How would it be found the Gibbs energy at a certain temperature, if they just give you another Gibbs energy at a temperature? For example: Given $\Delta G = -230 Kcal/mol$ at $773K$ for the reaction $4/3Al + O_2 > 2/3Al_2O_3$, find the Gibbs energy at $573K$

With that data I can only think of doing $(-230*573)/773$, but as far as I know the Gibbs energy is not only related to the temperature.


You have to know the entropy change of the reaction $\Delta S$, because the variation of $\Delta G$ with temperature is equal to -$\Delta S$. So when you know $\Delta S$, you multiply it by the $\Delta T$, here $200 K$, and subtract the result from your original value of $\Delta G$. This calculation is a first approach. To be more precise, you would have to take into account the change of heat capacity of the three substances ($\ce{Al, O2, Al2O3}$) when $T$ goes from $773 K$ to $573 K$. Usually this change is small and can be neglected.

  • $\begingroup$ Yeah I know how the $\Delta G$ varies, the problem is the exercise, just determining the $\Delta G$ at two different temperatures. But as you say, maybe I could assume the change of the heat capacity and entropy can be neglected. $\endgroup$ – brt88 Jan 20 at 10:55
  • $\begingroup$ No. You should know the entropy of your substances, calculate the difference product minus reagent, multiply by $\Delta T$ and substance it to $\Delta G$. This is sufficient. And do ignore the changes of heat capacity, which is much smaller than the effect of$\Delta S$. $\endgroup$ – Maurice Jan 20 at 11:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.