One of the great applications of thermodynamics is its role in predicting the direction in which a process will go. Thermodynamics is not able to give any idea about how long it will take for the process to be completed. But knowing the directionality helps in setting up proper temperature and pressure conditions for reactants as well as assembling appropriate electrodes in batteries. From equations (21.12) and (21.14), since TdS dq
dS – dq/T 0
(21.26)
Here dS and dq refer to the system only and not to the surroundings. When no PV work is done PdV = 0,
dU = (dq) V and
dS – dU/T0 or
(dS) U, V0 or
(21.27)
(dU) S, V0
(21.28)
Thus, for all natural processes which are irreversible, eq. (21.28) is valid. This is a criterion for a spontaneous or an irreversible process .The criteria are in terms of constant energy or constant entropy, which are difficult to realize in practice. Constant volume/pressure and temperature processes are readily accessible and we look for such suitable criteria now.
At constant pressure (dq)P = dH and eq.(21.26) becomes
dq 
