Wednesday, May 18, 2011

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    =    H2 – 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 = qp
                           \               DH = qp.              
               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.

No comments:

Post a Comment