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Chemistry · Glossary

What is Exothermic, endothermic, athermic?

Also known as: exothermic · endothermic · athermic

Definition 1.4 University Chemistry — Year 2 · Chapter 1 — Enthalpies of Reaction

A reaction is exothermic in a given state if ΔrH<0\Delta_r H < 0: run forward at constant TT and pp, it gives heat to the surroundings. It is endothermic if ΔrH>0\Delta_r H > 0, and athermic if ΔrH=0\Delta_r H = 0.

Examples

Example 1.8 (Formation reactions)

The formation reaction of water is HX2(g)+12 OX2(g)→HX2O(l)\ce{H2(g) + 1/2O2(g) -> H2O(l)}, with ΔfH∘=−285.83 kJ/mol\Delta_f H^\circ = -285.83\,\mathrm{kJ}/\mathrm{mol} at 298.15 K298.15\,\mathrm{K}; that of methane is C(s)+2 HX2(g)→CHX4(g)\ce{C(s) + 2H2(g) -> CH4(g)}, ΔfH∘=−74.9 kJ/mol\Delta_f H^\circ = -74.9\,\mathrm{kJ}/\mathrm{mol}; that of nitrogen monoxide, 12 NX2(g)+12 OX2(g)→NO(g)\ce{1/2N2(g) + 1/2O2(g) -> NO(g)}, ΔfH∘=+90.3 kJ/mol\Delta_f H^\circ = +90.3\,\mathrm{kJ}/\mathrm{mol}: an endothermic compound, which the hot gases of an engine nevertheless make. The half coefficients are not a problem: the formation reaction is a bookkeeping device, not a mechanism.

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