Reversible reactions and equilibrium Edexcel International GCSE Chemistry revision
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In plain words
Most reactions go one way. A reversible reaction can go both ways, forwards and backwards. Seal one in a container and the two directions end up going at the same speed, so the amounts stop changing even though both reactions are still happening. That balance is equilibrium.
Three things to know
- The sign ⇌ means reversible. Heating blue hydrated copper(II) sulfate turns it white (anhydrous) and drives off water; adding water turns it blue again.
- At equilibrium, in a closed system, the forward and reverse reactions happen at the same rate, and the concentrations stay constant.
- Changing conditions moves the balance. Raising the temperature favours the endothermic direction. Raising the pressure favours the side with fewer gas molecules. A catalyst changes neither: it just gets you there sooner.
Worked example
N₂(g) + 3H₂(g) ⇌ 2NH₃(g). What does raising the pressure do to the amount of ammonia at equilibrium?
- Count the gas molecules: 1 + 3 = 4 on the left, 2 on the right.
- Higher pressure favours the side with fewer gas molecules, which is the right.
- More ammonia is made.
Tips and tricks
- The equilibrium always moves to undo what you did: heat it and it moves in the direction that takes heat in.
- Catalysts speed up both directions equally. They never change the position of equilibrium or the yield.
It lands in your notebook with its questions as flashcards.
Reversible reactions and equilibrium: 6 questions and answers
These are the quiz’s questions. Do the quiz first, then come back here for the ones that got you.
What does the symbol ⇌ show?
It can go in both directions.
What is true at equilibrium?
That is why the amounts stop changing.
What effect does a catalyst have on the position of equilibrium?
It speeds up both directions equally.
For a reaction with gases on both sides, raising the pressure favours which side?
Fewer molecules take up less space, which eases the pressure.
Water is added to anhydrous copper(II) sulfate. What colour change is seen?
Blue hydrated copper(II) sulfate forms again.
The forward reaction is exothermic. What happens to the yield of product when the temperature is raised?
Heating favours the endothermic direction, which is the reverse reaction.
Quiz
6 questions
Tap an answer and you’ll see straight away whether it’s right, and why.
Worksheet
4 questions, 9 marks. Write your answers on paper, then check them.
Reversible reactions and equilibrium
Edexcel International GCSE Chemistry 4CH1 · 9 marks · papermunch.org
Name ______________________________ Date ______________
State two features of a reaction that is at equilibrium.[2]
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The forward and reverse reactions are happening at the same rate, and the concentrations of the reactants and products stay constant.
Water is added to white anhydrous copper(II) sulfate. State what is seen, and one use of this change.[2]
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It turns blue. It is used as a test for water.
N₂(g) + 3H₂(g) ⇌ 2NH₃(g). The forward reaction is exothermic. State and explain what raising the temperature does to the amount of ammonia at equilibrium.[3]
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It decreases. Raising the temperature favours the endothermic direction, which here is the reverse reaction.
State the effect of a catalyst on (a) the rate of reaching equilibrium and (b) the position of equilibrium.[2]
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(a) Equilibrium is reached faster. (b) No effect.
Answers: Reversible reactions and equilibrium
- 1. The forward and reverse reactions are happening at the same rate, and the concentrations of the reactants and products stay constant.
- 2. It turns blue. It is used as a test for water.
- 3. It decreases. Raising the temperature favours the endothermic direction, which here is the reverse reaction.
- 4. (a) Equilibrium is reached faster. (b) No effect.