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Chemistry · Glossary

What is Exergonic and endergonic?

Also known as: exergonic · endergonic

Definition 2.15 University Chemistry — Year 2 · Chapter 2 — Entropy and Free Energy of Reaction

A reaction is exergonic in a given state when ΔrG<0\Delta_r G < 0 and endergonic when ΔrG>0\Delta_r G > 0. With every species in its standard state these words apply to the sign of ΔrG∘\Delta_r G^\circ.

Examples

Example 2.16 (The cold pack)

For NHX4NOX3(s)→NHX4X+(aq)+NOX3X−(aq)\ce{NH4NO3(s) -> NH4+(aq) + NO3-(aq)} at 298.15 K298.15\,\mathrm{K}, standard states: ΔrH∘=−339.87+365.56=+25.69 kJ/mol\Delta_r H^\circ = -339.87 + 365.56 = +25.69\,\mathrm{kJ}/\mathrm{mol} and ΔrS∘=259.8−151.08=+108.7 J/(K mol)\Delta_r S^\circ = 259.8 - 151.08 = +108.7\,\mathrm{J}/(\mathrm{K}\,\mathrm{mol}), so ΔrG∘=25.69−298.15×0.1087=−6.7 kJ/mol\Delta_r G^\circ = 25.69 - 298.15 \times 0.1087 = -6.7\,\mathrm{kJ}/\mathrm{mol}: the dissolution is endothermic and exergonic. The ions, free in the water, are more disordered than in the crystal, and the entropy term TΔrS∘=32.4 kJ/molT\Delta_r S^\circ = 32.4\,\mathrm{kJ}/\mathrm{mol} outweighs the enthalpy cost.

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