Page 79 - Instant notes

C1
FUNDAMENTALS OF EQUILIBRIA

Key Notes
An equilibrium is established when the rate of the forward and
backward reactions are equal. This is denoted for a general
reaction by:

At equilibrium, the Gibbs free energy changes for both the
forward and backward reactions are zero. The equilibrium
constant, K, is given by:

where a i is the activity of species i.
For systems with negligible interaction between the reacting
species (gases, neutral molecules and ions at low dilution) the
equilibrium constant is given by:

where c i is the concentration of species i and , the standard
−3
concentration, is 1 mol dm . The equilibrium constant can
therefore be calculated from the concentrations of the reacting
−3
species in mol dm .
The equilibrium constant for the general reaction with all species
in the gaseous state is:

where p i is the partial pressure of the gaseous species i and is
the standard pressure of 1 atmosphere. The equilibrium constant
can therefore be calculated from the partial pressures of the
reacting species in atmospheres. Pure solids and pure liquids have
unit activity; this means there are no partial pressure or
concentration terms for pure solid or pure liquid species in the
equilibrium constant expression.