Is exergonic the same as endothermic or exothermic?

1 Answer
May 25, 2014

Answer:

Exergonic refer to changes in the Gibbs free energy. Exothermic and endothermic refer to changes in enthalpy.

Explanation:

Exothermic and endothermic refer to changes in enthalpy #ΔH#. Exergonic and endergonic refer to changes in the Gibbs free energy #ΔG#.

"Exo" and "exer" mean "out of". "Endo" and "ender" mean "into".

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#ΔH# decreases for an exothermic process and increases for an endothermic process.

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#ΔG# decreases for an exergonic process and increases for an endergonic process.

For a given reaction, the change in Gibbs free energy is

#ΔG = ΔH − TΔS#.

#ΔG# is a measure of the spontaneity of a reaction. If #ΔG# is negative, the process is spontaneous. If #ΔG# is positive the process is not spontaneous.

We have four possibilities:

1. #ΔH# < 0 and #ΔS# > 0 always gives #ΔG# < 0.

The process is both exothermic and exergonic. It is always spontaneous.

2. #ΔH# > 0 and #ΔS# < 0 always gives #ΔG# > 0.

The process is both endothermic and endergonic. It is never spontaneous.

3. #ΔH# > 0 and #ΔS# > 0.

This gives #ΔG# > 0 at low temperatures. The process is both endothermic and endergonic.

At high temperatures, #ΔG# < 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.

CaCO₃(s) → CaO(s) + CO₂(g).

ΔS is positive because the reaction produces a gas from a solid. CaCO₃ is stable at room temperature but decomposes at high temperatures.

4. #ΔH# < 0 and# ΔS# < 0.

This gives #ΔG# < 0 at low temperatures. The process is both exothermic and exergonic.

At high temperatures, #ΔG# > 0. The process is still exothermic but it has become endergonic. It is no longer spontaneous.

An example is the exothermic synthesis of ammonia.

N₂(g) + 3H₂(g) ⇌ 2NH₃(g)

Increasing the temperature increases the yield of ammonia. But it drives the position of equilibrium to the left.