When a reaction at equilibrium is subjected to a change of temperature, pressure or concentration of one (or more) of the reacting substances, then the system adjusts to a new equilibrium stage as dictated by the change of variable. These changes can be predicted qualitatively by the general principle known as Le Chatelier's principle. This principle may be stated as follows.
If a system at equilibrium is subjected to a change, the system adjusts to a new equilibrium stage in such a way so as to oppose or reduce the change.
The applications of the above principle may be discussed one by one.
Change in Concentration
If the concentration of one of the reacting substances is increased, then according to Le Chatelier principle, the system will adjust in such way so as to decrease the concentration of the substance.
Taking an example of the reactionFe3+(aq) + SCN-(aq) [Fe(SCN)]2+(aq)
we may discuss the effects of adding more Fe3+(aq), SCN-(aq) and Fe(SCN)2+ one by one.
Adding More of Fe3+(aq) or SCN-(aq)
The concentration of the ions Fe3+ or SCN- is increased. To reduce the concentration, the equilibrium is shifted to right-hand side, i.e. more of Fe(SCN)3 is formed. This conclusion can also be verified from the expression for the equilibrium constant. We have
If concentration of Fe3+ or SCN- is increased, the denominator of the above expression is increased. Now since the equilibrium constant has a constant value at a given temperature, the only possibility is that the value of the numerator (i.e. [Fe(SCN)]2+ should be increased, i.e. more [Fe(SCN)]2+ is formed with the consumption of added Fe3+ or SCN- ions.
Adding More of [Fe(SCN)]2+
To reduce the concentration of this species, the equilibrium is shifted to the left-hand side, i.e. more Fe3+ and SCN-are formed. This conclusion is also in agreement with the expression for the equilibrium constant. In this case, numerator is increased and in order to keep the equilibrium constant to the same value, the concentration terms appearing in the denominator are increased.
It may be noted here that the reaction achieves a new equilibrium stage, the new concentrations of Fe3+, SCN- and [Fe(SCN)]2+, when substituted in the expression for equilibrium constant, yield the same value of equilibrium constant, i.e. here equilibrium stage is adjusted and not its equilibrium constant as the latter depends only on the temperature of the system.
On the other hand, if concentration of a reacting species is decreased, the equilibrium is readjusted to produce more of this substance. For example, in the above equilibrium reaction if we add a little NaF, the concentration of Fe3+ is reducing owing to the formation of more stable complexes [FeF6]4-. In this case, the complex [Fe(SCN)]2+3 dissociates to supply more Fe3+ and SCN- ions. This also follows from the expression of equilibrium constant of the reaction.
Since for a gaseous substance, the concentration is directly proportional to its partial pressure (p = CRT), the effect of changing the partial pressure of the gaseous substance involved in a reaction at equilibrium can be likewise discussed. Increasing the partial pressure of a reacting constituent shifts the equilibrium in a direction so that is partial pressure is reduced.
Taking an example of formation of NH3(g) in Haber's process,
we have N2(g) + 3H2(g) 2NH3(g).Here, increasing the partial pressures of N2 and H2 shifts the equilibrium to the right-hand side, whereas increasing the partial pressure of NH3 shifts the equilibrium to the left-hand side. These partial pressures may be increased by adding more of the reacting constituent from outside to the reaction already existing at equilibrium. In the Haber process, NH3 is continuously removed by liquefaction so that more and more ammonia is formed as predicted by Le Chatelier's principle.