Page 86 - Instant notes

Physical Chemistry 72

equilibrium constant expression above and the value of K remains unchanged. In contrast,
if C or D is added, this again perturbs the equilibrium, Q increases, ∆G becomes positive
and the backward reaction is favored over the forward reaction. Equilibrium is again re-
established with the consumption of C, D and the production of A, B until the
concentrations are related by the equation for K above, with the value of K remaining
unchanged. A further perturbation to the system could be to increase the overall pressure
of a system involving gases. For example, for:

the equilibrium constant for the reaction as written is given by:

Increasing the overall pressure causes an increase in all of the partial pressures. As the
equilibrium constant involves more moles of gas on the RHS of the equation than the
LHS, equilibrium is lost and the reaction quotient, Q, becomes larger than K. From the
equation:

∆G becomes positive, the backward reaction becomes spontaneous and N 2 and H 2 react
to form NH 3 until the partial pressures are again related by the equilibrium constant
expression given above. In contrast, if more moles of gas were present overall on the
LHS of an equilibrium compared with the RHS, Q would decrease when the overall
pressure was increased, ∆G would become negative, the forward reaction would become
spontaneous and would occur, decreasing the amount of gas in the system. Equilibrium
would again be re-established when the equilibrium partial pressures were related by the
equilibrium constant expression, with an overall decrease in pressure.
The system could also be perturbed by a change in temperature rather than
concentration. In this case, the equilibrium constant would change value according to the
expression , so that if , the change in enthalpy of the
forward reaction were endothermic (see Topic B3) an increase in the temperature, T, of
the system would increase the value of the equilibrium constant K. The equilibrium
condition would therefore be lost and the existing reaction quotient, Q (the value of the
old equilibrium constant, K) would be less than this new equilibrium constant, and ∆G
for the forward reaction would be negative. This means that C, D would be produced at
the expense of A, B until equilibrium were re-established. This would be an endothermic
process, with heat being taken up from the system during the reaction, reducing the initial
temperature rise.
In contrast, if the forward reaction were exothermic (see Topic B3), increasing T
would decrease the value of K, making ∆G positive for the forward reaction (the reverse
reaction spontaneous) and A, B would be produced at the expense of C, D until
equilibrium were re-established with the concentrations related by the value of the new
equilibrium constant. Again this process would be endothermic, and heat would be taken
up during the reaction, which would again reduce the initial temperature rise.