Reversible reactions & dynamic equilibrium – GCSE Chemistry exam questions with mark scheme

1Grade 1–3Foundation3 marks

When blue hydrated copper(II) sulfate is heated, it forms white anhydrous copper(II) sulfate and water.

hydrated copper(II) sulfate ⇌ anhydrous copper(II) sulfate + water

(a) What does the symbol ⇌ show? [1]

(b) The forward reaction is endothermic. What happens to the temperature when water is added to anhydrous copper(II) sulfate? Give a reason. [2]

(a) the reaction is reversible allow the reaction can go in both directions / forwards and backwards; allow the products can react to re-form the reactants; ignore 'equilibrium' on its own(1)
(b) temperature increases / it gets hot allow gets warmer(1)
(b) the reverse reaction is exothermic / the same amount of energy is transferred (released) in the reverse direction allow adding water to anhydrous copper(II) sulfate is exothermic / releases energy (to the surroundings)(1)
Answer: (a) reversible reaction (b) temperature rises because the reverse reaction is exothermic
Examiner tip: If one direction is endothermic, the reverse is exothermic by the same amount — always state this link.

2Grade 6–7Higher5 marks

Dinitrogen tetroxide (colourless) and nitrogen dioxide (brown) form a dynamic equilibrium in a sealed syringe.

N₂O₄(g) ⇌ 2NO₂(g) the forward reaction is endothermic

(a) State what is meant by a dynamic equilibrium. [1]

(b) The plunger is pushed in, increasing the pressure. Predict and explain what happens to the position of equilibrium. [2]

(a) the forward and reverse reactions are happening at the same rate ignore 'concentrations stay constant' on its own; do not accept 'the reaction has stopped' or 'equal amounts of reactants and products'(1)
(b) position of equilibrium shifts to the left / towards N₂O₄ allow more N₂O₄ forms(1)
(b) because the left-hand side has fewer (gas) molecules (1 compared with 2) allow fewer moles of gas(1)
(c) the mixture becomes darker brown / more brown(1)
(c) the forward reaction is endothermic, so equilibrium shifts to the right / more NO₂ is made allow the endothermic reaction is favoured(1)
Answer: (a) forward and reverse rates equal (b) shifts left — fewer gas molecules (c) darker brown — endothermic forward reaction favoured
Examiner tip: Count gas molecules on each side for pressure questions; for temperature questions name the endothermic/exothermic direction.

3Grade 1–3Foundation4 marks

A student heats some white ammonium chloride in a test tube, as shown in the diagram.

The white solid at the bottom of the tube slowly disappears. A white solid appears in the cool part of the tube near the top.

(a) Complete the equation for the reaction.

ammonium chloride ⇌ ammonia + ______________________ [1]

(b) Explain why a white solid forms in the cool part of the tube. [2]

(c) The direction of this reaction depends on the conditions. Which condition causes the change in direction in this experiment?

Tick (✓) one box.

☐ colour ☐ pressure ☐ temperature ☐ volume [1]

(a) hydrogen chloride allow HCl; ignore hydrochloric acid; do not accept chlorine / hydrogen(1)
(b) the gases / ammonia and hydrogen chloride cool down (near the top of the tube) allow it is colder at the top(1)
(b) (and) react (together) to re-form ammonium chloride / the reverse reaction happens allow the reaction is reversible so ammonium chloride is made again; do not accept 'ammonium chloride condenses' for this mark(1)
(c) temperature (only box ticked) if more than one box ticked, no mark(1)
Answer: (a) hydrogen chloride (b) the gases cool and react to re-form solid ammonium chloride (reverse reaction) (c) temperature
Examiner tip: Saying the ammonium chloride 'condenses' or 'sublimes' misses the chemistry — the mark is for the two gases reacting to make it again.

4Grade 4–55 marks

Hydrated cobalt(II) chloride is pink. Anhydrous cobalt(II) chloride is blue.

hydrated cobalt(II) chloride ⇌ anhydrous cobalt(II) chloride + water

A student adds a few drops of water to some anhydrous cobalt(II) chloride in a boiling tube. The student measures the temperature before and after adding the water.

Temperature before adding water in °C	Temperature after adding water in °C
19.5	27.0

(a) Calculate the temperature change. [1]

(b) Give the colour change the student would see when the water is added. [1]

(d) Adding the water to the anhydrous cobalt(II) chloride released 1.8 kJ of energy.

The student then heats the pink solid until it is completely blue again.

How much energy is taken in by the reaction when the pink solid turns back to blue? Give a reason for your answer. [2]

(a) 7.5 (°C) allow +7.5 / an increase of 7.5; ignore missing unit(1)
(b) blue to pink both colours needed, in the correct order(1)
(c) exothermic and the temperature increased allow energy / heat is released to the surroundings; no mark for 'exothermic' with no reason(1)
(d) 1.8 kJ(1)
(d) the same amount of energy is transferred in each direction of a reversible reaction allow the reverse reaction is endothermic and takes in the same amount (as the forward releases); ignore 'it is reversible' on its own(1)
Answer: (a) 7.5 °C (b) blue → pink (c) exothermic — temperature rose (d) 1.8 kJ, the same energy is transferred in both directions
Examiner tip: For (d), the number alone is not enough — you must say the energy transferred is the same in both directions.

5Grade 4–55 marks

Two gases, P and Q, are sealed in a container. They react to form gas R.

P(g) + Q(g) ⇌ R(g)

At the start there is no R in the container.

The graph shows how the rate of the forward reaction and the rate of the reverse reaction change with time. After a certain time the two lines are on top of each other.

(a) Which line, A or B, shows the rate of the reverse reaction? Give a reason for your answer. [2]

(b) Use the graph to find the time when the reaction reaches equilibrium. [1]

(d) Why does the reaction need to be done in a sealed container to reach equilibrium? [1]

(a) B(1)
(a) (because) at the start there is no R / no product, so the reverse reaction cannot happen (rate is zero) allow its rate starts at zero and increases as R is made; dependent on B(1)
(b) 40 s allow any value from 39 to 41 s(1)
(c) stays the same / does not change / constant do not accept 'no more R is made' / 'the reaction stops'(1)
(d) so that none of the reactants or products can escape allow it is a closed system so the gases cannot get out; ignore 'to stop anything getting in' on its own(1)
Answer: (a) B — no R at the start so reverse rate starts at zero (b) 40 s (c) stays constant (d) so no gas can escape (closed system)
Examiner tip: For (c) don't say the reaction has stopped — at equilibrium both reactions continue at the same rate, so amounts stay constant.

6Grade 6–7Higher5 marks

Yellow chromate ions and orange dichromate ions are in equilibrium in a solution.

$$\underset{\text{yellow}}{2\text{CrO}_4^{\;2-}(aq)} + 2\text{H}^+(aq) \rightleftharpoons \underset{\text{orange}}{\text{Cr}_2\text{O}_7^{\;2-}(aq)} + \text{H}_2\text{O}(l)$$

(a) State Le Chatelier's Principle. [1]

(b) A student adds dilute sulfuric acid to a yellow solution of the mixture. Predict and explain the colour change. [2]

(c) The student then adds sodium hydroxide solution. The hydroxide ions react with and remove hydrogen ions. Predict and explain the colour change. [2]

(a) when a change is made to the conditions of a system at equilibrium, the system responds to counteract / oppose the change allow the position of equilibrium shifts to reduce the effect of the change(1)
(b) yellow to orange allow it becomes (more) orange(1)
(b) concentration of H⁺ (a reactant) increases so the position of equilibrium shifts to the right / more dichromate is made allow the forward reaction is favoured; ignore 'more acid' without reference to H⁺ or reactant(1)
(c) orange to yellow allow it becomes (more) yellow(1)
(c) concentration of H⁺ decreases so the position of equilibrium shifts to the left (to replace H⁺) / more chromate is made allow the reverse reaction is favoured / more dichromate reacts(1)
Answer: (a) a system at equilibrium responds to counteract a change (b) turns orange — more H⁺ shifts equilibrium right (c) turns yellow — H⁺ removed so equilibrium shifts left
Examiner tip: Link the change to a named species: say the concentration of H⁺ goes up or down, then which way the equilibrium shifts.

7Grade 6–7Higher5 marks

The table shows four gaseous equilibria. Each one is in a closed container at constant temperature.

Equilibrium	Equation
A	H₂(g) + I₂(g) ⇌ 2HI(g)
B	2SO₂(g) + O₂(g) ⇌ 2SO₃(g)
C	PCl₅(g) ⇌ PCl₃(g) + Cl₂(g)
D	CO(g) + 3H₂(g) ⇌ CH₄(g) + H₂O(g)

(a) The pressure is increased. For which two equilibria does the amount of product increase? [2]

(b) Which equilibrium has a position that is not changed by increasing the pressure? Explain your answer. [2]

(a) B(1)
(a) D if more than two letters given, deduct one mark for each extra letter(1)
(b) A(1)
(b) (because) there are the same number of (gas) molecules on each side (2 on each side) allow same number of moles of gas on each side; dependent on A(1)
(c) increases (because the position shifts to the side with more gas molecules / to the right) allow more chlorine is made(1)
Answer: (a) B and D (b) A — 2 gas molecules on each side (c) the amount of chlorine increases
Examiner tip: Count gas molecules using the balancing numbers: 2SO₂ + O₂ is 3 molecules, not 2.

8Grade 8–9Higher6 marks

Methanol is made from carbon monoxide and hydrogen.

CO(g) + 2H₂(g) ⇌ CH₃OH(g)

The graph shows the percentage yield of methanol at equilibrium at different temperatures, at two different pressures.

(a) Give the percentage yield of methanol at 300 °C and 100 atm. [1]

(b) At 100 atm the temperature is increased from 250 °C to 350 °C. Calculate the decrease in the percentage yield of methanol. [1]

(d) Explain why the yield is higher at 100 atm than at 50 atm. [2]

(a) 38 (%) allow 37–39(1)
(b) 44 (percentage points) allow 62 − 18; allow 43–45 from candidate's correct readings(1)
(c) exothermic(1)
(c) (because) the yield decreases as temperature increases, so increasing temperature favours the (endothermic) reverse reaction allow increasing temperature shifts the equilibrium to the left; dependent on exothermic(1)
(d) there are fewer (gas) molecules on the right-hand side / product side (1 compared with 3) allow fewer moles of gas(1)
(d) (so) increasing pressure shifts the position of equilibrium to the right / towards methanol allow the forward reaction is favoured(1)
Answer: (a) 38% (b) 44 (62 → 18) (c) exothermic — yield falls as temperature rises (d) 3 gas molecules → 1, so higher pressure shifts equilibrium to the right
Examiner tip: 'Yield goes down when it gets hotter' is only half the mark — say that higher temperature favours the endothermic reverse reaction.

9Grade 8–9Higher6 marks

Hydrogen can be made by reacting carbon monoxide with steam. The reaction reaches a dynamic equilibrium in a closed reactor.

CO(g) + H₂O(g) ⇌ CO₂(g) + H₂(g)

The forward reaction is exothermic.

A company wants to increase the equilibrium yield of hydrogen.

Explain how changing the temperature, the pressure and the concentrations of the substances in the reactor would affect the equilibrium yield of hydrogen. [6]

Temperature: decreasing the temperature increases the yield of hydrogen(1)
because the forward reaction is exothermic, so a lower temperature shifts the position of equilibrium to the right / favours the exothermic direction(1)
increasing the temperature would decrease the yield (favours the endothermic reverse reaction)(1)
Pressure: changing the pressure has no effect on the equilibrium yield(1)
because there are the same number of gas molecules on each side (2 and 2)(1)
Concentration: adding more steam / carbon monoxide (increasing a reactant concentration) shifts the equilibrium to the right, making more hydrogen(1)
removing carbon dioxide (or hydrogen) as it forms (decreasing a product concentration) shifts the equilibrium to the right(1)
the system responds to counteract each change (Le Chatelier's Principle)(1)
ignore references to rate or catalysts unless linked to yield; a statement that a catalyst increases yield is incorrect(1)

Level	Marks	What it looks like
Level 3	5–6	Explains the effect of all three factors correctly, each linked to a reason: lower temperature favours the exothermic forward reaction, pressure has no effect because the numbers of gas molecules are equal, and increasing a reactant or removing a product shifts the equilibrium to the right. Reasoning is clear and logically linked.
Level 2	3–4	Correct effect of at least two factors, with a reason for at least one. Or all three effects stated correctly with little or no explanation. May contain a minor error, such as an incorrect claim about pressure.
Level 1	1–2	At least one correct effect of a change in conditions on the yield, with little or no explanation. Ideas may be confused, e.g. rate and yield mixed up.
No relevant content	0	Nothing worthy of credit.

Answer: Lower temperature increases the yield (forward reaction exothermic); pressure has no effect (2 gas molecules on each side); adding more CO or steam, or removing CO₂/H₂, shifts the equilibrium to the right.
Examiner tip: Many students say 'high pressure increases yield' out of habit — count the gas molecules first: here it is 2 on each side, so pressure has no effect on yield.

10Grade 1–3Foundation4 marks

A reversible reaction is left in a closed flask until it reaches equilibrium.

(a) Which two statements about the reaction at equilibrium are correct?

Tick (✓) two boxes.

☐ The forward reaction has stopped.
☐ The forward and reverse reactions happen at the same rate.
☐ There are always equal amounts of reactants and products.
☐ The amounts of reactants and products stay the same.
☐ The reverse reaction happens faster than the forward reaction. [2]

(b) A student says: "At equilibrium, the reactions have finished."

Explain why the student is wrong. [2]

(a) The forward and reverse reactions happen at the same rate (ticked)(1)
(a) The amounts of reactants and products stay the same (ticked) if more than two boxes ticked, deduct one mark for each extra tick(1)
(b) both the forward and reverse reactions are still happening / the reactions have not stopped allow it is a dynamic equilibrium(1)
(b) (but) they happen at the same rate, so there is no overall change (in the amounts) allow products are made as fast as they are used up(1)
Answer: (a) same rate; amounts stay the same (b) both reactions continue, at equal rates, so amounts do not change
Examiner tip: 'Equal amounts of reactants and products' is a classic wrong answer — at equilibrium the amounts are constant, not equal.

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