Standard enthalpy - Definition and Overview

The standard enthalpy change of reaction (denoted H⁰ or H^O)is the enthalpy change that occurs in a system when 1 equivalent of matter is transformed by a chemical reaction under standard conditions.

For a generic chemical reaction

n_A A + n_B B + ... → n_P P + n_Q Q ...

the standard enthalpy change of reaction ΔH_r is related with the standard enthalpy change of formation ΔH_f of the reactants and products by the equation

<math> \Delta\!H_r = \sum_{products}{n_i \Delta\!H_f^{(i)}} - \sum_{reactants}{n_i \Delta\!H_f^{(i)}} <math>

Reactions with standard values

A common standard enthalpy change is the standard enthalpy change of formation, which has been determined for a vast number of substances. The enthalpy change of any reaction under any conditions can be computed, given the standard enthalpy change of formation of all of the reactants and products. Other reactions with standard enthalpy change values include combustion (standard enthalpy change of combustion) and neutralisation (standard enthalpy change of neutralisation).