AQA GCSE Chemistry Chemistry Paper 2 (Foundation), June 2023: Question 8

10 marks · Standard Demand difficulty · Short Answer

Investigate the rate of reaction between zinc and sulfuric acid, calculate the mean time taken excluding anomalies, identify factors that decrease rate, and describe the test for hydrogen gas.

Practise this question

Question

Question text

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

Hydrogen gas is produced during this reaction.

Figure 7 shows the apparatus.

Figure 7

This is the method used.

1. Add 50 cm3 of sulfuric acid to a conical flask.

2. Add 2.0 g of zinc to the conical flask.

3. Quickly put a stopper in the conical flask and start a timer.

4. Measure the time taken to collect 20 cm3 of gas.

5. Repeat steps 1 to 4 three more times.

08.1 Suggest why the stopper must be put in the conical flask as quickly as possible in

step 3.

[1 mark]

08.2 The student calculated the rate of the reaction for each trial.

Table 7 shows the results of the calculations.

Table 7

Trial 1 Trial 2 Trial 3 Trial 4

Rate of reaction in cm3/s 0.78 0.81 0.68 0.81

Determine the mean time taken to collect 20 cm3 of gas.

Do not include any anomalous results.

Use the equation:

volume of gas collected

mean rate of reaction =

mean time taken

[5 marks]

Mean time taken = s

08.3 The student changed the investigation so that the mean time taken to collect 20 cm3

of gas was greater.

Which two changes would increase the mean time taken to collect 20 cm3 of gas?

[2 marks]

Tick ( ) two boxes.

Use a catalyst

Use a larger conical flask

Use a lower temperature

Use smaller pieces of zinc

Use sulfuric acid of a lower concentration

08.4 Hydrogen gas is produced during this reaction.

Describe the test for hydrogen gas.

Give the result of the test.

[2 marks]

Test

Result

Mark scheme

Show the mark scheme

AO /

Question Answers Extra information Mark

Spec. Ref.

08.1 to reduce the escape of gas 1 AO3

4.6.1.1

RPA5

AO /

Spec. Ref.

08.2 (mean rate =) allow

0.78 + 0.81 + 0.81

30.78 + 0.81 + 0.68 + 0.81 1 AO3

0.80 (cm /s) = 0.77 (cm3/ s) for 1 mark

1 AO2

20 allow correct use of incorrectly 1 AO2

0.80 =

mean time taken determined mean rate

20 1 AO2

(mean time taken =)

0.80

= 25 (s) 1 AO2

4.6.1.1

RPA5

alternative approach:

0.78 =

time

0.81 = (1)

time

(trial 1 time = =) 25.6 (1)

0.78

(trial 2 and 4 time =

=) 24.7 (1) – HEMISTRY – –

0.81

(mean time =) allow correct use of incorrectly

25.6 + (2 × 24.7) determined value(s) for time

(1)

25.6 + 29.4 + (2 × 24.7)

allow

= 25 (s) (1) = 26.1 (s)

for 1 mark

AO /

Spec. Ref.

08.3 use a lower temperature 1 AO1

4.6.1.2

use sulfuric acid of a lower 1

concentration RPA5

AO /

Spec. Ref.

08.4 AO1

MP2 is dependent upon MP1

4.8.2.1

being awarded

(test)

burning / lit splint allow flame 1

do not accept glowing splint

(result)

burns with a (squeaky) pop allow pops 1

sound

– – 8462/2F –

Total Question 8 10

Question 9

How to answer it

Investigating Rates of Reaction: Zinc and Sulfuric Acid

📋 What This Question Tests

This question assesses practical and mathematical skills from Topics 6 & 8 (Rate and Extent of Chemical Change & Chemical Analysis):

Required Practical 5: Investigating methods for measuring rates of reaction via gas production.
Experimental Design & Errors: Understanding practical precautions (preventing loss of gaseous product).
Data Processing: Spotting anomalies, calculating a mean, rearranging rate equations, and calculating time taken.
Collision Theory: Identifying factors that decrease reaction rate / increase the time taken for a reaction.
Gas Identification: Recalling the standard chemical test and observation for hydrogen gas (H₂).

Question 08.1

Experimental Precautions: Inserting the Stopper

1 Mark • Assessment Objective: AO3 (Practical Skills)

✅ Acceptable Answers [1 mark]

To reduce the escape of gas
To stop/prevent gas escaping

🧠 Exam Technique

The reaction starts the moment zinc touches the acid. Any delay means gas is lost into the laboratory instead of being measured in the inverted measuring cylinder.

❌ Common Errors

Too vague: Writing "to keep it airtight" or "so nothing gets out" without specifying gas.
Incorrect physics: Stating "to stop air getting in" or "to prevent pressure changes".

Mark Scheme Guidance: 1 mark is awarded specifically for the idea of preventing or minimising gas escape.

Question 08.2

Calculating the Mean Time Taken

5 Marks • Assessment Objective: AO3 & AO2 (Handling Data & Calculations)

📐 Step-by-Step Calculation

Step 1: Identify and discard the anomalous result [1 mark]
Look at Table 7: Trial 1 = 0.78 , Trial 2 = 0.81 , Trial 3 = 0.68 , Trial 4 = 0.81 .
Trial 3 (0.68 cm³/s) is noticeably lower than the others and must be excluded.
Step 2: Calculate the mean rate of reaction [1 mark]
Mean rate = (0.78 + 0.81 + 0.81) ÷ 3
Mean rate = 2.40 ÷ 3 = 0.80 cm³/s
Step 3: Substitute known values into the equation [1 mark]
Given: mean rate = volume of gas collected ÷ mean time taken
0.80 = 20 ÷ mean time taken
Step 4: Rearrange the equation to make time the subject [1 mark]
mean time taken = 20 ÷ 0.80
Step 5: Solve for mean time taken [1 mark]
mean time taken = 25 s

💡 Alternative Valid Method

You can also calculate the time for each valid trial first, then find the mean time:

Trial 1: time = 20 ÷ 0.78 = 25.64 s
Trial 2 & 4: time = 20 ÷ 0.81 = 24.69 s
Mean time = (25.64 + 24.69 + 24.69) ÷ 3 = 25 s

❌ Common Errors & Pitfalls

Including the anomaly: Dividing all four numbers by 4 gives (0.78 + 0.81 + 0.68 + 0.81) ÷ 4 = 0.77 cm³/s. This caps your score at max 4 marks (yielding 26.0 s or 26.1 s).
Rearrangement error: Multiplying 20 × 0.80 = 16 s instead of dividing. Remember:
Time = Volume ÷ Rate.

Mark Breakdown: [1] Excluding 0.68 & writing correct mean sum; [1] Finding mean rate = 0.80; [1] Correct substitution (0.80 = 20 / time); [1] Correct rearrangement (time = 20 / 0.80); [1] Final answer = 25 (s).

Question 08.3

Factors Affecting the Rate: Increasing Time Taken

2 Marks • Assessment Objective: AO1 (Recall & Understanding)

🧠 Read the Question Carefully: Inverse Thinking!

The question asks what makes the time taken GREATER. A greater time means a SLOWER reaction rate! Do not make the mistake of selecting changes that make the reaction faster.

✅ Correct Ticks [2 marks]

☑ Use a lower temperature [1 mark]
☑ Use sulfuric acid of a lower concentration [1 mark]

💡 Collision Theory Behind Why:

Lower temperature: Particles have less kinetic energy, move slower, and collide less frequently with less energy, leading to fewer successful collisions per second.
Lower concentration: Fewer acid particles per unit volume, leading to less frequent collisions between zinc atoms and H⁺ ions.

❌ Incorrect Options Explained

Use a catalyst: Speeds up the reaction (decreases time taken).
Use smaller pieces of zinc: Increases surface area to volume ratio, speeding up the reaction (decreases time taken).
Use a larger conical flask: Has no effect on concentration, collision frequency, or reaction rate.

Mark Scheme Guidance: Exactly 2 boxes must be ticked. If more than 2 boxes are ticked, marks are deducted for incorrect choices.

Question 08.4

Chemical Test for Hydrogen Gas

2 Marks • Assessment Objective: AO1 (Chemical Analysis)

✅ Correct Answers [2 marks]

Test: Burning / lit splint (or flame) [1 mark]

Result: Burns with a (squeaky) pop sound [1 mark]

💡 The Reaction in the Test

When hydrogen ignites in air, it rapidly reacts with oxygen to produce water vapour, creating a mini-explosion and the characteristic sound:

2H₂(g) + O₂(g) → 2H₂O(g)

❌ Common Errors (Zero Marks)

Glowing splint: This is the test for oxygen (which relights a glowing splint). Examiner rule: "Do not accept glowing splint".
Limewater: This is the test for carbon dioxide (turns cloudy).
Missing the splint condition: Writing just "splint" without specifying that it is lit or burning.

Crucial Examiner Rule: Mark 2 (the result) is dependent on Mark 1. If you write "glowing splint" for the test, you cannot get the mark for "squeaky pop" even if it is written!

Topics

Chemistry · Required Practicals · C6: The Rate and Extent of Chemical Change · C8: Chemical Analysis · Required Practicals

Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 2 (Foundation), June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.