at constant (standard) pressure and constant temperature (usually 298 K) may be written as

As shown above, at constant pressure the heat of the reaction is exactly equal to the enthalpy change, , of the reacting system.

Variation with temperature or pressure

The variation of the enthalpy of reaction with temperature is given by Kirchhoff's Law of Thermochemistry, which states that the temperature derivative of ?H for a chemical reaction is given by the difference

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