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Reactions that go in both directions, what dynamic equilibrium means, and how Le Chatelier’s principle predicts the effect of changing concentration, pressure and temperature.
Specification references this topic covers: AQA 5.5.1, 5.6.1, 5.6.2; Edexcel C1.29, C4.13, C4.14, C4.17, C7.6, C7.10, C7.11.
Some reactions only go one way, like burning a log. Once the log is ash, you cannot turn it back. Other reactions can go forwards and backwards. These are called reversible reactions.
This matters because many useful chemicals are made this way. Ammonia, which is used to make fertiliser, is one. Chemists need to know how to get more of the product they want.
By the end you will be able to describe a reversible reaction and say what dynamic equilibrium means. Higher tier students will also predict how changing conditions moves an equilibrium.
Course note: reversible reactions and the idea of dynamic equilibrium are for both tiers. Predicting the effect of changing concentration, temperature and pressure (Le Chatelier) is Higher tier in most courses.
In a reversible reaction, the products can react to make the reactants again. We show this with the symbol ⇌. Heating blue hydrated copper sulfate drives off water and leaves a white powder. Adding water turns it blue again and gives out heat.
That shows an important rule. If a reaction is exothermic in one direction, it is endothermic in the other. The amount of energy is the same each way.
A closed system is one where nothing can escape. In a closed system, a reversible reaction reaches equilibrium. This is when the forward and reverse reactions happen at the same rate.
The amounts of reactants and products then stay constant. They are not usually equal. Think of a busy escalator with people walking up and others walking down at the same speed. The crowd on each floor stays the same, but everyone is still moving.
This is why it is called dynamic. Both reactions are still happening, but there is no overall change.
The position of equilibrium tells you how much reactant and product there is. If the position is to the right, there are more products. If it is to the left, there are more reactants.
Le Chatelier’s principle says that if you change a system at equilibrium, it moves to oppose the change. Add more reactant and the system uses some up, so the position shifts right. Remove a product and it makes more, so it also shifts right. Add product and it shifts left.
Raising the temperature shifts the equilibrium in the endothermic direction. The system takes in the extra energy. Lowering the temperature shifts it in the exothermic direction, which gives out energy.
Take N₂ + 3H₂ ⇌ 2NH₃. The forward reaction is exothermic. A higher temperature shifts the position to the left, so there is less ammonia. A lower temperature gives more ammonia.
Use the energy rule from earlier. The direction that takes in energy is the one that wins when you heat.
Pressure only matters when there are gases. It also matters only if the two sides have different numbers of gas molecules. A higher pressure shifts the position towards the side with fewer gas molecules.
In N₂ + 3H₂ ⇌ 2NH₃, there are 1 + 3 = 4 molecules on the left. There are 2 on the right. A higher pressure shifts the position right and gives more ammonia.
Lower pressure does the opposite. If both sides have the same number of gas molecules, pressure has no effect.
A catalyst speeds up the forward and reverse reactions by the same amount. The system reaches equilibrium faster. The position and the yield do not change.
A low temperature gives more ammonia, but the reaction would be far too slow. So the Haber process uses a compromise of about 450 °C and an iron catalyst. Think of choosing a walking pace that gets you to school on time without wearing you out.
A reversible reaction goes both ways. If one way is exothermic, the other is endothermic.
At dynamic equilibrium in a closed system, the forward and reverse rates are equal.
A change is opposed. Higher pressure favours fewer gas molecules. Higher temperature favours the endothermic direction.
In an exam answer, state the change. Say which way the equilibrium shifts and why. Then say what happens to the product.
What does a reversible reaction mean?
Answer: The products can turn back into the reactants.
N₂ + 3H₂ ⇌ 2NH₃ is exothermic in the forward direction. State and explain the effect of increasing the temperature on the yield of ammonia.
Answer: The yield decreases because the equilibrium shifts to the left, in the endothermic direction.
For 2SO₂ + O₂ ⇌ 2SO₃ (exothermic forward), explain the conditions that give the highest yield of SO₃ and why a lower temperature is not used in industry.
Answer: High pressure and a low temperature favour the products, but a low temperature is too slow, so a compromise and a catalyst are used.
Reversible reactions and equilibrium practice questions for GCSE Combined Science Chemistry. Try each one, then open the answer. Easy questions come first.
What is the name for the symbol ⇌ used in an equation?
Answer: reversible arrow
What colour is hydrated copper sulfate?
Answer: blue
A reaction is exothermic in the forward direction. Is the reverse reaction exothermic or endothermic?
Answer: endothermic
At dynamic equilibrium, what is the same for the forward and reverse reactions?
Answer: the rate
What colour is copper sulfate after it has been heated and the water driven off?
Answer: white
What do we call a system in which nothing can enter or leave?
Answer: closed system
In N₂ + 3H₂ ⇌ 2NH₃, how many gas molecules are on the left side?
Answer: 4
In N₂ + 3H₂ ⇌ 2NH₃, what happens to the yield of ammonia if the pressure is increased?
Answer: it increases
Longer questions in the style of a written paper, with the marking points. These are Yumio practice questions.
Hydrated copper sulfate crystals are heated: CuSO₄·5H₂O ⇌ CuSO₄ + 5H₂O. (a) State what you see when the blue crystals are heated. [1 mark] (b) Water is added to the product. State two observations. [2 marks] (c) State why the reaction is described as reversible. [1 mark]
Answer: (a) The blue crystals turn white. (b) The white powder turns blue and the mixture gets hot. (c) The reaction can go in both directions.
The equilibrium N₂ + 3H₂ ⇌ 2NH₃ is exothermic in the forward direction. Explain the effect of changing the pressure and the temperature on the yield of ammonia, and why a catalyst is used. [6 marks]
Answer: There are four gas molecules on the left and two on the right, so a higher pressure moves the equilibrium to the right and increases the yield of ammonia. The forward reaction is exothermic, so a lower temperature moves the equilibrium to the right and increases the yield, but a low temperature makes the reaction too slow. A compromise temperature is therefore used. A catalyst does not change the position of equilibrium or the yield, but it makes the reaction reach equilibrium faster.
Saying the concentrations are equal at equilibrium
They are constant, not necessarily equal.
Saying the reactions stop at equilibrium
Both reactions continue at the same rate.
Saying a catalyst increases the yield
It only speeds up the approach to equilibrium.
Saying pressure changes always matter
Only when the number of gas molecules is different on each side.
Saying the amounts of reactants and products are equal at equilibrium
They are constant, not necessarily equal. The position of equilibrium decides how much of each there is.
Reversible reactions and equilibrium is on both Foundation and Higher papers for GCSE Combined Science Chemistry, aimed at around grade 6.
Saying the concentrations are equal at equilibrium. They are constant, not necessarily equal.
Open Show answer under each question on this page. The questions are Yumio practice questions, not past exam questions.
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