Le Chatelier's Principle (Cambridge International A-Level Chemistry)

Spec point — 7.1.2

Le Chatelier's Principle

Position of the equilibrium

• The position of the equilibrium refers to the relative amounts of products and reactants in an equilibrium mixture.
• When the concentration of reactants increases, the position of equilibrium shifts to the right
• When the concentration of products increases, the position of equilibrium shifts to the left

Le Chatelier’s principle

• Le Chatelier’s principle says that if a change is made to a system at dynamic equilibrium, the position of the equilibrium moves to minimise this change
• The principle is used to predict changes to the position of equilibrium when there are changes in temperature, pressure or concentration

Effects of concentration

• If the concentration of a reactant is increased:

  • Equilibrium shifts to the right

• If the concentration of a reactant is decreased:

  • Equilibrium shifts to the left

• If the concentration of a product is increased:

  • Equilibrium shifts to the left

• If the concentration of a product is decreased:

  • Equilibrium shifts to the right

Worked Example Changes in equilibrium position

Use the reaction below:

CH₃COOH (l) + C₂H₅OH (l) $\rightleftharpoons$ CH₃COOC₂H₅ (l) + H₂O (l)

Explain what happens to the position of equilibrium when:

1. More CH₃COOC₂H₅ (l) is added
2. Some C₂H₅OH (l) is removed

Answer

• Answer 1: More CH₃COOC₂H₅ (l) is added

  • The position of the equilibrium moves to the left and more ethanoic acid and ethanol are formed
  • The reaction moves in this direction to oppose the effect of added ethyl ethanoate, so the ethyl ethanoate decreases in concentration

• Answer 2: Some C₂H₅OH (l) is removed

  • The position of the equilibrium moves to the left and more ethanoic acid and ethanol are formed
  • The reaction moves in this direction to oppose the removal of ethanol so more ethanol (and ethanoic acid) are formed from ethyl ethanoate and water

Worked Example Changes in equilibrium position

Use the reaction below:

Ce⁴⁺ (aq) + Fe²⁺ (aq) $\rightleftharpoons$ Ce³⁺ (aq) + Fe³⁺ (aq)

Explain what happens to the position of equilibrium when:

1. Water is added to the equilibrium mixture.

Answer

• There is no effect as the water dilutes all the ions equally so there is no change in the ratio of reactants to products

Effects of pressure

• Changes in pressure only affect reactions where the reactants or products are gases

Effects of pressure table

• If the pressure is increased

  • Equilibrium shifts in the direction that produces a smaller number of molecules of gas to decrease the pressure again

• If the pressure is decreased

  • Equilibrium shifts in the direction that produces a larger number of molecules of gas to increase the pressure again

Worked Example Changes in pressure

Predict the effect of increasing pressure on the following reactions:

1. N₂O₄ (g) $\rightleftharpoons$ 2NO₂ (g)
2. CaCO₃ $\rightleftharpoons$ CaO (s) + CO₂ (g)

Answer

• Answer 1:

  • The equilibrium shifts to the left as there are fewer gas molecules on the left
  • This causes a decrease in pressure

• Answer 2:

  • The equilibrium shifts to the left as there are no gas molecules on the left but there is CO₂ on the right
  • This causes a decrease in pressure

Worked Example Changes in pressure

Predict the effect of decreasing pressure on the following reaction:

2NO₂ (g) $\rightleftharpoons$ 2NO (g) + O₂ (g)

Answer

• The equilibrium shifts to the right as there is a greater number of gas molecules on the right
• This causes an increase in pressure

Effects of temperature

• If temperature is increased

  • Equilibrium moves in the endothermic direction to reverse the change

• If temperature is decreased

  • Equilibrium moves in the exothermic direction to reverse the change

Worked Example Changes in temperature

Predict the effect of increasing the temperature on the following reaction:

H₂ (g) + CO₂ (g) $\rightleftharpoons$ H₂O (g) + CO (g) ΔH = +410.2 kJ mol^-1

Answer

• The reaction will absorb the excess energy
• Since the forward reaction is endothermic, the equilibrium will shift to the right

Worked Example Changes in temperature

For the following reaction, increasing the temperature increases the amount of CO₂ (g) at constant pressure.

Ag₂CO₃ (s) $\rightleftharpoons$ Ag₂O (s) + CO₂ (g)

Explain whether the reaction is endothermic or exothermic.

Answer

• The reaction absorbs the excess energy
• Since more CO₂ (g) is formed, the equilibrium has shifted towards the right
• Therefore, the reaction is endothermic
• Remember: Endothermic reactions favour the products

Effects of catalysts

• A catalyst is a substance that increases the rate of a chemical reaction (they increase the rate of the forward and reverse reaction equally)
• Catalysts only cause a reaction to reach its equilibrium faster
• Catalysts therefore have no effect on the position of the equilibrium once this is reached