen.
A more quantitative description for the difference in energies is obtained by measuring the UV-vis spectrum for a solvated electron in both water and ammonia. One will quickly notice that the band appears at higher energies in water than it does in ammonia and there are considerable differences in the band shapes (the band is wider in ammonia).
**So Why Do They Differ**
So here I have to make the disclaimer that no current theories quantitatively reproduce the observed phenomena, such as the absorption spectrum for a solvated electron. This question has no established/accepted answer as it is an ongoing area of research.
The self-ionization of water has an equilibrium constant on the order of K = $10^{-14}$ and that of ammonia is on the order of $10^{-30}$. Hydronium formation occurs in the case of water and ammonium in the case of ammonia. Hydronium has a considerably lower pKa than ammonium and so it's reasonable to see why a reaction in water would be more likely with an electron. (Ammonia rarely produces a weakly acidic species, but water often produces a strongly acidic complex.)
A slightly deeper reason may be that water forms more ordered local domains/structures in solution than ammonia and this influences the rate by resulting in a larger entropy of activation when the hydrolyzed electron breaks up these larger structures, recall that $k \propto \exp (\Delta S ^\ddagger /R) $.
Just speculation though.
[1]: http://www.youtube.com/watch?v=o9KT7lM1UHU
[2]: http://www.youtube.com/watch?v=HY7mTCMvpEM |
Mercury is toxic. It forms amalgams and inorganic mercury compounds/salts are available.
My question is on toxicity. I am interested in its toxicity, not the mode of action (as it will be part of biology SE)
1. Is amalgam toxic? (sort of yes / no)
2. Mercury salts are toxic? (sort of Yes / no)
3. If mercury salts / amalgams are non-toxic, what makes them non-toxic. Is it because of the ionic nature / other metal or any other reason? |
I have been reading about resolution in Mass Spectrometry and there are a few things I do not understand.
The resolving power is determined by m/(m2-m1) at full width half height of a peak. The higher the value obtained the better. This is where I struggle to form a link between the resolution of the Mass spectrometer and the resolving power. In image A below the resolution of the Mass Spectrometer is 1000 and would have a very low mass resolution value, but in image B the resolution is 5000 and the nominal peaks masses are separated, therefore providing a high mass resolution value.
Why can a mass spectrometer with a resolution of 5000 resolve the nominal masses and the mass spectrometer with a resolution of 1000 cannot? and how does the mass resolution come into play? Also, what do the values of 1000 and 5000 mean?
![enter image description here][1]
[1]: https://i.stack.imgur.com/xKyH9.png |
In [Density Functional Theory](http://en.wikipedia.org/wiki/Density_functional_theory) courses, one is often reminded that [Kohn-Sham orbitals](http://en.wikipedia.org/wiki/Kohn%E2%80%93Sham_equations) are often said to bear no any physical meaning. They only represent a noninteracting reference system which has the same electron density as the real interacting system.
That being said, there are plenty of studies in that field’s literature that given KS orbitals a physical interpretation, often after a disclaimer similar to what I said above. To give only two examples, KS orbitals of [H<sub>2</sub>O][1]<sup>\[1]</sup> and [CO<sub>2</sub>][2] closely resemble the well-known molecular orbitals.
Thus, I wonder: **what good (by virtue of being intuitive, striking or famous) examples can one give as a warning of intepreting the KS orbitals resulting from a DFT calculation?**
---
\[1] “What Do the Kohn-Sham Orbitals and Eigenvalues Mean?”, R. Stowasser and R. Hoffmann, *J. Am. Chem. Soc.* **1999**, *121*, 3414–3420.
[1]: http://www.roaldhoffmann.com/pn/modules/Downloads/docs/444s.pdf
[2]: https://wiki.fysik.dtu.dk/dacapo/Solution_3#kohn-sham-wavefunctions-of-a-co-molecule |
In a thesis I am reading, it is said that one of the reasons for using plane-wave basis sets for first-principles molecular dynamics (aka *ab initio* MD) is that the Pulay forces<sup>\[1,2]</sup> that arise from an MD using atomic basis sets are computationally expensive to calculate.
While I understand that having additional terms means more code to write, i.e. they make writing the software harder to write, is it true that they are CPU-intensive to compute? The criterion I would use to quantify this subjective statement is:
> Given that you have already calculated the energy and forces at that point, you have already computed a large number of integrals required for this task and involving basis functions: overlap integrals, terms of the form $\left\langle\phi_\alpha\left|\hat{A}\right|\phi_\beta\right\rangle$ where operator $\hat{A}$ is either the hamltonian, its gradient, or any other operator necessary for the calculation of energy or forces. Would the calculation of Pulay forces require any more integrals to be evaluated, or can it be straightforwardly computed from those previously-calculated integrals alone?
---
\[1] P. Pulay, *Molec. Phys.* 19, 197 (1969)<br>
\[2] See also slide 6 of [this](http://tfy.tkk.fi/~asf/physics/lectures/PDF/lect11.pdf) |
The most stable cyclohexane form is the chair conformation but on the other hand, the bigger the side-chain of the cyclohexane is, its hydrogen atoms become more equatorial rather than axial, which brings them closer to the cyclohexane's hydrogens.
Shouldn't they arrange in a less energetic, but a more stable conformation? |
Why is it that the bigger the side-chain is, its hydrogen atoms tend to be more equatorial? |
What is the difference between melting and dissolving? I am looking some general features.
The answer should be adaptable to the melting/dissolving of ice cube (water) in a class of pure alcohol (ethanol) just below(or at the) melting point of ice, or similar phenomena.
I am now assuming that the ice is dissolving and melting at the same time.
**Edit:**
In other words which reaction energy is higher in the following reactions:
$H_2O(s) \to H_2O(l)$
$H_2O(s) + n EtOH(l) \to H_2O \centerdot EtOH_n$
Or, are there substances that release more energy in desolvation than consume in melting?
|
Are there reactions like
$X(s) + n Y(l) \to X \centerdot Y_n(l)$
that are endothermic? What are X and Y then?
The material X should also be soluble to material Y. |
Which desolvation reactions are endothermic? |
I very much like the first answer, especially the superb visuals!
However, I will nonetheless add a simpler approach to answering your question, which is this: Pauli exclusion holds as long as there is _something_ in space or in spin (spin being the pairs of electrons) that firmly distinguishes two "stable" states.
What that means is that your initial intuition is essentially correct if _all_ the electron pairs shared exactly the same molecular orbital.
So the answer to your question in general terms is that electron pairs settle into molecule-sized orbitals (molecular orbitals) that are physically spread out over the entire ring, but which nonetheless have unique geometric features that keep them distinguished in real space. I would add that they are even more unique when represented in an abstract space called momentum space, where the geometric differences translate into actual distances between the electron pairs. These abstract distances in momentum space satisfy the Pauli exclusion principle just as well as physical separation in real space, so that even very similar looking orbitals can remain distinct under Pauli exclusion.
For more details and some easy visual analogies, you can read more in the addendum below.
----------
The same general problem of how to distinguish electron pairs to meet Pauli exclusion exists in metals. In that case, the conduction electrons are free to drift over the entire macroscopic metal. The result is an even more extreme need than in benzene for the pairs to distinguish themselves firmly from each other, since for conduction electrons _all_ of these gazillions [technical term] of pairs must find "unique places" in which they can maintain their unique identity. That's tough!
Now it turns out that these two problems of benzene electrons and metallic conduction electron pairs are related in a rather deep fashion, since a benzene ring can be interpreted (quite correctly) as a special tiny case of a one-dimensional metal. That means that if you understand what happens conceptually in metals, you are well on the way to understanding all forms of hybridization in organic molecules. So, let's take a quick look at how that works.
The situation for electron pairs in metals is most easily understood by imagining the pairs moving back and forth along the $x$ length of a very narrow, highly conductive, and unusually regular metallic wire. Next, borrow the $y$ and $z$ plane to represent the complex plane (complex phase of a wave function). The amplitude for finding an electron from the pair becomes the radius of the complex plain at each point along $x$. For stable electron pair states, these radii and their phases will exhibit a lot of continuity as you move their as along the $x$ length of the wire. They will also rotate at the same speed in the complex plane of $y$ and $z$.
In fact... if you calculate the stable or "stationary" states possible in such a configuration, you will find that they bear a striking resemblance both visually and mathematically to skip rope solutions, that is, to the ways that you can twirl a skip rope so that it maintains stable, unmoving (the "stationary" part) loops.
So let's look at that situation as a good analog solution to the quantum equations. Anyone can make one large loop, and that in fact represents the electron pair state with the lowest possible energy along the length of the wire, or the base state. Incidentally, the volume enclosed by the skip rope is proportional to the square of the radius, which as I mentioned earlier is the "amplitude" of the wave function. So... that means that if you simply look at the volume enclosed by the skip rope as it turns, you pretty much instantly know what the probability of finding an electron from the pair is. In the case of the base state, it means that the electron pair will tend to be found in the center of the wire, and almost never at the ends.
In chemistry such varying probabilities of finding electrons are usually represented as densities in ordinary space, without the benefit of the dynamics of the complex plane. And for good reason, since for anything except wires, trying to show the complex plane would result in five-dimensional spaces. Those are tough to visualize! Instead, the idea of a _phase_ is attached to tell the relative positions of the rotating points in the unseen complex plane.
The skip rope in the one-dimensional wire makes the idea of phase a lot easier to visualized. For example, good skip-ropers can easily create double loop skip ropes. These correspond to the first excited state of the electron pair, one with a bit more energy added (you have to twirl the rope faster) and two distinct loops. Phase just means this for such a double loop: When one loop is towards you, the other is opposite of you. You can label one such loop positive and one negative, but of course it's all arbitrary, and really just a technique for roughly describing those invisible but rope-like complex plane dynamics.
Notice that the volume-equals-probability rule still applies to the double loop. That means that in the first excited state of the electron pair, the odd are good and roughly equal for finding it in the left loop or right loop, but close to zero at the ends _and_ at the center. So, this really is a unique state for the excited electron pair, one that is quite distinct from the first single-loop or base state.
Now there's an interesting bit of mathematics, Fourier transforms, in which _any_ very long sinusoidal shape can always be distinguished very sharply from any other sinusoidal, even if they only differ slightly in wavelength. People use this idea all the time, since in the case of radio waves (which are sinusoidal waves, mostly) it's called tuning a radio!
The uniqueness of long sinusoidal waves is relevant here because it turns out that Pauli exclusion is met equally well if two pairs of electrons are separated from each other in _frequency_ space.
So, another way of saying all of this is that when electrons are "delocalized" either in bonds or in metals -- it really is the same idea -- they can achieve uniqueness by bumping around _either_ in ordinary space _or_ in a more abstract frequency space called momentum space. Both work equally well. The "extra quantum number" you speculated about in your question could be equated to this idea of additional separation in momentum space for example.
So, the benzene orbitals maintain Pauli exclusion uniqueness by finding specific stable solutions that are similar but geometrically distinct, and which are even more unique when interpreted in terms of frequencies or loop counts. Those equate to a quite different-looking, distance-like separations in the abstract space called momentum space.
So to finish up, what about that extreme case I mentioned of metals that have an electron or two for _every_ atom in what may be a very large crystal?
That's a truly fascinating case, a sort of "benzene gone bad" scenario in which the need for Pauli exclusion uniqueness pushes the electron pairs into absolutely bizarre behaviors, ones very important to modern (and ancient) technologies: Some of the electrons are forced to get very hot. Now that's weird!
What happens is easy to see with the above background. The first pair of electrons in a silver wire is very, very cold, near absolute zero, moving so slowly that it forms only a single loop in the center of the wire. To maintain Pauli exclusion -- the momentum space version of it to be precise -- the next pair must look like a skip rope with two loops, and the third three, and so forth. You get the picture.
But each such loop has more energy, even if only by a little bit! So what happens when you have a gazillion such electron pairs all vying for uniqueness?
Someone gets pushed very, _very_ high up in frequency, and thus also in energy, while other lurk in the deeper and colder regions of this pile of moving electrons. The result is almost like a sea in momentum space, and that is in fact what it is called: A Fermi sea, with cold electron pairs at the bottom, and some truly _hot_ ones at the surface. Fermi seas are also referred to by the more common phrase of "conduction bands," meaning the range of energies that the electrons occupy in order to maintain Pauli uniqueness.
You see Fermi surfaces all the time, since it is _only_ the hottest electrons in a metal that interact with photons and bounce them back at you. You are in effect looking directly at a hot Fermi surface every time you look into a silver or aluminum backed mirror.
How hot? Well, believe it or not, X-ray hot. A typical piece of ordinary shiny metal, say a fork or spoon, would kill you from X-ray exposure if all of its hot Fermi surface and near-surface electrons could suddenly fall back into a "ground state" of just one loop. They are tamed and prevented from doing so by all those other cooler electrons that refuse to let them fall to the bottom of the Fermi sea.
And with that I'll close. I realize the parts about metals are a bit remote from the original chemistry of benzene rings. But then again, as you get into longer and longer carbon chains such as the ones that carry off electrons from chlorophyll molecules, you may find that recognizing the "almost metallic" nature of multiple pairs and the way they interact in momentum space can be a good shorthand for understanding how some pretty complicated forms of large-molecule dynamics play out. |
> Is this even how GC/MS results work?
As cbeleites said the method you described is a proper technique but not likely to be appropriate given the information you cited.
In GC/MS you should have two sets of information. The first is the GC Total Ion Chromatograph (TIC) which will have time as the x-axis and response (abundance) as the y-axis. For each retention time on the TIC there will be a corresponding Mass Spectra (MS). In the MS the x-axis is m/z (ion) and y-axis is also response (abundance).
Different compounds have different responses so if you inject the exact same amount of two different compounds you could get a much larger response from one compared to the other. For example tramadol gives much higher response compared to hydrocodone. This is why you want your internal standard to be structurally similar to your analyte.
Quantification of a compound is often done by running 3 - 5 calibrators at known concentrations in order to make a calibration curve. Once an acceptable calibration curve is made the sample along with controls can be fit onto the curve to calculate the amount of the compound of interest in the sample and controls. If the controls are correct then the sample value can be used.
To make a calibration curve you need an internal standard in each calibrator, control and sample. You can then make the curve either with ion ratios between paired internal standard and analyte ions OR if your internal standard response is consistent in all the calibrators, controls and sample you can use the GC peak response.
In conclusion if you only have one GC/MS run with the two dissimilar compounds in it you will not be able to easily calculate the quantity of the second compound based on the GC response of the first unless you have additional information that was not listed in this question.
|
Many computational chemistry packages permit the calculation of vibrational and electronic spectra. These spectra are obtained as a set of discrete eigenvalues however they are often convolved with some distribution to give a continuous spectrum that is realistic for a finite temperature, rather than a sequence of impulse functions.
The ADF package allows the user to convolve the spectrum with either a Gaussian or Cauchy (Lorentzian) function. My understanding is that the latter affords a more realistic line broadening, however the Gaussian must be there for a reason.
Which spectral broadening scheme is preferred? Why the choice? |
When simulating spectral line broadening, which convolution is preferred? |
The identification of point groups of a molecule is usually done following a strict scheme, either manually or algorithmically. In all textbooks I could find, however, the first step of the scheme is actually not explicited: in this example scheme drawn from Housecroft and Sharpe,
![enter image description here][1]
you can see that there is a very unhelpful step *“Does this molecule have $I_h$, $O_h$ or $T_d$ symmetry?”*. It is implied that one would recognize e.g. an icosahedral molecular compound on sight. However, I wonder: how can one strictly identify these “special” point groups? What set of rules is to be followed (again, either manually or algorithmically)?
[1]: https://i.stack.imgur.com/W05SA.png |
How does one recognized Td/Oh symmetry in molecules? |
`Is desolvation always exothermic?`
No it is not always
Dissolving $\ce{NaOH}$ is Exothermic while $\ce{NH4NO3}$ is an endothermic reaction(Fromm my personal experience)
`Is melting always endothermic?`
Yes it always is (except for the exception F'x gave). THis is because during melting the body to be melted takes in `latent heat of fusion` from the surroundings thus bringing down the temperature.
`Can alcohol dissolve ice?`
Lower alcohol like methanol and ethanol may be able to dissolve ice ad they can effectively form H-bond with water but higher alcohols may not be able to do that. |
Why does ice cream make soda fizz? |
Some additional information relevant to this question.
I'm going to use "forces" and "particles" in this explanation to generalize it. Particles can be ions (ionic compounds held together by strong electrostatic forces), metal atoms (held together by electrostatic forces from attraction of the metal atom nuclei for the "sea of electrons") or molecules (for covalent compounds held together in the solid or liquid by intermolecular forces: London dispersion forces, dipole-dipole attractions, and hydrogen bonding. Note: we are not talking about the intramolecular covalent bonds in this explanation.) So, solids and liquids are held together in that form by forces between the particles.
The reason that melting is always endothermic (except for the exceptions given by F'x...which I also did not know about) is that when you melt a substance, you have to break up some of the forces holding the particles together in the solid so that they can move past each other and form a liquid. This takes energy, the "heat of fusion" (usually given on a per mole basis) mentioned above. The stronger the forces, the more energy it would take to melt the substance. As long as you are melting a substance and holding the temperature at the melting point (i.e. not adding enough energy to raise the temperature) then you will have an equilibrium between the solid and liquid.
I think there is some confusion in the terms you are using. **Desolvation** means removing the solvent from a solute-solvent mixture. (This can happen in crystallization, for example.) **Solvation** (which I think you're asking about) is the interaction of solvent particles with solute particles. **[Dissolving or dissolution][1]** is the process in which a solid, liquid, or gas forms a solution with a solvent.
So, as answered by Ashu, dissolution and be either exothermic or endothermic as he's noted above. This is true because in order to dissolve a substance in a solute, it takes energy to break up the solvent-solvent interactions, it takes energy to break up the solute-solute interactions, and you get energy back from new solvent-solute interactions. If you have stronger attractions between the solvent and solute than you had originally, then the process releases energy and is exothermic. The same concepts can be applied to mixing two liquids...which is usually referred to by saying they are "miscible" instead of speaking of one dissolving in the other. Ethanol and water are miscible in all proportions.
Can alcohol dissolve ice ("at or just below the melting point of ice" as phrased in your original question)? There are a couple of ways to answer this. If you put ice in ethanol at 0 degrees (its melting point) it will melt because the melting point of ice in a mixture of water and ethanol is lower than the melting point of pure water. The melting point depends on the proportions of water and ethanol.(That is to say, as some melted, it could not refreeze as you would expect to happen in an equilibrium mixture, so it would continue to melt.) See [colligative properties][2] for more information.
You've given two equations, H2O(s)→H2O(l)
H2O(s)+nEtOH(l)→H2O⋅EtOHn
In either case, you will need to provide the heat of fusion to melt the ice. So the question you asked is really about the energy of solvation of water in ethanol. When you mix water and ethanol, the flask feels warmer, so this is an exothermic process; you get more energy back than you put into it because of good interactions (hydrogen bonding) between water and ethanol particles. That solvation energy would provide some of the energy needed to melt the ice.
For the broader question "can something happen at a given temperature", you're generally asking if it happens spontaneously. To answer this you'd have to calculate the [Gibbs Free Energy][3]. This would take into account not only the energy considerations spoken of above, but also entropy considerations (which would matter here). If the Gibbs Free Energy value is negative, then the process or reaction is spontaneous at the temperature given. So, the answer to which would be the preferred process would also depend on entropy considerations.
[1]: http://en.wikipedia.org/wiki/Dissolution_%28chemistry%29
[2]: http://en.wikipedia.org/wiki/Colligative_properties
[3]: http://en.wikipedia.org/wiki/Gibbs_free_energy |
We have a well that delivers hard water and we do not have a water softener. (There was a “green sand filter” that utilizes permanganate to oxidize other metals on the system, but it has been disconnected.) Analysis of the well water shows the presence of iron in noticeable amounts. No other metal ions were specifically identified. The system does have an ozonator to oxidize Fe (II) to Fe (III). Over the last couple of years, the porcelain toilet bowls have developed a blue ring at the air/water interface. When you scrub on the blue ring, it doesn’t feel like there is a deposit present (like normal hard water deposits). The ring is unresponsive to ammonia (household), strong acid (6 M HCL) or strong base (Drano), as well as to hydrogen peroxide (30 %). (We haven’t tried a good reducing agent yet.) So, my question is, does anyone have an idea of what is causing the blue color? Is it possible that porcelain reacted chemically with salts, urea, uric acid, oxygen, etc? Not an earth-shattering theoretical question, but I’m chemically curious as to what this could be if anyone has any ideas. |
Brands like Brita and Pur (in the U.S.) have made a name for themselves for the ability of their product (essentially a large-pore filter with activated carbon/charcoal) to extract the added chlorine from tap water. I visualize $\ce{C}^*$ as a vast network of channels in which ions are "trapped". I had assumed that most cations and anions were trapped effectively, but it seems to favor chlorine and iodine.
According to [Wikipedia](http://en.wikipedia.org/wiki/Activated_carbon):
>Activated carbon does not bind well to certain chemicals, including alcohols, glycols, strong acids and bases, metals and most inorganics, such as lithium, sodium, iron, lead, arsenic, fluorine, and boric acid.
Why would it not bind effectively to fluorine? Is it an issue of atomic radius? Does $\ce{F-}$, with a radius of [0.136 nm](http://boomeria.org/chemlectures/textass2/table10-9.jpg) sneak by, while $\ce{I-}$ and $\ce{Cl-}$ at 0.181 and 0.216 nm, repectively, get caught up in the matrix? Why do the cations get passed through? |
Why does activated carbon preferentially adsorb anions? |
The ideal gas equation (daresay "law") is a fascinating combination of the work of dozens of scientists over a long period of time.
I encountered Van der Waal's interpretation for non-ideal gases early on, and it was always somewhat "closed-form"
$${\bigl(p + \frac{n^2a}{V^2}\bigr)(V - nb) = nRT}$$
with `a` being a measure of the charge interactions between the particles and `b` being a measure of the volume interactions.
Understandably, this equation is only still around for historical purposes, as it is largely inaccurate.
Fastforwarding to the 1990s, [Wikipedia](http://en.wikipedia.org/wiki/Equation_of_state#Elliott.2C_Suresh.2C_Donohue_equation_of_state) has a listing of one of the more current manifestations (of Elliott, Suresh, and Donohue):
$$\frac{PV{_m}}{RT} = 1 + Z^{rep}+Z^{att}$$
where the repulsive and attractive forces between the molecules are proportional to a shape number (c = 1 for spherical molecules, a quadratic for others) and reduced number density, which is a function of Boltzmann's constant, etc (point being, a lot of "fudge factors" and approximations getting thrown into the mix).
Rather than seeking an explanation of all of this, I am wondering whether a more "closed form" solution lies at the end of the tunnel, or whether the approximations brought forth in the more modern models will have to suffice?
|
Why are equations of state for a non-ideal gas so elusive? |
When drawing a skeletal formula, what is the difference between an angular version and a linear version?
I was asked to draw the Z isomer of Reversatol:
![E Reversatol][1]
For which I drew:
![My Z Reversatol][2]
However the markscheme states that:
> skeletal structure must be correct **and angular not linear**
I haven't come across the difference between the two before and can't find anything on google to suggest one. Their drawing of the correct answer is equivalent to mine, however I am concerned that I may have drawn the linear version as my benzene rings are in a line and theirs are not. Can anyone put my mind at ease?
Their Version:
![Their Z Reversatol][3]
[1]: https://i.stack.imgur.com/sKlxu.png
[2]: https://i.stack.imgur.com/5gRaA.jpg
[3]: https://i.stack.imgur.com/2qYhs.jpg |
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