0
$\begingroup$

One of the parts in a question I'm trying to solve for an assignment goes like this:

A key issue in the Hydrogen Economy is Hydrogen Storage. This can be restored if a substance $X$ can bind to $H_2$. The binding should be strong enough so that $XH_2$ is a liquid or solid. Necessarily, $X$ must also be a liquid or solid. Consider the chemical reaction at constant pressure:

$ X(l) + H_2(g) \rightarrow XH_2 (l) $

Write a relation between $\Delta G^{\circ}(T)$ and $\Delta S^{\circ}$ for this reaction. Take $\Delta S^{\circ}(T)$ to be a constant of temperature, equal to $ -85 kJ/mol $. Justify this approximation.

(and then there are more parts)

I know how to write the relation ($\Delta G^{\circ}(T)$ = $\Delta H^{\circ}(T)$ - T$\Delta S^{\circ}$ since $S$ is constant), but how do I justify the approximation?

I'm confused, because the value given seems to be the one given by Trouton's rule - but if you use Trouton's rule, then entropy of $X$ and $XH_2$ are both approximately equal, so $\Delta S^{\circ}(T) \approx -S_{H_2}$. But entropy of $H_2$, being a gas, would massively change with temperature, and would definitely not be $-85 kJ/mol.$

Does that mean the approximation is wrong, or am I thinking in the wrong direction?

$\endgroup$
2
  • $\begingroup$ Entropy must have unit J/K, respectively molar entropy J/K/mol. Additionally, respective Delta S and delta H are either both functions of T, either considered both constant, as they are linked by the difference of heat capacity of reactants and products. $\endgroup$
    – Poutnik
    Commented Nov 11, 2023 at 13:17
  • $\begingroup$ Further parts of the question require calculation of delta H at different temperatures, so they don't expect us to take that as a constant. How, then, do I justify the constant delta S? $\endgroup$
    – poirot_06
    Commented Nov 11, 2023 at 13:32

0

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Browse other questions tagged or ask your own question.