AQA GCSE Chemistry Chemistry Paper 2 (Foundation), November 2021: Question 4
8 marks · Low Demand difficulty · Short Answer
Answer questions on reversible reactions, dynamic equilibrium, and the effect of pressure and other factors on the rate of reaction between gases.
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Question text
04 This question is about reactions between gases.
When hydrogen gas is heated with iodine gas, hydrogen iodide gas is produced.
The equation for this reversible reaction is:
hydrogen + iodine ⇌ hydrogen iodide
This reversible reaction reaches equilibrium in a sealed container.
04.1 How does the equation show that the reaction is reversible?
[1 mark]
04.2 Which two statements are correct when the reaction reaches equilibrium?
[2 marks]
Tick ( ) two boxes.
The forward reaction and reverse reaction are both exothermic.
The gases have escaped from the container.
The hydrogen no longer reacts with iodine.
The mass of each substance does not change.
The rates of the forward reaction and reverse reaction are equal.17
04.3 The initial mixture of hydrogen and iodine in the sealed container is purple.
Hydrogen iodide is colourless.
How will the colour of the mixture in the sealed container have changed when
equilibrium is reached?
[1 mark]
Tick ( ) one box.
The mixture will have become a deeper purple.
*16* The mixture will have become a paler purple.
The mixture will have become colourless.
04.4 The rate of reaction between gases is affected by changing the pressure.
Complete the sentences.
[3 marks]
When the pressure of the reacting gases is increased,
the rate of reaction .
This is because at higher pressures the distance
between the particles .
This means that the frequency of collisions .
04.5 Give one other way of changing the rate of reaction between gases.
You should not refer to pressure in your answer.
[1 mark]
Mark scheme
Show the mark scheme
Question 4
AO /
Question Answers Extra information Mark
Spec. Ref.
04.1 (equation contains the symbol) allow description of arrow / 1 AO1
⇌ symbol 4.6.2.1
AO /
Spec. Ref.
04.2 the mass of each substance 1 AO3
does not change
the rates of the forward reaction 1 AO1
and reverse reaction are equal
4.6.2.3
AO /
Spec. Ref.
04.3 the mixture will have become a 1 AO3
paler purple 4.6.2.3
AO /
Spec. Ref.
04.4 must be in this order AO1
increases 1 4.6.1.2
4.6.1.3
decreases 1
increases 1
AO /
Spec. Ref.
04.5 ignore references to pressure AO1
change the temperature 1 4.6.1.2
4.6.1.3
or
add a catalyst
Total 8
How to answer it
Reversible Reactions, Dynamic Equilibrium & Reaction Rates
This question assesses fundamental concepts from AQA C6 (The Rate and Extent of Chemical Change):
- Recognising the reversible reaction symbol (⇌).
- Recalling the two core conditions defining dynamic equilibrium in a closed system.
- Predicting observable colour changes in an equilibrium mixture.
- Using collision theory to explain step-by-step how changing pressure changes reaction rate.
- Recalling alternative factors that alter the rate of gaseous reactions (temperature and catalysts).
Identifying Reversible Reactions
Question: How does the equation show that the reaction is reversible? [1 mark]
✅ Correct Answer
The equation contains the reversible symbol ⇌ .
🧠 Exam Technique
- Keep it concise! You do not need to explain what a reversible reaction is—the question only asks how the equation shows it.
- Drawing the symbol ⇌ is completely acceptable and saves time.
Key Features of Dynamic Equilibrium
Question: Which two statements are correct when the reaction reaches equilibrium? [2 marks]
✅ Correct Checkboxes
- ☑ The mass of each substance does not change.
- ☑ The rates of the forward reaction and reverse reaction are equal.
💡 Key Knowledge
At dynamic equilibrium in a closed system:
- Rates are equal: Forward rate = Reverse rate.
- Amounts stay constant: The amounts/concentrations/masses of reactants and products stay constant (they do not have to be equal to each other).
- Reactions are still happening continuously in both directions.
❌ Common Errors
- Misconception: Thinking the reaction has "stopped" or that hydrogen no longer reacts with iodine. It is dynamic—molecules are still reacting!
- Confusion: Believing both directions are exothermic. If forward is exothermic, reverse must be endothermic by the exact same amount.
Deducing Colour Changes at Equilibrium
Question: Initial mixture (H₂ + I₂) is purple. Hydrogen iodide (HI) is colourless. How will the colour change when equilibrium is reached? [1 mark]
✅ Correct Answer
☑ The mixture will have become a paler purple.
🧠 Examiner Insight
- Why paler purple? Because some purple iodine is used up to make colourless hydrogen iodide, decreasing the iodine concentration.
- Why not colourless? Because it is an equilibrium mixture in a sealed container, so not all the iodine reacts—some reactant always remains!
Explaining Reaction Rate using Collision Theory
Question: Complete the sentences about increasing gas pressure. [3 marks]
✅ Completed Sentences
When the pressure of the reacting gases is increased, the rate of reaction increases . [1]
This is because at higher pressures the distance between the particles decreases . [1]
This means that the frequency of collisions increases . [1]
💡 Collision Theory Chain of Reasoning
- Higher Pressure: Gas particles are pushed closer together into a smaller volume.
- Particle Distance: Decreases (particles are closer together / more particles per unit volume).
- Collision Frequency: Particles collide more frequently (more successful collisions per second).
- Result: Faster rate of reaction.
❌ Common Errors
- Writing "number of collisions" instead of frequency of collisions (or "collisions per second"). Rate depends on time!
- Mixing up "distance between particles" with "particle speed". Increasing pressure pushes particles closer together, but does not make individual particles move faster (only temperature increases particle kinetic energy).
Other Ways to Change Reaction Rate
Question: Give one other way of changing the rate of reaction between gases (do not refer to pressure). [1 mark]
✅ Acceptable Answers (Choose ONE)
- Change the temperature (or increase / decrease the temperature)
- Add a catalyst (or use a catalyst)
❌ What NOT to Write
- Surface area: Only applies to solids, not gases!
- Pressure: The question explicitly told you: "You should not refer to pressure in your answer." Always read negative instructions carefully!
- Concentration: For gases, concentration is directly proportional to pressure, so examiners specifically look for temperature or catalyst.
Topics
Chemistry · C6: The Rate and Extent of Chemical Change
Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 2 (Foundation), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.