Exergonic refer to changes in the Gibbs free energy. Exothermic and endothermic refer to changes in enthalpy.
Exothermic and endothermic refer to changes in ##?H##. Exergonic and endergonic refer to changes in the ##?G##.
“Exo” and “exer” mean “out of”. “Endo” and “ender” mean “into”.
##?H## decreases for an exothermic process and increases for an endothermic process.
##?G## decreases for an exergonic process and increases for an endergonic process.
For a given reaction, the change in Gibbs free energy is
##?G = ?H ? T?S##.
##?G## is a measure of the spontaneity of a reaction. If ##?G## is negative, the process is spontaneous. If ##?G## is positive the process is not spontaneous.
We have four possibilities:
1. ##?H## < 0 and ##?S## > 0 always gives ##?G## < 0.
The process is both exothermic and exergonic. It is always spontaneous.
2. ##?H## > 0 and ##?S## < 0 always gives ##?G## > 0.
The process is both endothermic and endergonic. It is never spontaneous.
3. ##?H## > 0 and ##?S## > 0.
This gives ##?G## > 0 at low temperatures. The process is both endothermic and endergonic.
At high temperatures, ##?G## < 0. The process is still endothermic but it has become exergonic. The process is spontaneous only at high temperatures.
An example is the endothermic decomposition of calcium carbonate.
CaCO?(s) ? CaO(s) + CO?(g).
?S is positive because the reaction produces a gas from a solid. CaCO? is stable at room temperature but decomposes at high temperatures.
4. ##?H## < 0 and## ?S## < 0.
This gives ##?G## < 0 at low temperatures. The process is both exothermic and exergonic.
At high temperatures, ##?G## > 0. The process is still exothermic but it has become endergonic. It is no longer spontaneous.
An example is the exothermic synthesis of ammonia.
N?(g) + 3H?(g) ? 2NH?(g)
Increasing the temperature increases the yield of ammonia. But it drives the position of equilibrium to the left.