The Effect of Changing Temperature (Oxford AQA IGCSE Chemistry)
Revision Note
Written by: Alexandra Brennan
Reviewed by: Stewart Hird
The Effect of Changing Temperature on Equilibrium
We can predict the effect of changes in temperature on systems in equilibrium
To make this prediction it is necessary to know whether the reaction is exothermic or endothermic
If the temperature is raised:
The yield from the endothermic reaction increases
The yield from the exothermic reaction decreases
If the temperature is lowered:
The yield from the endothermic reaction decreases
The yield from the exothermic reaction increases
When a change in temperature is made to a system, the system will oppose the change
E.g. If the temperature is increased, the system will oppose the change by decreasing the temperature
It will do this by favouring the endothermic reaction
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
Examiner Tips and Tricks
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.
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