What is more stable: a carbonate ion or a sulphate ion? [closed]








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The sulphate ion has a higher number of resonance structures than the carbonate ion. Hence, it should be more stable than the carbonate ion.
But my book says that the carbonate ion is stronger than the sulphate ion. Where am I going wrong?




*Stability* is a type of unicorn. You do not know what it means *a priori* and, most importantly, you cannot deduce it from a structural formula *a priori*. The only valid measure of stability is thermodynamic stability according to standard formation enthalpies. The following values are taken from the [NIST Chemical Webbook](http://webbook.nist.gov/chemistry/):


$$\begin{array}{ccc}\hline
\text{compound} & \Delta\_\mathrm{f}H^0\_\text{(liquid)} & \text{source}\\
\hline\ce{Na2CO3} & -1108.51~\mathrm{kJ/mol} & \text{[1]}\\
\ce{Na2SO4} & -1356.38~\mathrm{kJ/mol} & \text{[2]}\\
\hline
\end{array}$$


Gas phase heats of formation would be even better for comparison because then you could consider ‘single’ species, but at least the liquid phase does not contain a significantly skewing lattice enthalpy term.


According to this data, sulphate should be more stable than carbonate. **This is not simply because** of simple explanations such as:


* ‘more resonance structures’ (sulphate actually has only one significant resonance structure, carbonate three)
* geometrical features such as being tetrahedral
* electronegativity (carbon is less electronegative, one would assume its anion to be more stable)
* resistance to reduction
* an equilibrium with a gaseous compound
* whatever else your teacher may tell you.


The thermodynamic stabilities of carbonate and sulphate do not compare well with each other since there are far too many differences between the two molecular ions. You might as well ask whether iron or carbon are more stable.




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**Sources:**


[1]: M. W. Chase Jr. *J. Phys. Chem. Ref. Data, Monograph 9*, **1998**, 1.  

[Link to the relevant Webbook page](http://webbook.nist.gov/cgi/cbook.cgi?ID=C497198&Units=SI&Mask=8#Thermo-React).


[2]: M. W. Chase Jr. *J. Phys. Chem. Ref. Data, Monograph 9*, **1998**, 1.  

[Link to the relevant Webbook page](http://webbook.nist.gov/cgi/cbook.cgi?ID=C7757826&Units=SI&Mask=2#Thermo-React).


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