Which substances have the highest/lowest enthalpy of formation per gram?

Which substances have the most extreme enthalpies of formation per gram at standard temperature and pressure?




Jonathan Ray has indicated he has used data from either [Wikipedia](https://en.wikipedia.org/wiki/Standard_enthalpy_of_formation) or [NIST Webbook](https://webbook.nist.gov/) in his comment. Actually, NIST Webbook is an excellent choice because it has listed wide variety of thermochemical data. However, there were few important references and points he did not mention. For example, in that tabulated data set, the state of compound(s) wasn't clear. If you look at the data reported for calcium fluoride (as liquid and solid), you may understand my point (see below table). I'm here trying to correct those facts and some other errors in above table given by my colleague. One such erroneous data is the value given for cyclopropene (g). Actually, value given above is more closed to that of cyclopropane than for cyclopropene (see below table). For readers' benefits, I also want to emphasize that Ref.1 has tabulated a lot of data, which are NIST Webbook incorporated (in case if you don't trust NIST Webbook).


$$
\begin{array}{ccc}
\hline
\text{Compound} &\text{Formula} & M\text{, g/mol} & \Delta\_\mathrm fH^\circ\text{, kJ/mol} & \Delta\_\mathrm fH^\circ\text{, kJ/g} & \text{Ref.} \\\hline
\text{Beryllium oxide (s)} & \ce{BeO} & \pu{25.01} & \pu{−609.43} & \pu{-24.57} & \pu{1} \\
\beta\!-\!\text{Beryllium hydroxide (aq)} & \ce{Be(OH)2} & \pu{43.03} & \pu{−905.84} & \pu{-21.05} & \pu{2} \\
\alpha\!-\!\text{Beryllium hydroxide (aq)} & \ce{Be(OH)2} & \pu{43.03} & \pu{−902.70} & \pu{-20.98} & \pu{2} \\
\text{Lithium Hydroxide (s)} & \ce{LiOH} & \pu{23.95} & \pu{-484.93} & \pu{-20.25} & \pu{1} \\
\alpha\!-\!\text{Aluminium oxide (s)} & \ce{Al2O3} & \pu{101.96} & \pu{-1675.69} & \pu{-16.43} & \pu{1,3}\\
\text{Calcium fluoride (s)} & \ce{CaF2} & \pu{78.07} & \pu{-1225.91} & \pu{-15.70} & \pu{1} \\
\text{Calcium fluoride (l)} & \ce{CaF2} & \pu{78.07} & \pu{-1186.07} & \pu{-15.19} & \pu{1} \\
\text{Hydrogen fluoride (g)} & \ce{HF} & \pu{20.01} & \pu{-272.55} & \pu{-13.62} & \pu{1} \\
\text{Cyclopropane (g)} & \ce{C3H6} & \pu{42.08} & \pu{53.30} & \pu{1.27} & \pu{4}\\
\text{Nitric oxide (g)} & \ce{NO} & \pu{30.01} & \pu{90.29} & \pu{3.01} & \pu{1}\\
\text{Cubane (g)} & \ce{C8H8} & \pu{104.15} & \pu{622.2} & \pu{5.97} & \pu{5}\\
\text{Cubane (g)} & \ce{C8H8} & \pu{104.15} & \pu{597.1} & \pu{5.73} & \pu{6}\\
\text{Cyclopropene (g)} & \ce{C3H4} & \pu{40.06} & \pu{277} & \pu{6.91} & \pu{7}\\
\text{2-Butynedinitrile (g)} & \ce{NCCCCN} & \pu{76.06} & \pu{533.46} & \pu{7.01} & \pu{1}\\
\text{Acetylene (g)} & \ce{HCCH} & \pu{26.04} & \pu{226.73} & \pu{8.71} & \pu{1}\\
\hline
\end{array}
$$


**References:**


1. M. W. Chase, Jr., *NIST-JANAF Themochemical Tables, Fourth Edition, J. Phys. Chem. Ref. Data, Monograph 9*, 1998, 1–1951.
2. I. J. Bear, A. G. Turnbull, “The Heats of Formation of Beryllium Compounds. I. Beryllium Hydroxides,” *J. Phys. Chem.* **1965**, *69(9)*, 2828–2833 (DOI: 10.1021/j100893a004).
3. R. C. King, G. T. Armstrong, “Heat of Combustion and Heat of Formation of Aluminum Carbide,” *Journal of Research of the National Bureau of Standards–A. Physics and Chemistry* **1964**, *68A(6)*, 661–668 (<http://dx.doi.org/10.6028/jres.068A.066>).
4. J. W. Knowlton, F. D. Rossini, “Heats of combustion and formation of cyclopropane,” *Journal of Research of the National Bureau of Standards* **1949**, *43(2)*, 113–115 (<http://dx.doi.org/10.6028/jres.043.013>).
5. B. D. Kybett, S. Carroll, P. Natalis, D. W. Bonnell, J. L. Margrave, J. L. Franklin, “Thermodynamic Properties of Cubane,” *J. Am. Chem. Soc.* **1966**, *88(3)*, 626–626 (DOI: 10.1021/ja00955a056).
6. A. Bashir-Hashemi, J. S. Chickos, W. Hanshaw, H. Zhao, B. S. Farivar, J. F. Liebman, “The enthalpy of sublimation of cubane,” *Thermochimica Acta* **2004**, *424(1-2)*, 91–97 (<https://doi.org/10.1016/j.tca.2004.05.022>).
7. K. B. Wiberg, W. J. Bartley, F. P. Lossing, “On the strain energy in cyclopropene and the heat of formation of the $\ce{C3H4+}$ ion,” *J. Am. Chem. Soc.* **1962**, *84(20)*, 3980–3981 (DOI: 10.1021/ja00879a049).


 4

* Beryllium oxide −23.96 kJ/g
* Lithium fluoride −23.75 kJ/g
* Beryllium fluoride −21.84 kJ/g
* Beryllium hydroxide −21.01 kJ/g
* Lithium hydroxide −20.24 kJ/g
* Lithium oxide −19.93 kJ/g
* Boron trioxide −18.29 kJ/g
* Magnesium fluoride −18.04 kJ/g
* Aluminum fluoride −17.98 kJ/g
* Boron trifluoride −16.76 kJ/g
* Aluminum oxide −16.43 kJ/g
* Water −15.86 kJ/g
* Magnesium hydroxide −15.85 kJ/g
* Magnesium oxide −14.92 kJ/g
* Hydrogen fluoride −13.66 kJ/g
* Lithium hydride −11.4 kJ/g
* Boron nitride −10.24 kJ/g
* Carbon dioxide −8.941 kJ/g
* Lithium nitride −4.73 kJ/g
* Methane −4.6 kJ/g
* Ammonia −2.71 kJ/g
* Diborane 1.31 kJ/g
* Cyclopropane 1.33 kJ/g
* Cubane 5.73 kJ/g
* Dicyanoacetylene 6.579 kJ/g
* Various unstable nitrogen allotropes >10 kJ/g
* Free hydrogen (not bound in diatomic molecules) 216.34 kJ/g


 4

This is a fun question. Given that you are looking for kJ/g rather than kJ/mol, it seemed most fruitful to explore the possible compounds that can be formed from the lightest-weight elements. 


So: I was unable to find any compounds to beat those in the lists provided by Jonathan and Matthew for exothermic heats of formation. But, *considering compounds with endothermic heats of formation*, gaseous beryllium monohydride, $\ce {BeH\_{(g)}}$, has a standard enthlapy of formation (at 298 K) of 321 kJ/mol = **32 kJ/g**.


Source: Chase, M.W., Jr., NIST-JANAF Themochemical Tables, Fourth Edition, J. Phys. Chem. Ref. Data, Monograph 9, 1998, 1-1951.


[Secondary source: <https://webbook.nist.gov/cgi/cbook.cgi?ID=C13597972&Units=SI&Mask=FFFF> ]


And considering polyatomic ions, gaseous helium hydride, $\ce {HeH\_{(g)}^+}$, has a standard enthlapy of formation (at 298 K) of 1348.3 kJ/mol = **269.1 kJ/g**. 


Source: <https://atct.anl.gov/Thermochemical%20Data/version%201.122g/species/?species_number=1213>


Of course, the substance with the highest enthlapy of formation is probably the omega molecule:
<https://memory-alpha.fandom.com/wiki/Omega_molecule>


 0