Change in Enthalpy

The absolute value of E cannot be calculated; therefore, from above equation it follows that absolute value of H is undetermined. Hence it is meaningful to calculate change in enthalpy. The change in enthalpy is given as; 

                               DH    =    H_{2 –} H₁

Where H₁ denoted enthalpy of system in initial state and H₂ in final enthalpy of the system.

Therefore,         DH  =  (E₂  +  P₂V₂) – (E₁ + P₁V₁)

                                               =   E₂   + P₂V₂  –   E₁ – P₁V₁

                                               =   E₂ – E₁ + (P₂V₂ – P₁V₁)

                                               =   DE + (P₂V₂ – P₁V₁)

               If the pressure is kept constant i.e. P₁ = P₂= P, then

                               DH = DE + P (V₂ – V₁)

                               DH = DE + PDV                                                                             

                              But,         DE + PDV = q_p

                           \               DH = q_p.              

               Hence, the change in enthalpy of a system (DH) may be defined as, the amount of heat absorbed or evolved at constant pressure, provided that only work done is of (pressure – volume) type.