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A LevelChemistryCIERevision Notes7. Equilibria7.1 Chemical Equilibria: Reversible Reactions & Dynamic EquilibriumLe Chatelier's Principle

Le Chatelier's Principle (CIE A Level Chemistry)

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Caroline

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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

Effects of concentration table

Change How the equilibrium shifts  

Increase in concentration of a reactant

Equilibrium shifts to the right

Decrease in concentration of a reactant 

Equilibrium shifts to the left 

Increase in concentration of a product

Equilibrium shifts to the left 

Decrease in concentration of a product

Equilibrium shifts to the right

Worked example

Changes in equilibrium position

Use the reaction below:

CH3COOH (l) + C2H5OH (l) rightwards harpoon over leftwards harpoon 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) rightwards harpoon over leftwards harpoon 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

Change How the equilibrium shifts  

Increase in pressure

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

Decrease in pressure

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) rightwards harpoon over leftwards harpoon 2NO2 (g)
  2. CaCO3 rightwards harpoon over leftwards harpoon 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) rightwards harpoon over leftwards harpoon 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

Effects of temperature table

Change How the equilibrium shifts  

Increase in temperature

Equilibrium moves in the endothermic direction to reverse the change

Decrease in temperature

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) rightwards harpoon over leftwards harpoon 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) rightwards harpoon over leftwards harpoon 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

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    Caroline

    Author: Caroline

    Expertise: Physics Lead

    Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.