ABSTRACT
Many processes are carried out at constant pressure, usually that of the atmosphere. For most chemical reactions, the only kind of work done by or on the system is the PV-work that is associated with the volume changes that occur during the process. We saw earlier that we can’t use the expression –P6V if the pressure does not stay constant during the process; instead, we must use –PdV, which indicates that P can be considered constant only over the small volume change dV of the system. To obtain the total PV-work done during the process we must integrate, to get
On the other hand, if Pext is constant during the process, then Eqn [1] reduces to
= < <P (V V )ext 2 1 = < P Vext syst6 . . . [2]
which is just the same expression we had before; however, if the external pressure varies during the process,1 then the value of the integral must be determined in order to evaluate W.
