Heat of reaction is the change in enthalpy or internal energy as represented by a balanced thermochemical equation. The amount of energy released during a chemical change depends upon the state of reactants and products, the conditions of pressure and volume at which reaction is carried out, and temperature. The variation of heat of reaction (Delta H or Delta E) with temperature is given as DeltaH_(2) - Delta H_(1) = Delta C_(p) [T_(2) - T_(1)] or Delta E_(2) - Delta E_(1) = Delta C_(v) (T_(2) - T_(1)) . Standard heat enthalpy of elements in their most stable state is assumed to be zero whereas standard heat enthalpy of compound is referred as heat of formation of that compound at 1 atm pressure and 25^(@)C . Oxidation of N_(2) to N_(2) O , NO , NO_(2) showsabsorption of energy whereas heat of combustion of N_(2) is exothermic like other heat of combustion. Heat of vaporisation of H2O is 627.78 cal/g. If heat of formation of H_(2) O_((l)) is -68.3 kcal , heat of formation of H_(2) O_((g)) is :