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
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 equation