AQA GCSE Chemistry Chemistry Paper 2 (Higher), 2022: Question 2

8 marks · Standard Demand difficulty · Short Answer

Analyze the thermal decomposition of hydrated cobalt chloride, including mass changes, reaction completion, energy calculations, and reaction type.

Practise this question

Question

An exam question showing a method and results table for heating hydrated cobalt chloride. The word equation is hydrated cobalt chloride in equilibrium with anhydrous cobalt chloride and water. Table 1 shows total heating time in seconds (0 to 120) and mass of test tube and contents in grams (26.5 down to 25.6). Five sub-questions follow: 02.1 asks to determine the mass of the empty test tube; 02.2 asks to explain why the mass decreased; 02.3 asks why it was heated until the mass did not change; 02.4 asks to calculate the energy taken in when 2.00 g is heated, given that 238 g takes in 88.1 kJ, to 3 significant figures; 02.5 asks what type of reaction takes place.
Question text

02 A student investigated the change in mass when hydrated cobalt chloride was heated.

The word equation for the reaction is:

hydrated cobalt chloride ⇌ anhydrous cobalt chloride + water

This is the method used.

1. Add 2.0 g of hydrated cobalt chloride to an empty test tube.

2. Measure the mass of the test tube and contents.

3. Heat the test tube and contents gently for 30 seconds.

4. Allow the test tube and contents to cool.

5. Measure the mass of the test tube and contents.

6. Repeat steps 3 to 5 until the mass of the test tube and contents does not change.

Table 1 shows the results.

Table 1

Mass of test tube and

Total heating time in seconds

contents in grams

0 26.5

30 26.2

60 25.9

90 25.6

120 7 25.6

02.1 Determine the mass of the empty test tube.

[1 mark]

Mass of empty test tube = g

02.2 Explain why the mass of the test tube and contents decreased.

[2 marks]

02.3 Suggest why the test tube and contents were heated until the mass did not change.

[1 mark]

Energy is taken in from the surroundings when hydrated cobalt chloride is heated.

02.4 When 238 g of hydrated cobalt chloride is heated until the mass does not change,

88.1 kJ of energy is taken in.

The student heated 2.00 g of hydrated cobalt chloride until the mass did not change.

Calculate the energy taken in during this reaction.

Give your answer to 3 significant figures.

[3 marks]

Energy taken in (3 significant figures) = kJ

02.5 What type of reaction takes place when hydrated cobalt chloride is heated?

[1 mark]

Mark scheme

Show the mark scheme The mark scheme for Question 2. For 02.1, the answer is 24.5 g. For 02.2, the marking points are water vapour was produced and escaped from the tube. For 02.3, the answer is so that the reaction was complete. For 02.4, the calculation steps are 2.00 divided by 238 times 88.1, giving 0.740336... kJ, rounded to 0.740 kJ for 3 marks. For 02.5, the answer is endothermic reaction.

Question 2

AO /

Question Answers Extra information Mark

Spec. Ref.

02.1 24.5 (g) 1 AO2

4.6.2.2

AO /

Spec. Ref.

02.2 allow steam for water vapour AO2

4.3.1.3

water vapour was produced allow water was produced as a 1 4.6.2.2

gas

(so) water (vapour) escaped allow (so) the mass of the water 1

(from the tube) (vapour) was not measured

AO /

Spec. Ref.

02.3 (so that) the reaction was allow (so that) no more water 1 AO3

complete (vapour) was produced 4.6.2.2

AO /

Spec. Ref.

02.4 (energy =) AO2

2.00 1 4.6.2.2

× 88.1

= 0.740336134 (kJ) 1

= 0.740 (kJ) allow an answer correctly 1

calculated to 3 significant figures

from an incorrect calculation

which uses all the values in the

question

AO /

Spec. Ref.

02.5 endothermic (reaction) allow reversible (reaction) 1 AO1

allow (thermal) decomposition 4.6.2.2

(reaction)

Total Question 2 8

How to answer it

Heating Hydrated Cobalt Chloride

What this question tests

This question assesses your understanding of conservation of mass in open systems, the practical technique of heating to constant mass, quantitative chemistry calculations using proportionality (ratios), and identifying endothermic reactions.

Part 02.1

Determining the mass of the empty test tube

Correct Answer

24.5 g

[1 Mark] Awarded for the correct value. No working out is strictly required for this mark, but it helps prevent silly mistakes!

Exam Technique

Always read the method steps carefully before looking at the data table!

  • Step 1 tells us that 2.0 g of hydrated cobalt chloride was added to the empty tube.
  • Table 1 shows that at 0 seconds (before heating), the total mass of the tube + contents was 26.5 g.
  • Subtract the mass of the chemical from the total mass:
    26.5 g - 2.0 g = 24.5 g

Part 02.2

Explaining the decrease in mass

Correct Answer

  • Water vapour / steam was produced [1 mark]
  • The water vapour escaped from the test tube [1 mark]

Key Knowledge

The reaction equation is:

hydrated cobalt chloride ⇌ anhydrous cobalt chloride + water

When heated, the water of crystallisation turns into water vapour (gas). Because the test tube is open, this gas escapes into the air, reducing the measured mass of the tube's contents.

❌ Common Errors & Pitfalls

Students often lose marks here by being too vague. Avoid these incomplete answers:

  • "Water was lost" — You must state that it turned into gas/vapour or steam.
  • "The reaction lost mass" — You must explain how (the gas escaped into the surroundings).

Part 02.3

Why heat until the mass does not change?

Correct Answer

To ensure that the reaction was complete (or that all the water had been driven off/produced).

[1 Mark] for stating completion or that no more water vapour was produced.

Exam Technique: Constant Mass

Heating a sample, cooling it, weighing it, and repeating until the mass remains constant is a classic GCSE practical technique.

In Table 1, the mass stops changing between 90 seconds and 120 seconds (remaining at 25.6 g). This constant mass is the experimental proof that the reaction has finished.

Part 02.4

Energy calculation (3 Significant Figures)

📐 Step-by-Step Calculation

The Problem: If heating 238 g takes in 88.1 kJ of energy, how much energy is taken in when heating 2.00 g?

  1. Find the scaling factor (ratio): Divide the mass used by the reference mass.
    2.00 g / 238 g = 0.00840336... [1 Mark]
  2. Calculate the energy: Multiply this factor by the reference energy.
    0.00840336... × 88.1 kJ = 0.740336134... kJ [1 Mark]
  3. Round to 3 significant figures:
    Look at the first three non-zero digits: 0.740 (the 3 after the zero rounds down).
    Energy = 0.740 kJ [1 Mark]

❌ Calculation Traps

The Significant Figure Trap: Writing 0.74 instead of 0.740 will lose you the final mark! The trailing zero is required to show the value is accurate to three significant figures.

💡 Tutor Tip

Even if you make an arithmetic error in Step 1 or 2, you can still get the final mark for correctly rounding your incorrect answer to 3 significant figures. Never leave the final answer line blank!

Part 02.5

Identifying the reaction type

Correct Answer

endothermic (reaction)

[1 Mark] Also accepts: reversible reaction or thermal decomposition.

Key Knowledge

The text right above the question states: "Energy is taken in from the surroundings..."

  • Endothermic reactions take in thermal energy from the surroundings (causing temperature to drop).
  • Exothermic reactions release thermal energy to the surroundings (causing temperature to rise).

Topics

Chemistry · C3: Quantitative Chemistry · C5: Energy Changes · C6: The Rate and Extent of Chemical Change

Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 2 (Higher), 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.