gibbs free energy equation

I've got a question regarding the classical gibbs or helmholtz free
energy equations. I understand that they yield the maximum work
extractable for a quasistatic process as constrained by the second
law, i.e DSuniv = 0 for the quasistatic case. I'm having difficulty
however intuiting why it works.

For instance, I know that the oxidation or rusting of iron to form
iron oxide is both exothermic and entropy reducing. Therefore by
Gibbs' logic you cannot extract 100 percent of the energy release of
the reaction as work since the DS for system is negative and by
extracting all of the chem. pot. energy released as work, the net DS
for the universe would be -ve which is impossible. Therefore the
equation would suggest that you must waste enough of that energy
release as heat to the environment to increase the entropy of the
universe by the amount which the reaction decreased it. My issue
however is that in trying to visualize the situation dynamically I
can't figure out why that's necessary.

I'm visualizing sold iron being attacked by oxygen gas molecules which
together are transforming into iron oxide solid molecules. Of course
I understand why the entropy of the products is lower and that thermal
energy is being released via the bond breakage of the reactants vs.
those formed by the products, but why couldn't in principle all of
that thermal energy be soaked up as mechanical work by the
surroundings via say expansion without letting any leak out as heat.
As I said, I know the 2nd law says this can't work but I'm not
intuiting why when visualizing the process step by step.

Any thoughts?

Cheers,
Renai
.

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