Reversible reactions and equilibrium

Some reactions are reversible: the products can react to form the reactants again. The symbol ⇌ shows this. In a closed system a reversible reaction reaches equilibrium, where the forward and backward reactions go at the same rate, so the amounts of everything stay the same. If the conditions change, the position of equilibrium moves to oppose the change (Le Chatelier's principle).

Key facts

  • A reversible reaction can go both ways: A + B ⇌ C + D.
  • If a reversible reaction is exothermic one way, it is endothermic the other way, and the same amount of energy is transferred.
  • Dynamic equilibrium: in a closed system the forward and backward reactions happen at the same rate, so the concentrations stay constant.
  • Le Chatelier's principle: if a condition is changed, the position of equilibrium shifts to oppose the change.
  • Raising the temperature favours the endothermic direction; lowering it favours the exothermic direction.
  • Raising the pressure favours the side with fewer gas molecules. Adding a reactant or removing a product shifts the equilibrium to the right.
  • A catalyst does not change the position of equilibrium; it only helps equilibrium be reached sooner.

Worked examples

Example 1

Blue hydrated copper(II) sulfate is heated: CuSO4·5H2O ⇌ CuSO4 + 5H2O. What is seen, and what happens when water is added to the product?

  1. Heating removes the water of crystallisation: blue crystals turn to white powder. This direction is endothermic.
  2. Adding water to the white solid reverses the reaction: it turns blue again.
  3. The reverse reaction is exothermic: it gives out the same amount of energy as was taken in, so it gets hot.

Answer: blue turns white; adding water turns it blue again and it gets hot

Example 2

N2(g) + 3H2(g) ⇌ 2NH3(g), ΔH = −92 kJ/mol. How does increasing the pressure, and increasing the temperature, change the yield of ammonia?

  1. Pressure: there are 4 molecules of gas on the left and 2 on the right.
  2. Higher pressure favours the side with fewer gas molecules, so the equilibrium shifts right: more ammonia.
  3. Temperature: the forward reaction is exothermic (ΔH is negative).
  4. Higher temperature favours the endothermic, backward reaction, so the equilibrium shifts left: less ammonia.

Answer: higher pressure: more ammonia; higher temperature: less ammonia

Example 3

In a closed flask, a reversible reaction reaches equilibrium. Sketch how the rates of the forward and backward reactions change.

  1. At the start there are only reactants, so the forward reaction is fast and the backward reaction has not started.
  2. As reactants are used up, the forward rate falls. As products build up, the backward rate rises.
  3. When the two rates are equal, equilibrium is reached. Both reactions keep going, so it is dynamic.

Answer: forward rate falls, backward rate rises, until they are equal