The two French chemists, Le-Chateliers and Braun in 1884 made a certain generalizations to explain the effect of changes of system in equilibrium due to concentration, temperature or pressure. The generalization is known as Le-Chateliers’s principle. When a system is subjected to a change in one of the factors, the equilibrium gets disturbed and the system readjusts itself until it returns back to equilibrium.
Le-Chateliers’s principle states that:
Change in any of the factors like temperature, concentration or pressure that determine the equilibrium conditions of a system will shift the equilibrium in such a manner to reduce or to counteract the effect of the change.
Let us consider the effect of these variables in the light of Le-Chateliers’s principle in the case of a few well known equilibria.
1. Effect of change of concentration:
When the concentration of one substance in a system at equilibrium is increased, then the equilibrium will shift so as to partially use up the added substance. For example:
Fe3+ (aq) + SCN– (aq) ⇌ [FeSCN] 2+ (aq)
Suppose some potassium sulphocyanide, capable of giving SCN– ions are added to the above equilibrium. The color of the solution darkens immediately showing thereby the formation of [FeSCN] 2+ ion. In other words, the equilibrium shifts towards forward direction. Similarly, if the concentration of Fe3+ ions is increased by adding some ferric salt like FeCl3, color deepens again due to the formation of more of [FeSCN] 2+ ions. Thus, the increase in concentration of either Fe3+ ions or SCN– ions pushes the system in the forward direction so as to undo the effect of the increase in the concentration of Fe3+ ions or SCN– ions. This is what Le-Chateliers’s principle explains.
Hence it is concluded that increase in concentration of any of the reactants shifts the equilibrium to forward direction and increase in concentration of any of the products shifts the equilibrium to backward direction.
2. Effect of change of temperature:
The equilibrium will shift in the direction of endothermic reaction with increase in temperature and with decrease in temperature will shift the equilibrium towards the direction in which heat is produced i.e. towards exothermic reaction.
If the forward reaction is exothermic then backward reaction will be endothermic and vice versa. The effect of temperature on equilibrium can be easily understood by performing the dimerization reaction of NO2 gas to N2O4. For example:
Consider the reaction:
2 NO2 (g) ⇌ N2O4 (g) ………………. ∆H = -57.2 kJ mol-1
Prepare NO2 gas by adding copper turnings to concentrated HNO3 and collect the gas though a delivery tube in two test tubes each of 5 ml . Check equal intensity of the color of the gas in each test tube with araldite. Now take three 250 ml beakers and label them as 1, 2 and 3. Fill beaker 1 with freezing mixture, beaker 2 with water a t room temperature and beaker 3 with hot water. When both the test tubes are placed in beaker 2, intensity of brown color of the gas is almost equal. After keeping the test tubes for about 10 to 15 minutes in water maintained at room temperature, remove them and place one test tube in beaker 1 which is containing freezing mixture and other in beaker 3 containing hot water. We will observe that in beaker 1, the intensity of color decreases whereas in beaker 3, intensity of color increases.
2 NO2 (g) Red brown ⇌ N2O4 (g) colorless………………. ∆H = -57.2 kJ mol-1
Hence, at low temperature N2O4 is favored and at high temperature, NO2 is favored.
