AQA GCSE Combined Science: Trilogy Chemistry Paper 2 (Foundation), 2022: Question 4

14 marks · Standard Demand difficulty · Short Answer

Analyze an investigation into the effect of concentration on the rate of reaction between sodium thiosulfate and hydrochloric acid, including data plotting and mean calculation.

Practise this question

Question

The image depicts a chemistry experiment investigating reaction rates. It includes a diagram of a conical flask on a cross, a thermometer reading 19.5°C, a data table showing concentration versus time, a blank graph grid for plotting, and a second table with four time measurements for calculating a mean.
Question text

04 A student investigates the effect of concentration on the rate of the reaction between

sodium thiosulfate solution and hydrochloric acid.

Figure 6 shows the experiment.

The experiment was done in a fume cupboard.

Figure 6

This is the method used.

1. Pour 50 cm3 of sodium thiosulfate solution into a conical flask.

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

3. Pour 10 cm3 of hydrochloric acid into the conical flask and start a timer.

4. Stop the timer when the cross can no longer be seen.

5. Repeat the experiment with different concentrations of sodium thiosulfate solution.

04.1 Draw one line from each type of variable to the correct example of the variable

in this investigation.

[2 marks]

Type of variable Example of variable

Concentration of sodium

thiosulfate solution

Dependent

Temperature of reaction

mixture

Time taken for the cross

to no longer be seen

Independent

Volume of acid

14 Volume of the flask

04.2 The experiment is done at room temperature.

Figure 7

*13* What is the temperature shown on the thermometer in Figure 7?

[1 mark]

Temperature = °C

Table 1 shows the student’s results.

Table 1

Concentration of sodium Time in

thiosulfate solution in mol/dm3 seconds

0.1 82

0.2 40

0.3 20

0.4 13

0.5 10

150.6 8

04.3 Plot the data from Table 1 on Figure 8.

Draw a line of best fit.

[3 marks]

Figure 8

04.4 Predict the time taken for the cross to no longer be seen at a concentration

of 0.7 mol/dm3

Use your graph in Figure 8.

[1 mark]

Time = s

04.5 Complete the sentence.

[1 mark]

As the concentration of sodium thiosulfate solution increases, the time taken for the

cross to no longer be seen .

04.6 In one experiment 0.725 g of sulfur is produced in 20 seconds.

Calculate the mean rate of the reaction from 0 to 20 seconds.

Use the equation:

mass of sulfur produced in grams

mean rate of reaction =

time in seconds

[2 marks]

Mean rate of reaction =

04.7 What is the unit for the mean rate of reaction calculated in Question 04.6?

[1 mark]

Tick ( ) one box.

g g/s 17 s s/g

04.8 The student did the experiment with 0.15 mol/dm3 sodium thiosulfate solution and

repeated the experiment three more times.

Table 2 shows the results.

Table 2

Test 1 Test 2 Test 3 Test 4

Time in seconds for the cross to no

60.5 63.2 82.3 65.7

longer be seen

Calculate the mean time for this reaction.

Do not include the anomalous result in your calculation.

Give your answer to 3 significant figures.

[3 marks]

Mean time for the reaction (3 significant figures) = s

Mark scheme

Show the mark scheme The mark scheme details the correct answers: identifying independent and dependent variables, reading the thermometer as 19.5°C, plotting points on a graph with a curve of best fit, calculating a reaction rate of 0.03625 g/s, and calculating a mean time of 63.1 seconds after excluding an anomalous result and rounding to three significant figures.

Question 4

AO /

Question Answers Extra information Mark

Spec. Ref.

04.1 AO2

5.6.1.1

RPA11

do not accept more than one line from a box on the left

AO /

Spec. Ref.

04.2 19.5 (°C) 1 AO2

5.6.1.2

RPA11

AO /

Spec. Ref.

04.3 all points correctly plotted allow a tolerance of ± ½ a small 2 AO2

square 5.6.1.1

allow 1 mark for 3 / 4 / 5 points RPA11

correctly plotted

line of best fit 1

AO / 13

Spec. Ref.

04.4 allow ecf from question 04.3 AO3

5.6.1.1

7 (s) allow a tolerance of ± ½ a small 1 RPA11

square

AO /

Spec. Ref.

04.5 decreases 1 AO2

5.6.1.1

RPA11

AO /

Spec. Ref.

04.6 (mean =) AO2

0.725 5.6.1.1

20 RPA11

= 0.03625 ignore units 1

AO /

Spec. Ref.

04.7 g/s 1 AO1

5.6.1.1

RPA11

AO /

Spec. Ref.

04.8 (mean =) 1 AO3

60.5 + 63.2 + 65.7 allow for 1 mark

3 60.5 + 63.2 + 65.7 + 82.3

= 67.925

41 AO2

= 63.13333 (s)

= 63.1 (s) allow an answer correctly 1 AO2

rounded to 3 significant figures

from an incorrect calculation 5.6.1.1

which uses the values in the RPA11

question

Total Question 4 14

How to answer it

Investigating Reaction Rates: The Disappearing Cross

What this question tests

This question assesses your understanding of Required Practical 11. You need to identify experimental variables, read precision instruments, plot and interpret graphical data, and perform multi-step calculations involving rates, anomalies, and significant figures.

Part 04.1

Identifying Variables

Correct Answers

  • Independent: Concentration of sodium thiosulfate solution
  • Dependent: Time taken for the cross to no longer be seen

Exam Technique

Use the "I Change" rule: The Independent variable is the one I change. The Dependent variable is the Data you collect (what you measure).

Part 04.2

Reading Scales

Correct Answer

19.5 °C

Common Error

Students often misread the scale as 19.4 or 20.5. Look closely: the liquid is exactly halfway between the 19 and 20 marks.

Parts 04.3 & 04.4

Graphing and Prediction

Drawing the Graph

You should draw a smooth curve of best fit. In rate experiments involving time, the relationship is rarely a straight line.

Visual Guide: The points should start high on the left (82s) and curve downwards steeply, flattening out towards the right (8s).

04.4 Prediction

7 s (Accept 6.5 to 7.5 based on your curve)

Top-level responses used a sharp pencil and ensured the curve passed through or very near all plotted points.
Part 04.5

Describing Trends

Correct Answer

As concentration increases, the time taken decreases.

Don't Confuse Time and Rate!

If the question asked about rate, the answer would be "increases". Because it asks about time, the answer is "decreases". Higher concentration = Faster reaction = Less time.

Parts 04.6 & 04.7

Calculating Rate

Step-by-Step Calculation

  1. Identify Mass: 0.725 g
  2. Identify Time: 20 s
  3. Divide Mass by Time: 0.725 / 20
  4. Result: 0.03625

04.7 Units

The correct unit is g/s (grams per second).

Part 04.8

Mean and Significant Figures

Handling Anomalies

An anomaly is a result that does not fit the pattern. Looking at the data: 60.5, 63.2, 82.3, 65.7. The value 82.3 is much higher than the others and must be excluded.

Calculation Steps

  1. Remove anomaly: Ignore 82.3.
  2. Sum remaining: 60.5 + 63.2 + 65.7 = 189.4
  3. Divide by count: 189.4 / 3 = 63.1333...
  4. Round to 3 Sig Figs: 63.1

Common Pitfalls

  • Including the 82.3 in the mean (this limits you to 1 mark).
  • Dividing by 4 instead of 3.
  • Incorrect rounding (e.g., writing 63 or 63.13).

Significant Figures Tip

To get 3 significant figures, count the first three non-zero digits from the left.
6 3 . 1 | 3 3...
The 4th digit is 3, so we round down.

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 (Foundation), 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.