Reversible reactions and equilibrium

O Level / IGCSE Chemistry · Chemical reactions. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

A reversible reaction can go in both directions, shown by the symbol ⇌. Heating blue hydrated copper(II) sulfate gives white anhydrous copper(II) sulfate and water; adding water reverses it.

In a closed system, where nothing can escape, a reversible reaction reaches equilibrium. The forward and reverse reactions are both still going on, at equal rates, so the concentrations of reactants and products stop changing.

If the conditions change, the position of equilibrium shifts in the direction that opposes the change. Industry uses this in the Haber process and the Contact process, choosing conditions that balance yield, rate, cost and safety.

Rules to remember

Common mistake

Students say the reactions stop at equilibrium, or that a catalyst increases the yield. Both reactions carry on at equal rates, and a catalyst changes only how quickly equilibrium is reached.

Worked example

Methanol is made by CO(g) + 2H2(g) ⇌ CH3OH(g). The forward reaction is exothermic. What happens to the equilibrium yield of methanol if (a) the pressure is raised, (b) the temperature is raised?

  1. Count gas molecules: 1 + 2 = 3 on the left, 1 on the right.
  2. (a) Higher pressure favours the side with fewer gas molecules, the right, so the yield rises.
  3. (b) Higher temperature favours the endothermic direction. The forward reaction is exothermic, so the equilibrium shifts left and the yield falls.

Answer: (a) The yield of methanol increases. (b) The yield of methanol decreases, although the reaction is faster.

Practice questions

Try each one, then open the answer.

1. At equilibrium in a closed system

  1. A
    only the forward reaction still occurs, but very slowly
  2. B
    the forward and backward reactions happen at the same rate
  3. C
    the concentrations of reactants and products are equal to each other
  4. D
    the reaction has stopped because the reactants are used up
Show answer

Answer: B. Dynamic equilibrium: both reactions continue at equal rates, so the concentrations stay constant (not necessarily equal).

2. Why is a pressure of only about 2 atm used in the Contact process, even though a higher pressure would favour sulfur trioxide?

  1. A
    the vanadium(V) oxide catalyst stops working when the pressure rises above 2 atm
  2. B
    the yield is already high at 2 atm, so higher pressure would cost more for little gain
  3. C
    the reaction goes faster at low pressure, so the yield is higher at 2 atm
  4. D
    a higher pressure would move the equilibrium to the left and lower the yield
Show answer

Answer: B. Three moles of gas form two, so high pressure would help slightly, but the equilibrium already lies far to the right at 2 atm. High-pressure plant is expensive and more dangerous.

3. When water is added to white anhydrous copper(II) sulfate, the mixture becomes hot and turns blue. What can be deduced about heating blue hydrated copper(II) sulfate?

  1. A
    it has no energy change
  2. B
    it cannot be reversed
  3. C
    it is exothermic
  4. D
    it is endothermic
Show answer

Answer: D. For a reversible reaction the energy change in one direction is equal and opposite to the other. Hydration is exothermic, so dehydration by heating is endothermic.

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