Reversible reactions and equilibria

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What are reversible reactions?

• In some chemical reactions, the products of the reaction can react to produce the original reactants — these are called reversible reactions

• The direction of the reaction can be changed by changing the conditions the reaction takes place in

• E.g. The Haber Process: nitrogen + hydrogen ⇌ ammonia

What is dynamic equilibrium?

• Dynamic equilibrium is reached in a closed system when the rate of the forward reaction is equal to the rate of the backward reaction

• This makes the concentrations of reactants and products remain constant

• It occurs in reversible reactions, where the reaction continues in both directions but there is no overall change in the concentration of the substances present

The Haber Process

• Used to manufacture ammonia, which is used to produce nitrogen-based fertilisers

• Equation: N₂ + 3H₂ ⇌ 2NH₃

• Nitrogen is obtained from the air and hydrogen may be obtained from natural gas

• The purified gasses are passed over an iron catalyst at a high temperature (450^∘C) and a high pressure (200 atm)

• The reaction is reversible to ammonia breaks down again into nitrogen and hydrogen and fed back into the reactor

How would dynamic equilibrium be affected by a change in temperature?

• If the temperature is increased, the equilibrium is moved in the direction of the endothermic reaction

• If the temperature is decreased, the equilibrium is moved in the direction of the exothermic reaction

• E.g. if the forwards reaction is endothermic and temperature is increased, equilibrium shifts right to produce more product

How would dynamic equilibrium be affected by a change in pressure?

• An increase in pressure would favour the side of the equilibrium with the fewer amount of molecules

• A decrease in pressure would favour the side of the equilibrium with the larger amount of molecules

How would dynamic equilibrium be affected if the concentration was changed?

• If the concentration of the reactants is increased, the position of the equilibrium would shift towards the products

• If the concentration of the products is increased, the equilibrium would shift towards the reactants