What is Thermochemistry?

      Chemical reactions are almost invariably accompanied by energy changes which, ordinarily, appear in the form of evolution or absorption of heat.  The branch of chemistry which deals with energy changes involved in chemical reactions is called Thermochemistry.  Thermochemistry, in fact, applies the First law of thermodynamics to chemical reactions.  The energy change that occurs in a chemical reaction is largely due to change of bond energy, it results from the breaking of bonds in reactants and formation of new bonds in products.

Change of Internal Energy in a Chemical Reaction:

     Let us consider a chemical reaction taking place at constant temperature and at constant volume.  In such a case, w=0 and hence from the equation of the First law (viz.,  𝞓U = q+w),

                                                        𝞓U =qv

where qv is the heat exchanged at constant volume.

suppose Ur is the internal energy of the reactants and Up that of the products, then, 

                                                       𝞓U =Up - Ur

  We may represent this conclusion schematically as below:

↓ 𝞓U = Up - Ur = qv = Heat of reaction at constant volume

products (Up)