Physical Chemistry 2.pdf - OER@AVU - African Virtual University

1.8 Ideal solutions
µA(g)
A(g), B(g)
µA(l)
A(l), B(l)
African Virtual University
Figure 1.6 The chemical potential of the gaseous form of the pure substance A,
µA(g), equals that of the pure liquid A, µA).
To consider the chemical potential of liquids we first consider the equilibrium that
exists between a liquid and its vapour pressure. We make use of the fact that at equilibrium,
the chemical potential of a substance in the liquid is equal to the chemical
potential of the substance in the vapour. Let us consider a pure liquid A with chemi-
* *
cal potential µ A ( l)
with a vapour pressure P A . The chemical potential for the pure
liquid A can be written as
*
* ° PA µ A (l) = µA +RT ln
⎛ ⎞
⎝
⎜
P° ⎠
⎟
(1.18)
If another solute was introduced into to the liquid then the vapour pressure above
is no longer that of the pure substance
potential is thus given by
µ
A
( l)
µ
= °
A
⎛ PA
⎞
+ RT ln⎜
⎟
⎝ P°
⎠
*
P A but the partial pressure P . The chemical
A
(1.19)
By combining equations (1.18) and (1.19) we obtain the equation (1.20), without the
standard chemical potential of the gas.