In a chemical reaction, chemical equilibrium is a state of a system in which rate of forward reaction is equal to reverse reaction. Also, it can be defined as a state of the system in which the concentration of reactants and products don’t show the tendency to change with time.
To understand the effect of change of factors like the concentration of reactants, temperature, pressure, and catalyst understanding Le Chatelier’s principle is very essential.
We use Le Chatelier’s principle to know the effect of following factors on a system in equilibrium
- change in concentration of reactants
- change in pressure
- change in temperature
Statement of Le Chatelier’s Principle – When a change in temperature, pressure or concentration of reactant is applied to the system in equilibrium, equilibrium will shift in direction to cancel the effect of an applied change.
Change in concentration of reactant
Consider reaction in which A and B are reactants and C and D are products which are in equilibrium. If we add more reactant A to the reaction, the concentration of reactant A in above reaction will increase. Now here Le Chatelier’s principle helps us to find out what will happen next.
Due to the increase in the concentration of A, equilibrium will shift to the right i.e in forward direction to cancel the effect of increased concentration. And again equilibrium is established between forward and reverse reaction. We obtain more product by increasing the concentration of reactants.
Also if we decrease the concentration of products, the system again tries to counteract the change and shift the equilibrium in the right direction. Again more product is obtained.
Change in pressure
A(g) + 2B(g) ⇌ C(g)+D(g)
Consider reaction in which both reactants and products are in gaseous state. As we know, there is no effect of pressure on solid and negligible on liquid. Since there is large intermolecular space of gas molecules, they are affected by a change in pressure.
If you applied pressure on the above equilibrium system, the equilibrium will shift in forward reaction. When there is an increase in pressure, the equilibrium will shift towards the side of the reaction with fewer moles of gas. When there is a decrease in pressure, the equilibrium will shift towards the side of the reaction with more moles of gas.
A + B ⇌ C+D + heat
Consider an exothermic reaction in which heat evolved. After establishing chemical equilibrium, the rate of forward and reverse reaction will become equal. If you remove the heat from the system, equilibrium is disturbed because for equilibrium particular amount of heat is required. But we have removed the heat.
So the system will try to increase the temperature of the system as Le Chatelier’s principle stated. It is possible only by shifting the equilibrium to the right so more heat will produce. Hence yield of products will be greater.
Adding catalyst increase the rate of forward and reverse reaction. It just helps to obtain equilibrium condition fast. But there is no effect of the catalyst on the final position of the equilibrium
Read more about Le Chatelier’s Principle with the advanced view