Standard enthalpy change of reaction

The standard enthalpy change of reaction (denoted ΔH° or ΔHo) is the enthalpy change that occurs in a system when one mole of matter is transformed by a chemical reaction under standard conditions.

For a generic chemical reaction


 * nA A + nB B + ... → nP P + nQ Q ...

the standard enthalpy change of reaction ΔHr is related with the standard enthalpy change of formation ΔHf of the reactants and products by the following equation:

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

In this equation, the ni corresponds to the stoichiometric coefficients.

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), neutralisation (standard enthalpy change of neutralisation), and solution (standard enthalpy change of solution).

Reaktionsenthalpie Enthalpie de réaction