Enthalpy

Since the change in internal energy is the heat energy exchanged between system and surroundings at constant volume but most of the chemical reactions and physical processes are carried in open tubes or containers since these are under the influence of atmospheric pressure which is constant at particular place. Thus when there undergoes a chemical change or physical change, the volume will not remain constant but either decreases or increases. In case the volume increases some work is done by system and its internal energy decreases. If there is contraction in volume, some work is done on the system. As a result, its internal energy increases.

               Thus it is concluded that at constant temperature and pressure the energy change in reaction is not only due to change in internal energy alone but also include, the energy changes due to expansion or contraction. To express these changes taking place at constant pressure, a term known as Enthalpy is introduced.

               The enthalpy of a system may be defined as “The sum of the internal energy and pressure volume (PV) energy.”

               i.e.  Enthalpy   =    Internal Energy   +    PV

               Enthalpy is also called Heat Content and is denoted by H.