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Thermochemical equations


Thermochemical equations:
A thermochemical equation may be defined as a chemical equation which includes the heat changes occurring the chemical reaction.In chemistry,we come across several reactions which are accompanied by heat changes. Some reactions are accompanied by the evolution of heat while others are accompanied by the absorption of heat.
The reaction which takes place with the evolution of heat is called an exothermic reaction and that which takes place with the absorption of heat is called an endothermic reaction.
For example,
                   c+o2 _co2+ heat (exothermic reaction)
                    c+2s_cs2-heat (endothermic reaction)
The heat energy evolved in an exothermic reaction can also be expressed as change in enthalphy i.e.,&. since energy has been evolved,heat contents of products(hp)becomes less than heat contents of reactants(hr).
                  &h=hp-hr = -ve (for exothermic reaction)
Thus, we conclude that in an exothermic reaction carried at constant temperature and pressure,                                           
                  heat evolved is given positive sign and
              change in enthalpy & is given negative sign.
Similarly,the heat energy absorbed in an endothermic reaction can also be expressed as change in enthalpy i.e.,&.Since energy has been absorbed, heat contents of the products(hp)becomes more than heat contents of the reactants(hr).
              &h=hp-hr = +ve (for endothermic reaction)
Thus, we conclude that in an endothermic reaction carried at constant temperature and pressure,
               heat absorbed is given negative sign and
          change in enthalpy (&h)is given positive sign.
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