Page 69 - Bruno Linder Elementary Physical Chemistry

August 18, 2010 11:36 9in x 6in b985-ch06 Elementary Physical Chemistry

54 Elementary Physical Chemistry

and

Π= −RT/V ln x A ≈−RT/[V ln(1 − x B )]
= x B RT/V ≈ n B /[(n A + n B )(RT/V m )]

=(n B /V )RT ≈ [B]RT (6.19)

6.2.6. Chemical Reaction Equilibria

The relation between K and ∆G o was already discussed in previous
chapters. Here, we will make a few remarks which were not sufficiently
emphasized before.

a) From the free energy or chemical potentials, we can derive for the
reaction

aA + bB ··· =cC + dD ··· (6.20)

producing the quilibrium constant

c
d
a
K = a a .../a a b (6.21)
A B
C D
where the activities are related to partial pressures or to concentrations,
etc. The expression of K in terms of the activities is exact, but the
relation of the activities to pressure, to concentration, as shown in
Eqs. (5.22a–c), are approximate. Activities are equal to the mole fraction
times activity coefficients, which are not easily calculated.
b) Consider the heterogeneous reaction,

CaCO 3(s) = CaO(s) + CO 2(g) (6.22)

It is found that K depends only on the partial pressure P(CO 2)of
the gas, i.e. K = P(CO 2 ) and not on the other substances. Reason:
the activity of each solid is 1, and the activity of the gas is to a good
approximation equal to the partial pressure.