Page 83 - Instant notes

Fundamentals of equilibria 69

where p i is the partial pressure of i. The approximation can be applied as gas molecules
are neutral and widely separated, which means that γ i≈1. The standard pressure for a
gas, , is defined as 1 atmosphere. Thus, for an equilibrium involving gases, such as

, the equilibrium constant is
given by:

and, in a similar fashion to concentrations, the values of the partial pressures of the gases
at equilibrium (in units of atmospheres, see Topic D1) can be used to obtain the
dimensionless value of K. For equilibria involving gases and species in solution, for
example the solubility of a gas in a liquid, the partial pressures and concentration terms of
the respective species, as appropriate, are combined to produce the equilibrium constant
expression.
When a pure solid or a pure liquid is present in the equilibrium, such as:

it is clearly meaningless to talk of a variation in concentration within a pure substance
such as A (see Topic D1). For these substances the activity is unity under all conditions.
This is equivalent to ignoring pure solids and liquids when drawing up the equilibrium
expression, so that no terms (activity, a, concentration, c, or pressure, p, as appropriate)
due to these species are present, which in the above example gives:

Physical transitions

An equilibrium can also exist between the same species in two different forms e.g. solid
and liquid, liquid and gas. This is a physical transition or a transition in the state of the
system. For an equilibrium between a liquid A and a gas A (the vaporization and
condensation of A):

the equilibrium constant is given by the expression: