# Where is my non-expansive work in combustion?

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?

• I think a question underlying this is "how can a combustion reaction be carried out reversibly?" Does that make sense to you? Jul 23, 2021 at 12:37
• It is the maximal work possible. Now if you had an octane fuel cell, you could capture some of this, sciencedirect.com/science/article/abs/pii/S2211285512000729
– Karsten
Jul 23, 2021 at 21:45
• – Karsten
Jul 23, 2021 at 21:50