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I have read that in a reversible process, entropy can be transferred but not generated. Such that the entropy of the universe stays constant. On the other side, there is the Gibbs Energy, which can be defined as $-TdS$, where $dS$ is the change of the entropy of the universe, meaning also an exergonic chemical reaction, for instance, increases the entropy of the universe. As far as I know, for Gibbs Energy Processes occur under the assumption that they are isothermal and isobaric, as well as proceeding in a reversible manner. Where do I have the mistake?

Here is the source from where I have the $-TdS$ definition

Urb
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Achilles Zehnter
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2 Answers2

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In a reversible process, a system is in thermal equilibrium with its surroundings at every instant of change. As a result the morsel of energy it looses at a temp. $T$ is the morsel of energy surroundings gain at the same temp. $T$, hence the net entropy change in reversible processes is zero.

Note that in a reversible process, while the entropy of the universe is unchanged - of the system's isn't.

If you know about state variables skip to What this means below

While its 'easiest' to calculate entropy change for reversible processes, its numerically same for irreversible processes too that take the system between the same initial and final state. However, its not as easy to calculate.

Observables like these that are path independent and only dependent on the initial and final system state are called state variables. Gibbs free energy is one of them.

What this means is that

during exergonic reactions, where $\Delta G_{sys}<0$, occurs regardless of the path the system takes, as reasoned above. So reversibility is not a pre-requisite for spontaneous reactions. In fact for reversible processes, $\Delta G_{sys}=0$ as they are always in equilibrium thus technically, not spontaneous.

Hence, for spontaneous reactions that occur irreversibly, there can be a change in entropy of the system and the universe without any contradictions.

lineage
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For the typical case of a reaction process that is carried out reversibly, going from pure reactants at T and P to pure products at T and P, the $\Delta G$ refers to the change in Gibbs free energy of the system (i,e, the chemicals participating in the reaction), not the universe. The change in free energy of the universe in such a situation is zero, with $\Delta H_{system}=-\Delta H_{surroundings}$ and $\Delta S_{system}=-\Delta S_{surroundings}$.

Chet Miller
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