Chemistry A level – Equilibria

Important definitions and concepts that need to be known for equilibrium

LE CHATELIER’S PRINCIPLE – a system as dynamic equilibrium will shift its equilibrium position to oppose any external change applied to it.

DYNAMIC EQUILIBRIUM – the rate of the forward reaction exactly equals the rate of the reverse reaction, maintaining constant concentration of reactants and products.

Factors affecting the equilibrium position:

Concentration – Le Chatelier’s principle predicts that if a reactant’s concentration is increased, the equilibrium position will shift to the right/the product’s side in order to decrease the concentration.

If the reactant’s concentration is decreased, the equilibrium position will shift to the left/the reactant’s side in order to increase the concentration.

if a product’s concentration is increased, the equilibrium position will shift to the left/the reactant’s side in order to decrease the concentration.

If the product’s concentration is decreased, the equilibrium position will shift to the right/the product’s side in order to increase the concentration.

Pressure – The pressure depends on the number of gas molecules in the system. Le Chatelier’s principle predicts that if the pressure is increased, the equilibrium position will shift to the side with fewer gas moles.

If the pressure is decreased, the equilibrium position will shift to the side with more gas moles.

If the number of gas moles is the same on both sides, then changing the pressure has no effect on the equilibrium position.

Catalyst – adding a catalyst will have no effect on the position of equilibrium. The addition of a catalyst will increase the rate of the forward and reverse reactions but by the same amount.

Temperature – in order to understand how temperature change may affect the equilibrium position, you must know how to identify whether a reaction is exothermic (gives heat out therefore, increasing the temperature of the system) or endothermic (takes heat in therefore, decreasing the temperature of the system).

Le Chatelier’s principle predicts that if the forward reaction is exothermic, an increase in temperature will cause the equilibrium position to shift to the left. If the forward reaction is exothermic, a decrease in temperature will cause the equilibrium position to shift to the right.

If the forward reaction is endothermic, an increase in temperature will cause the equilibrium position to shift to the right. If the forward reaction is endothermic, a decrease in temperature will cause the equilibrium position to shift to the left.

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