# Is exergonic the same as endothermic or exothermic?

May 25, 2014

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".

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

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.