Specification Point 3.22C (Paper 2C Only):
Know the effect of changing either temperature or pressure on the position of equilibrium in a reversible reaction: an increase (or decrease) in temperature shifts the position of equilibrium in the direction of the endothermic (or exothermic) reaction, an increase (or decrease) in pressure shifts the position of equilibrium in the direction that produces fewer (or more) moles of gas
Le Chatelier’s Principle
“When a reversible reaction is in equilibrium and you make a change, it will do what it can to oppose that change.”
This is used to predict changes to the position of equilibrium when there are changes in temperature or pressure.
Effects of Temperature
Iodine Monochloride reacts reversibly with Chlorine to form Iodine Trichloride.
ICl + Cl2 ⇌ ICl3
Dark Brown Yellow
When the equilibrium mixture is heated, it becomes darker brown in colour. Explain whether the backward reaction is exothermic or endothermic:
- Equilibrium has shifted to the left as the colour dark brown means that more of ICI is produced
- Increasing temperature moves the equilibrium in the endothermic direction
- so the backward reaction is endothermic
Effects of Pressure
Nitrogen Dioxide can form Dinitrogen Tetraoxide, a colourless gas
2NO2 ⇌ N2O4
Brown Gas Colourless Gas
Predict the effect of the increase in pressure on the position of equilibrium:
- Number of molecules of gas on the left = 2
- Number of molecules of gas on the right = 1
- An increase in pressure will cause equilibrium to shift in the direction that produces the smaller number of molecules of gas
- so equilibrium shifts to the right
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Jamie got a First class degree in Chemistry from Oxford University before going on to teach chemistry full time as a professional tutor. He’s put together these handy revision notes to match the Edexcel IGCSE Chemistry specification so you can learn exactly what you need to know for your exams.
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