Le Chatelier’s principle: Temperature change

In the last post we have seen how the system dealt with the concentration change and pressure change, today we will see what happens if we change the temperature or add some foreign substances like catalyst or noble gases to the system.

You know that equilibrium constant depends on temperature, if we change the temperature, system will no longer be in equilibrium. How can a system control its temperature itself? Energy is released when new bonds are formed and this provides heat to the system. And energy is needed when a bond is broken which is supplied by the system in the form of heat. Now you can guess how the system can deal with it.

When reactants combine to form products, some old bonds are broken and some new bonds are formed. And when we subtract the energies involved, we get to know how much energy is used or released in that particular reaction. If the energy of the reactants is more than that of the products, then energy will be released in the reaction, such reactions are called exothermic reactions. And its opposite is called endothermic reactions, here energy is required.

N_2(g) + 3H_2(g) ⇌ 2NH_3(g)  ......... ∆E = 92.38 kJ mol^-1

The above reaction is an example of exothermic reaction, which means some amount of energy is released. If we increase the temperature of the system, then the system will shift the reaction in backward direction so that it can consume some of the heat. And if we lower the temperature then system will make the forward reaction faster to produce more heat. That means if we want to produce more ammonia we have to keep the temperature low.  

Effect of Temperature change on Equilibrium

Effect of catalyst addition

Catalysts are those substances which speed up the reaction without being involved it in. Suppose you are participating in a race and suddenly you find that a furious dog is chasing you, then what will happen? Naturally you will run like hell. Here the dog is neither participating in the race nor is it involved the race, but its presence speeds up your running. So dog acts as a catalyst.

Catalysts can help a system to achieve equilibrium sooner but their presence don’t create any disturbance because they don’t participate in the reaction.

Similarly addition of noble gases don’t alter the equilibrium because they are noble in nature and do not participate in reaction.

So you have learnt how Le Chatelier’s principle help us to predict the direction of the reaction and help us to understand how a system deals with the changes. Now you will be able to understand what Le Chatelier’s principle states, it states that “a change in any of the factors that determine the equilibrium conditions of a system will cause the system to change in such a manner so as to reduce or to counteract the effect of the change”.

In the next post we will try to sum up all the findings of equilibrium.​

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