Le Chatelier's Principle (HT only)

Le Chatelier's Principle

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• Le Chatelier's Principle states that:
• If a a stress is applied to a system in equilibrium, the system will adjust so as to relieve that stress

Effect of pressure on equilibrium

• Pressure only affects the equilibrium of reactions involving gases
• One mole of gas of any substance takes up the same amount of space, if measured under the same conditions of temperature and pressure.
• Therefore, if one side of the reaction contains more moles of gas than the other, this is the side that will be favoured under low pressure
• Example: Ammonia is produced by the following reaction:\(N_2 + 3H_2 \rightleftharpoons 2NH_3 \)
• LHS has four moles of gas, RHS has two.
• Therefore, under high pressure, the rate of reaction in the forwards direction will be greater than that in the backwards direction because the RHS takes up less space
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Effect of temperature on equilibrium

• Exothermic reaction: Reaction that releases heat
• Endothermic reaction: Reaction that takes in heat
• Usually one direction of a reaction is exothermic, while the other is endothermic
• Under high temperatures, there is more heat energy available to be taken in by the reaction. Therefore, the endothermic direction is favoured.
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Effect of catalyst on equilibrium

• Catalysts do not change the position of equilibrium
• They only increase the rate of reaction by lowering the activation energy for a reaction!
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Effect of concentration on equilibrium

• If the concentration of reactants is increased, according to Chatelier, the system will try to counteract this increasing by increasing the rate of forward reaction.
• Similarly, if the concentration of reactants is decreased, then the rate of backwards reaction will increase, to try to make more reactants.
• This is commonly used in industry. The rate of forward reaction can be increased by removing the product from the reaction vessel as soon as it is produced. The system tries to relieve this change by producing more product.