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 `(DeltaH_(1) or DeltaE)` with temperature is given as `DeltaH_(2)-DeltaH_(1)=DeltaC_(P)[T_(2)-T_(1)] or DeltaE_(2)-DeltaE_(1)=DeltaC_(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)` shows absorption of energy whereas heat of combustion of `N_(2)` is exothermic like other heat of combustion.
Standard heat enthalpy has been assumed to be zero for `:`