Heat of reaction (or) enthalpy of reaction

The Heat of Reaction or Enthalpy of Reaction may be defined as, “The amount of heat evolved or absorbed in a chemical reaction when number of moles of the reactants as represented by balanced chemical reaction have completely reacted.”

For Example:

                C (s)   +   O₂ (g)         ------------>   CO₂ (g)  ;                           DH   =   – 393.5 KJ.

                CH₄ (g)  +   2O₂ (g)   ------------>   CO₂ (g)  +   2H₂O (l)  ;       DH   =  –390 KJ.

                N₂ (g)    +    3H₂ (g)   ------------>   2NH₃  (g)  ;                        DH  =  – 92.3 KJ