Chemical Equilibria and Le Chatelier's Principle

This lesson covers dynamic equilibrium, the characteristics of equilibrium, Le Chatelier's principle, and its application to important industrial processes.

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Dynamic Equilibrium

A dynamic equilibrium is established in a closed system when:

1. The rate of the forward reaction equals the rate of the reverse reaction.
2. The concentrations of reactants and products remain constant (but not necessarily equal).
3. Both forward and reverse reactions continue to occur — the equilibrium is dynamic, not static.

Key Definition: A dynamic equilibrium occurs in a closed system when the rate of the forward reaction equals the rate of the backward reaction, and the concentrations of reactants and products remain constant.

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Le Chatelier's Principle

Key Definition: Le Chatelier's principle states that if a system at equilibrium is subjected to a change in conditions, the position of equilibrium shifts to partially oppose the change.

The system cannot completely counteract the change — it only partially opposes it.

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Effect of Changes on Equilibrium

Consider the exothermic equilibrium: N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = −92 kJ mol⁻¹

Effect of Temperature

• Increasing temperature shifts the equilibrium in the endothermic direction (to the left in this case). This reduces the yield of NH₃ but increases the rate at which equilibrium is reached.
• Decreasing temperature shifts the equilibrium in the exothermic direction (to the right). Yield of NH₃ increases but equilibrium is reached more slowly.

Temperature is the only factor that changes the value of the equilibrium constant (Kc or Kp).

Effect of Pressure (gaseous equilibria)

• Increasing pressure shifts the equilibrium towards the side with fewer moles of gas. In the Haber process: left has 4 moles of gas, right has 2 moles. So increasing pressure favours NH₃ production.
• Decreasing pressure shifts the equilibrium towards the side with more moles of gas.

If there are equal moles of gas on both sides, changing pressure has no effect on the position of equilibrium.

Effect of Concentration

• Increasing the concentration of a reactant shifts the equilibrium to the right (towards products).
• Increasing the concentration of a product shifts the equilibrium to the left (towards reactants).
• The system shifts to partially use up the added substance.

Effect of a Catalyst