Dear User, The standard enthalpy of reaction: (denoted ΔrH⊖) is the enthalpy change (amount of heat absorbed or evolved) that occurs in a system when one mole of matter is transformed by a chemical reaction under standard conditions. The bond enthalpy can be used to calculate enthalpy of a reaction. Enthalpy of a reaction = Sum of bond enthalpy of reactants - Sum of bond enthalpy of products Regards