39
internal energy for the system under consideration. Recall
that, the enthalpy changes are commonly termed "heats of
reaction" for processes, entropy may be used as a measure ,
of the degree of order or disorder, and that the internal
energy is unavailable for work of any form. The change in
the free energy, dG, may now be expressed as
dG = dE + PdV + VdP TdS SdT (1-35)
For a reversible process in which no work other than
expansion occurs,
PdV = dw (1-36)
and
dE dq + dw = 0 (1-37)
The terms q and w refer to the heat absorbed, and work
done, by the system respectively. Equation (1-37) is a
direct consequence of the First Law of Thermodynamics and
states, in effect, that the total change in the internal
energy of a system is the sum (or difference) of the amount
of heat absorbed, dq, by the system and the amount of work
done, dw, by the system. Notice that heat and work are not
functions of state (which are conventionally assigned upper
case symbols). Equation (1-35) now becomes
dG = VdP SdT
(1-38)