AQA GCSE Chemistry Chemistry Paper 2 (Higher), 2024: Question 9

16 marks · High Demand difficulty · Extended Answer

Analyze the rate of reaction between zinc and sulfuric acid through experimental data interpretation, mole calculations, tangent drawing, and collision theory explanations.

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Question

Question text

09 A student investigated the rate of the reaction between zinc and sulfuric acid.

The equation for the reaction is

Zn(s) + H2SO4(aq) → ZnSO4(aq) + H2(g)

Figure 10 shows the apparatus.

Figure 10

This is the method used.

1. Pour 50 cm3 of sulfuric acid into the conical flask.

2. Add excess zinc to the conical flask.

3. Insert the stopper and start a timer.

4. Measure the volume of hydrogen collected in the 50 cm3 measuring cylinder

every 20 seconds for 180 seconds.

09.1 Explain why the volume of hydrogen collected in the 50 cm3 measuring cylinder is

less than the volume of hydrogen produced.

[2 marks]

Figure 11 shows the volumes of hydrogen collected in the 50 cm3 measuring cylinder

after 40 seconds and after 100 seconds.

*34* Figure 11

09.2 Determine the number of moles of hydrogen collected between 40 seconds and

100 seconds.

The volume of one mole of any gas at room temperature and pressure is 24 dm3.

[4 marks]

Moles of hydrogen =

A different student investigated how the concentration of sulfuric acid affected the rate

of the reaction.

09.3 The student did a different experiment using sulfuric acid of

concentration 0.40 mol/dm3.

The student calculated the number of moles of hydrogen collected after every

20 seconds.

Figure 12 shows the results.

Figure 12

Determine the rate of reaction at 45 seconds.

You should draw a tangent on Figure 12.

Give your answer in standard form.

[5 marks]

Rate of reaction (in standard form) =38 mol/s

09.4 Figure 13 shows the results for 0.40 mol/dm3 sulfuric acid.

Figure 13

*37* 3

The student repeated the experiment using 0.20 mol/dm sulfuric acid instead of

0.40 mol/dm3 sulfuric acid.

Excess zinc was used in each experiment.

Sketch a line on Figure 13 to show the results you would expect.

[2 marks]

09.5 Explain how increasing the temperature would affect the rate of reaction between zinc

and sulfuric acid.

[3 marks]

Mark scheme

Show the mark scheme

Question 9

AO /

Question Answers Extra information Mark

Spec. Ref.

09.1 (some) hydrogen / gas escapes 1 AO3

(from the flask) 4.6.1.2

RPA5

(because the reaction starts) allow (because) stopper cannot 1

before the stopper is put in be inserted instantly

allow for 1 mark some air (from

the conical flask) is collected

allow some hydrogen remains in

the conical flask

allow some hydrogen remains in

the delivery tube– – –

AO /

Spec. Ref.

09.2 (volume = 39 − 25 =) 14 (cm3) 1 AO2

4.3.5

(14 cm3 =) 0.014 (dm3) allow correct use of an 1 4.6.1.1

incorrectly determined volume RPA5

(moles of hydrogen =)

0.014 allow correct use of an

incorrectly determined volume

allow correct use of an incorrect

/ no conversion of volume

allow 5.833333 x 10–4 correctly

= 5.8 x 10–4 (mol) 1

rounded to at least 2 significant

figures

allow 0.00058 (mol)

alternative approach 1:

(24 dm3 =) 24 000 (cm3) (1)

(volume = 39 − 25 =)

14 (cm3) (1)

allow correct use of an

(moles of hydrogen =)

28 14 incorrectly determined volume

(1)

24000 allow correct use of an incorrect

/ no conversion of volume

–4 allow 5.833333 x 10–4 correctly

= 5.8 x 10 (mol) (1)

rounded to at least 2 significant

figures

allow 0.00058 (mol)

– – –

alternative approach 2:

(24 dm3 =) 24 000 (cm3) (1)

(moles of hydrogen at 100 s =

39 allow correct use of an incorrect

=) 0.001625 / no conversion of volume

24000

and

(moles of hydrogen at 40 s =

=) 0.00104 (1)

24000

(moles 100 s – moles 40 s =) allow correct use of an

0.001625 – 0.00104 (1) incorrectly determined number

of moles

= 5.8 x 10–4 (mol) (1)

allow 5.833333 x 10–4 correctly

rounded to at least 2 significant

figures

allow 0.00058 (mol)

alternative approach 3:

(39 cm3 =) 0.039 (dm3)

and

(25 cm3 =) 0.025 (dm3) (1)

(moles of hydrogen at 100 s =

0.039 allow correct use of an incorrect

=) 0.001625

24 / no conversion of volume

and

(moles of hydrogen at 40 s =

0.025

=) 0.00104 (1)

(moles 100 s – moles 40 s =)

0.001625 – 0.00104 (1) allow correct use of an

incorrectly determined number

of moles

= 5.8 x 10–4 (mol) (1)

allow 5.833333 x 10–4 correctly

rounded to at least 2 significant

figures

allow 0.00058 (mol)– – –

AO /

30Question Answers Extra information Mark

Spec. Ref.

09.3 tangent drawn at 45 s 1 AO2

4.6.1.1

correct values for y step and allow correct use of an 1 RPA5

x step from tangent incorrectly drawn tangent

allow a tolerance of ± ½ a small

square for each coordinate

value for y step allow correct use of incorrectly 1

(rate =) determined value(s) from the

value for x step

tangent for y step and/or x step

correct calculation of rate 1

(mol/s)

rate given in standard form allow a correctly calculated 1

(mol/s) answer in standard form from an

incorrect attempt at rate

determination

Answers AO /

Question Extra information Mark

Spec. Ref.

09.4 line starting at 0,0.000 and less 1

AO2

steep than existing line

4.3.2.4

4.3.4

becomes level at 0.0084 mol allow a tolerance of ± ½ a small 1 4.6.1.1

square – – – RPA5

AO /

Spec. Ref.

09.5 (increasing the temperature) 1 AO1

increases the rate of reaction 4.6.1.2

4.6.1.3

(because) particles have more allow (because) particles move 1

energy faster

(so) the frequency of collisions allow (so) a greater proportion of 1

increases collisions have enough energy

to react

ignore successful 31

Total Question 9 16

How to answer it

Rates of Reaction: Zinc and Sulfuric Acid

What this question tests

Experimental Skills: Identifying sources of error in gas collection.
Quantitative Chemistry: Calculating moles of gas using molar volume (24 dm³).
Graph Skills: Drawing tangents to determine the rate of reaction at a specific time.
Collision Theory: Explaining the effects of concentration and temperature on reaction rates.

Part 9.1

Experimental Errors

Correct Answer

Some hydrogen gas escapes from the flask. [1 mark]
This happens because the reaction starts before the stopper can be inserted. [1 mark]

Exam Technique

When asked why a volume is less than expected in a gas experiment, always look at the start of the reaction. The delay between adding the solid and putting the bung in is a classic "loss of gas" error.

Part 9.2

Calculating Moles of Gas

Determining moles between 40s and 100s

Step-by-Step Calculation

Read the scale: At 40s, volume = 25 cm³. At 100s, volume = 39 cm³.
Find the difference: 39 - 25 = 14 cm³. [1 mark]
Convert to dm³: 14 / 1000 = 0.014 dm³. [1 mark]
Calculate moles: Moles = Volume / 24.
0.014 / 24 = 0.0005833... [1 mark]
Final Answer: 5.8 × 10⁻⁴ mol (or 0.00058 mol). [1 mark]

Common Errors

Unit Conversion: Forgetting to divide the cm³ by 1000 before using the 24 dm³ value.
Reading the Scale: The measuring cylinder is upside down. Ensure you are reading the meniscus correctly!

Key Knowledge

At room temperature and pressure (RTP), 1 mole of any gas occupies 24 dm³ (24,000 cm³).

Part 9.3

Rate from a Graph

Determining the rate at 45 seconds

How to get 5/5 Marks

Draw a Tangent: Use a ruler to draw a straight line that just touches the curve at exactly 45 seconds. [1 mark]
Create a Triangle: Make the tangent line long so it's easy to read the "rise" (change in y) and "run" (change in x). [1 mark]
Gradient Formula: Rate = Change in Moles / Change in Time. [1 mark]
Calculation: Perform the division based on your specific tangent. [1 mark]
Standard Form: Convert your answer (e.g., 0.00012 becomes 1.2 × 10⁻⁴). [1 mark]

Examiner Insight

Many students lose marks by not drawing the tangent long enough. A short tangent leads to inaccurate readings. Always ensure your tangent extends across a large portion of the grid.

Part 9.4

Sketching the Effect of Concentration

Using 0.20 mol/dm³ instead of 0.40 mol/dm³

What to draw

The Slope: A line starting at (0,0) that is less steep than the original line (showing a slower rate). [1 mark]
The Plateau: The line must level off at exactly 0.0084 mol (half the height of the original 0.0168 mol plateau). [1 mark]

Why half the height?

The concentration was halved (0.40 to 0.20). Since zinc was in excess, the sulfuric acid is the limiting reactant. Half the concentration of acid means half the total number of moles of hydrogen produced.

Part 9.5

Temperature and Collision Theory

The 3-Step Explanation [3 marks]

The Effect: Increasing temperature increases the rate of reaction.
The Energy: Particles have more kinetic energy (and move faster).
The Collisions: There is an increase in the frequency of collisions (or more particles have energy greater than the activation energy).

Common Error

Do not just say "there are more collisions." You must say "more frequent collisions" or "more collisions per second." The examiner needs to know you understand that time is the key factor in rate.

Topics

Chemistry · Required Practicals · C3: Quantitative Chemistry · C6: The Rate and Extent of Chemical Change

Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 2 (Higher), 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.