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Consider if I burn some octane.

The enthalpy released is $5430 \;\text{kJ/mol}$. The Gibbs free energy released is $5537 \;\text{kJ/mol}$.

From my understanding, enthalpy is the heat change in the reaction - it is the heat released in the reaction. Gibbs Free energy is the amount of non-expansive work done. I can accept that out of the $5537 \;\text{kJ}$, there will be $5430 \;\text{kJ/mol}$ of thermal energy output, which is "non-expansive". But how about the other $107 \;\text{kJ/mol}$? Where did this energy go? Where can I see it?

Typically, the example of non-expansive work done is electrical or magnetic work. I can appreciate that, but in this context, since there are no magnets or electric charges moving, where is that non-expansive work?

John Hon
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  • I think a question underlying this is "how can a combustion reaction be carried out reversibly?" Does that make sense to you? – Chet Miller Jul 23 '21 at 12:37
  • It is the maximal work possible. Now if you had an octane fuel cell, you could capture some of this, https://www.sciencedirect.com/science/article/abs/pii/S2211285512000729 – Karsten Jul 23 '21 at 21:45
  • Related: https://chemistry.stackexchange.com/questions/154236/does-entropy-contribute-work – Karsten Jul 23 '21 at 21:50

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