Endothermic reaction — heat is a “reactant”, DH = +242 kJ.

This does not occur spontaneously.

Consider the combustion of H2 to form water . .

H2(g) + 1/2 O2(g) ---> H2O(g) + 242 kJ

Exothermic reaction — heat is a “product”. DH = -242 kJ.

This is spontaneous and proceeds readily once initiated.

USING ENTHALPY

BUT . . . Decomposition of water can be made to occur

by coupling to another, spontaneous process . . .

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