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38 Chapter 4
Partial Pressures
When gases are involved in a reaction, the partial pressure of each gas
has to be considered. For the general reaction
where p is the partial pressure of the gas and pc is the standard
pressure. Kp denotes the equilibrium constant obtained by using the
partial pressures of the gases, instead of their concentrations. For this
reason, Kp is always dimensionless, as all pressures are divided by the
standard pressure. This is implicit in the definition of the standard
Gibbs free energy change, AGO. Therefore, the above equation can be
expressed in a more convenient form as
where p(A)‘ = p(A)/p+,p(B)‘ =p(B)/p+, etc.
For simplicity, the prime will be dropped in subsequent equations
involving partial pressures, but it is always implied. For example, for
the reaction involving the manufacture of ammonia by the Haber
process,
But, Kp is related to Kc: for the reaction
But, from Chapter 1, the equation of state of an ideal gas is defined
as:
