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Enthalpy and exothermic reactions

The reaction enthalpy ΔH\Delta H shows whether a reaction releases or absorbs heat at constant pressure. An exothermic reaction releases heat to the surroundings and has ΔH<0\Delta H < 0, like combustion, while an endothermic reaction absorbs heat and has ΔH>0\Delta H > 0.

ΔH<0\Delta H < 0exothermic: the reaction releases heat
ΔH>0\Delta H > 0endothermic: the reaction absorbs heat

Symbols

ΔH\Delta Hreaction enthalpykJ/mol

Example

Combustion of methane releases heat to the surroundings: the reaction is exothermic with ΔH<0\Delta H < 0.

Exothermic reactions can still proceed slowly without heat input — the sign of ΔH\Delta H says nothing about reaction rate.
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