# Le-Chatelier's Principle

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.