Le-Chatelier's Principle And Its Interpretation

Le-Chatelier’s Principle or more commonly known as the “Equilibrium Law” is one of the most important laws that help us to understand qualitatively, how the equilibrium reacts to any change that it encounters.

Le-Chatelier’s Principle states that “Once a dynamic equilibrium has been established, any change brought about in the conditions will be compensated by a balance shift so as to counter the original change.

Henry Louis Le Châtelier

In the late 19th century, Henry- Louis Le Chatelier experimentally found that a system at chemical equilibrium behaves similarly to inertia. Inertia, as we know, opposes any change that is brought about by the system. If a system is at rest, it will tend to stay at rest until and unless an external force is acted upon it and it will try to oppose the effects of the external force. Le-Chatelier found similar properties in reactions at the equilibrium position. Let us try to understand this with an example. Have a look at one of the most famous reactions used for the manufacturing of ammonia (NH3),

N2 (g) + 3H2 (g) ⇌ 2NH3 (g)             (ΔH = −92.4 kJ·mol−1)

In the above reaction, 1 molecule of nitrogen reacts with 3 molecules of hydrogen to give 2 molecules of ammonia along with the release of 92.4 kJ/mol of heat.

What we can observe from the above statement is that the reaction proceeds with a decrease in pressure and volume but with an increase in temperature.

He observed that at equilibrium altering concentration of reactants, any addition in the reactants drove the equilibrium position towards the formation of products so as to negate the addition of reactants.

If the temperature of the system was increased, the equilibrium position shifted towards the reactants as the formation of ammonia itself was an exothermic reaction. So the backward reaction would be an endothermic reaction and will lead to a decrease in temperature (due to heat being absorbed).

Pressure has more of an impact on the reaction in terms of the partial pressures of the reactants and products rather than the total system pressure. If the pressure of the system is reduced, the equilibrium shifts towards the reactants as backward reaction leads to the addition of moles, which results in an increase in pressure.

If we alter the volume of the system, there will be a change in the partial pressures of reactants and products and also affect the equilibrium concentrations. An increase in volume will lead to a pressure drop, then the side with a larger number of moles of substance will be more favored to shift the equilibrium to and vice versa.

We observed, how Le-Chatelier’s Principle can be used to examine any reaction at equilibrium qualitatively. But, did you know that another law of physics of utmost significance, called Lenz’s law, works similar to the equilibrium law?

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Practise This Question

For the gaseous reaction, pA+qBqC+pD, which of the following relationship is true?