Le Chatelier's Principle and Position of Equilibrium for a Reversible System

In this video we want to discuss the concept of Le Chatelier's Principle and the Position of Equilibrium for a system at dynamic equilibrium.

Dynamic Equilibrium

A reversible reaction is at dynamic equilibrium when the rate of forward and reverse reaction are the same.

The concentration of reactants and products stay constant so it seems like there are no changes going on, but the system is well and alive and can respond when it feels disturbed.

Le Chatelier's Principle

Le Chatelier's Principle states that when a system at dynamic equilibrium is subjected to a disturbance, it will shift to counter, oppose or reduce the change imposed partially.

We can treat the system as a stubborn person who will oppose any changes to it.

This makes the system highly predictable and we can determine how the system would respond when the following 4 changes are introduced.

For our discussion we will be using the Haber Process.

Le Chateliers Principle equation

1. Add reactant N2

When reactant N2 is added the position of equilibrium would shift towards the right to remove N2.

Hence concentration of reactants would decrease, concentration of products would increase and equilibrium constant remains unchanged.

Le Chateliers Principle add reactant

2. Increase Pressure

When pressure is increased the system will favour the formation of less moles of gas to lower pressure.

In this case since there are less gaseous products, position of equilibrium wil shift right.

Hence concentration of reactants would decrease, concentration of products would increase and equilibrium constant remains unchanged.

Le Chateliers Principle increase pressure

3. Increase Temperature

When temperature in increased the system will favour the endothermic process to absorb excess heat from the surrounding to lower the temperature.

In this case the forward reaction is exothermic and the reverse reaction is endothermic, hence position of equilibrium will shift left to favour the endothermic process.

Hence concentration of reactants would increase, concentration of products would decrease and equilibrium constant, which is only affected by changes in temperature, would decrease.

Le Chateliers Principle increase temperature

4. Add catalyst

When a catalyst is added the rate of forward and reverse reaction is increased to the same extent.

Position of equilibrium is not affected hence there are no changes to concentration of reactants and products. Equilibrium constant is also not affected.

Le Chateliers Principle add catalyst

For the detailed step-by-step discussion on how to apply Le Chatelier's Principle, check out this video!

Topic: Chemical Equilibria, Physical Chemistry, A Level Chemistry, Singapore

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