Now you are familiar with the term ‘equilibrium’. In this post we will try to understand its nature. You know that equilibrium is established under particular conditions of temperature, pressure and concentration. If you change any of these conditions it gets disturbed. But it has a peculiar quality that when anything disturbs it, it tries to overcome that disturbance and regain its peace. How does it overcome disturbances? Let’s try to learn it.
Equilibrium happens in reversible reactions or systems. When equilibrium gets disturbed due to any change, the system works to nullify those changes and regain its equilibrium. It is known as Le Chatelier’s principle (I am not giving its proper definition). In this post we will see how a system deals with the change in concentration and pressure?
Effect of Concentration Change
H2(g) + I2(g) ⇌ 2HI(g)
If we add some H2 or I2 in above reaction mixture, system will no longer be in equilibrium. To regain its equilibrium, the system will work to reduce the concentration of H2 or I2. So it works in forward direction to consume excess H2 or I2and regain its equilibrium.
If we add some HI, then the system will start working in backward direction and regain its equilibrium. If we remove some HI then what will it do? It will work in forward direction and produce more HI to cancel out the changes. And if we frequently remove some HI from the system it will continually produce HI.
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| Effect of concentration change on equilibrium |
Effect of Pressure Change
In previous post (Ideal Gas Equation) we have seen that pressure is inversely proportional to the volume and directly proportional to the number of moles. Pressure change affects only those systems or reactions which involve gaseous reactants and products but has no effect on solid and liquid reactants or products because pressure change does not cause much effect on them.
Pressure change affects those reactions in which total number of moles of reactants and total number of moles of product are different. Let’s take an example:
CO(g) + 3H2(g) ⇌ CH4(g) + H2O(g)
As you can see, in this reaction 4 moles of reactants are being converted into 2 moles of products.
If we reduce the volume of the reaction vessel by half then pressure of the system will be doubled (P∝V-1). By doing this, we have disturbed the equilibrium of the system. So it shifts the reaction in forward direction (pressure ∝ number of moles), thus it can reduce the number of moles and the pressure of the system.
If we reduce the volume of the reaction vessel by half then pressure of the system will be doubled (P∝V-1). By doing this, we have disturbed the equilibrium of the system. So it shifts the reaction in forward direction (pressure ∝ number of moles), thus it can reduce the number of moles and the pressure of the system.
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| Le Chatelier’s Principle: effect of pressure change |
Similarly if we reduce the pressure of the system, it will shift the reaction in backward direction and by increasing the number of moles the system will cancel the effect of pressure and resume its equilibrium.
Now you have seen how smartly a system reacts and regains its equilibrium. In the next post we will see how it deals with the other changes.
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