The Effect of Temperature Changes on Equilibrium (AQA GCSE Chemistry)

Iodine monochloride reacts reversibly with chlorine to form iodine trichloride and the forward reaction is exothermic:

ICl (l) + Cl₂ (g) ⇌ ICl₃ (s)

dark brown            yellow

What colour will the mixture turn when heated? Explain your answer.

Answer:

• The system will oppose the increase in temperature

  • Increasing the temperature of an equilibrium reaction favours the endothermic reaction

• If the forward reaction is exothermic, then the backward reaction must be endothermic

• Therefore, the equilibrium will move to the left and produce more reactants

• This means that the colour of the mixture will become increasingly brown as the temperature increases

Remember all reversible reactions are exothermic in one direction and endothermic in the other.

Mark schemes don't usually give you a mark for stating that the system opposes the change but for the explanation of which reaction is favoured and why.