Standard enthalpy change of reaction

The standard enthalpy change of reaction 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