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free energy
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Extended explanation
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Spontaneity of physical and chemical processes
For a physical or chemical process at constant temperature and pressure, the change in Gibbs free energy determines whether the process is spontaneous or not.
Consider the chemical reaction
A → B The reaction is spontaneous, i.e. the reaction will occur in the direction of the products, if the change in G is negative:
ΔG ≡ GB - GA < 0 For a more intuitive sense of why this is so, consider that
G = U + PV - TS
Since a lower value of G is favored, it means that a chemical reaction favors the direction that:
- Minimizes the internal energy U,
- Minimizes the volume, in the presence of nonzero pressure, and
- Maximizes the entropy
Moreover:- Increasing the pressure increases the tendency to minimize the volume
- Increasing the temperature increases the tendency to maximize the entropy
It can be shown that a negative ΔG always results in an entropy increase for the "universe", i.e. the entropy of the system plus the surrounding environment.
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Commentary
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