AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Higher), 2020: Question 2

11 marks · Standard Demand difficulty · Extended Answer

Describe improvements to a rate of reaction method using sodium thiosulfate and hydrochloric acid, calculate a mean rate of reaction from a mass-loss graph, and predict the graph at a higher temperature.

Practise this question

Question

The question page contains three linked parts about rate of reaction and temperature. Part 02.1 describes a practical where students react 50 cm³ of heated sodium thiosulfate solution with 10 cm³ hydrochloric acid in a conical flask placed over a black cross on paper, timing until the cross is no longer visible; a labelled diagram shows a thermometer in the flask, the flask on the cross, a student observing, and a digital timer reading 2:30. Part 02.1 asks for a better method to produce accurate results, ignoring safety, for 6 marks. Lower down, Figure 2 is a graph of loss of mass in grams against time in minutes at 40 °C, starting at 0.00 g at 0 minutes and curving down to about -0.35 g by around 4 to 5 minutes; part 02.2 asks for the mean rate of reaction between 1 and 3 minutes using the equation mean rate of reaction = change in mass of gas in g divided by time in mins, and part 02.3 asks students to draw the expected curve for 50 °C instead of 40 °C.
Question text

02 Some students investigated the effect of temperature on the rate of reaction.

02.1 The students reacted sodium thiosulfate solution with hydrochloric acid.

This is the method used.

1. Use a beaker to measure 50 cm3 of heated sodium thiosulfate solution into a

conical flask.

2. Measure the temperature of the room.

3. Put the conical flask on a black cross drawn on a piece of paper.

4. Start a timer.

5. Use the same beaker to measure 10 cm3 of hydrochloric acid into the conical flask.

6. Stop the timer when the cross is no longer visible.

The students repeated the experiment at a different room temperature.

Figure 1 shows the apparatus.

Figure 1

The method contains errors and does not produce accurate results.

Describe a method the students should use to produce accurate results.

You do not need to write about safety precautions.

[6 marks]

Some students investigated the effect of temperature on the rate of a

different reaction.

They recorded the loss of mass from their apparatus at 40 °C

Figure 2 shows the results.

Figure 2

02.2 Calculate the mean rate of reaction between 1 minute and 3 minutes at 40 °C

Use Figure 2 and the equation:

change in mass of gas in g

Mean rate of reaction =

time in mins

[3 marks]

Mean rate of reaction = g/min

02.3 Draw a curve on Figure 2 for the results you would expect at a temperature of

50 °C instead of 40 °C

[2 marks]

Mark scheme

Show the mark scheme The mark scheme shows a level-of-response table for question 02.1, with Level 3 worth 5 to 6 marks for a valid, logically sequenced improved method, Level 2 worth 3 to 4 marks, Level 1 worth 1 to 2 marks, and 0 for no relevant content. Indicative points include using measuring cylinders, pipettes or burettes for accurate volumes, starting the timer when acid is added, recording solution temperature after mixing and at the end, calculating mean temperature, repeating at different temperatures, keeping concentrations and volumes the same, and describing how to heat the solutions. For 02.2, the scheme reads values from the graph giving a change in mass of 0.15 g over 2 minutes and a mean rate of 0.075 g/min, ignoring signs. For 02.3, the expected curve starts at the same point, is steeper and further left than the original, and levels off at -0.35 g; an example graph with both curves is shown.

AO / Spec.

Question Answers Mark

Ref.

Level 3: The method would lead to the production of a valid outcome. 5–6 AO3

02.1

All key steps are identified and logically sequenced. 5.6.1.2

Level 2: The method would not necessarily lead to a valid outcome. 3–4

Most steps are identified, but the plan is not fully logically sequenced.

Level 1: The method would not lead to a valid outcome. Some 1–2

relevant steps are identified, but links are not made clear.

No relevant content 0

Indicative content:

• use a measuring cylinder / pipette / burette to measure 50 cm3 of

sodium thiosulfate

• use a different measuring cylinder / pipette / burette to measure

10 cm3 of hydrochloric acid

• start the timer when the acid is added

• record the temperature of the solution after the acid is added

• stop the timer when the cross is no longer visible

• record the temperature of the solution at the end of the reaction

• calculate the mean temperature of the reaction

• repeat the experiment at different temperatures

• use the same concentration of sodium thiosulfate and same

concentration of hydrochloric acid

• use the same volume of sodium thiosulfate and the same

volume of hydrochloric acid

• describe how to heat the thiosulfate / acid solutions

AO /

Question Answers Extra information Mark

Spec. Ref.

02.2 ignore signs AO2

5.6.1.1

5.6.1.4

(change in mass of gas = ) 0.15 1

and

(time =) 2

0.15

(rate of reaction =) allow correct use of incorrectly 1

2 determined values for mass

8 and / or time

0.075 (g/min) 1

02.3 curve starts at same point, to the left and steeper 1 AO2

5.6.1.1

line levels at −0.35 g 1 5.6.1.2

5.6.1.4

the curve below can be awarded 2 marks

Total 11

How to answer it

Rate of Reaction: Sodium Thiosulfate and Temperature

What this question tests

This question checks whether you can plan a fair test, identify improvements to an imperfect method, calculate a mean rate from a graph, and predict how temperature changes the shape of a reaction curve.

Key ideas: control variables, accurate measuring equipment, starting/stopping the timer correctly, reading values from a graph, and explaining that higher temperature gives a faster reaction rate.

Part 02.1

Improve the method so it gives accurate results

6 marks: describe a better method for the sodium thiosulfate and hydrochloric acid experiment

✅ Correct answer points

  • Use a measuring cylinder, pipette or burette to measure 50 cm³ of sodium thiosulfate.
  • Use a different measuring cylinder, pipette or burette to measure 10 cm³ of hydrochloric acid.
  • Start the timer when the acid is added.
  • Stop the timer when the black cross is no longer visible.
  • Record the temperature after the acid is added, or record the temperature at the end of the reaction.
  • Repeat at different temperatures and calculate a mean temperature if needed.
  • Keep the same concentration of sodium thiosulfate and hydrochloric acid for each repeat.
  • Keep the same volume of each solution for each repeat.
  • Describe how the solutions are heated to the chosen temperature before mixing.

💡 Key knowledge

  • The method in the question is inaccurate because it measures and mixes in a way that does not control variables well.
  • To compare temperature properly, only temperature should change.
  • Everything else must be kept the same: volume, concentration, and the same endpoint.
  • A valid endpoint is when the cross disappears, because sulfur makes the mixture cloudy.

🧠 Exam technique

  • To get full marks, give a linked sequence of steps, not just random improvements.
  • Show you know what to measure, when to start timing, when to stop, and what to keep constant.
  • Top answers often include: measure temperature before/after mixing and repeat for reliability.

❌ Common errors

  • Saying “use a thermometer” without explaining when to take the temperature.
  • Forgetting to say the same volumes and concentrations must be used.
  • Starting the timer too early or too late.
  • Stopping the timer before the cross disappears or after it has already disappeared.
  • Writing about safety precautions, even though the question says not to.
Why students lost marks: the mark scheme rewards clear method improvements that make the test fair. The best answers identified the important control variables and gave the correct timing point. Weak answers were too vague, or repeated the original method without fixing the problems.
Part 02.2

Calculate the mean rate of reaction between 1 minute and 3 minutes at 40 °C

3 marks: read values from the graph and use the rate equation

📐 Calculation steps

  1. Read the mass change at 1 minute: about −0.10 g.
  2. Read the mass change at 3 minutes: about −0.25 g.
  3. Find the change in mass between these times:
    0.25 − 0.10 = 0.15 g
  4. Find the time interval:
    3 − 1 = 2 minutes
  5. Use the equation:
    mean rate = change in mass of gas ÷ time
  6. Substitute the values:
    mean rate = 0.15 ÷ 2 = 0.075 g/min

✅ Final answer

0.075 g/min

Marks are awarded for:
1 mark for the correct change in mass,
1 mark for the correct time,
1 mark for the correct final rate.

🧠 Exam technique

  • Ignore negative signs when calculating the amount of mass lost.
  • Use the values from the graph carefully: the question asks for the interval between 1 and 3 minutes, not from 0 to 3 minutes.
  • Include units: g/min.
  • If your graph readings are slightly different but your method is correct, examiners can still award marks for a correct calculation.

❌ Common calculation traps

  • Using the total loss from 0 to 3 minutes instead of the change between 1 and 3 minutes.
  • Forgetting to subtract the two readings.
  • Writing the answer in grams instead of g/min.
  • Mixing up “loss of mass” with “mass remaining”.
Part 02.3

Draw the curve expected at 50 °C

2 marks: show the effect of a higher temperature on the rate graph

✅ What the curve should show

  • The curve should start at the same point as the 40 °C curve.
  • It should go further to the left and be steeper at the start.
  • It should level off at about −0.35 g.
  • The curve can be drawn completely and still gain full marks if it shows the correct shape and final level.

💡 Key knowledge

  • A higher temperature means particles have more kinetic energy.
  • Collisions happen more often, so the reaction is faster.
  • Because the reaction is faster, the graph drops more quickly at the start.
  • The final mass loss is the same because the amount of reactants is unchanged.

🧠 Exam technique

  • When drawing the curve, keep the same starting point.
  • Make the line steeper initially, then flatten out earlier.
  • End at the same final level as the original curve.
  • Examiner marking gives one mark for the correct shape and one mark for the correct final level.

❌ Common errors

  • Starting the curve at a different point.
  • Making the curve higher or lower at the end instead of ending at −0.35 g.
  • Drawing a straight line instead of a curve.
  • Only moving the curve left but not making it steeper.
What distinguished top-level responses: they showed both parts of the temperature change clearly — a faster initial rate and the same final mass loss. In graph questions, examiners reward shape and accuracy more than artistic neatness.

Quick revision checklist

✅ You should be able to...

  • Suggest improvements to a practical method.
  • Identify control variables in a rate experiment.
  • Read values from a graph accurately.
  • Calculate a mean rate using change ÷ time.
  • Predict how temperature changes the shape of a rate graph.

🧠 Easy marks to remember

  • Measure accurately with proper apparatus.
  • Start timing when the reactants are mixed.
  • Stop timing at the endpoint.
  • Keep volumes and concentrations the same.
  • Use units in calculations.

Topics

Chemistry · C6: The Rate and Extent of Chemical Change

Question and mark scheme from the AQA GCSE Combined Science: Trilogy examination, Chemistry Paper 2 (Higher), 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.