The Effect of Changing Temperature (Oxford AQA IGCSE Chemistry)

Worked Example

Iodine monochloride reacts reversibly with chlorine to form iodine trichloride.

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 of the reactants

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

Exam Tip

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.