AQA GCSE Chemistry Chemistry Paper 1 (Higher), November 2021: Question 8

10 marks · High Demand difficulty · Short Answer

Interpret state symbols, determine reacting gas volumes, complete an exothermic reaction profile, and calculate an unknown bond energy from given data.

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Question

Question text

08 This question is about the reaction between hydrogen sulfide (H2S) and oxygen.

The equation for the reaction is:

2H2S(g) + 3O2(g) → 2H2O(g) + 2SO2(g)

08.1 What does H2O(g) represent?

[1 mark]

08.2 Calculate the volume of oxygen required to react with 50 cm3 of hydrogen sulfide.

[1 mark]

Volume = cm3

08.3 Figure 6 shows part of the reaction profile for the reaction.

The reaction is exothermic.

Complete Figure 6.

You should:

• complete the profile line

• label the activation energy

• label the overall energy change.

[3 marks]

Figure 6

08.4 Figure 7 shows the displayed formula equation for the reaction of hydrogen sulfide

with oxygen.

*24* Figure 7

Table 5 shows some of the bond energies.

Table 5

Bond

Energy in kJ/mol 364 498 464 X

In the reaction the energy released forming new bonds is 1034 kJ/mol greater than

the energy needed to break existing bonds.

Calculate the bond energy X for the bond.

Use Figure 7 and Table 5.

[5 marks]

X = kJ/mol

Mark scheme

Show the mark scheme

Question 8

AO /

Question Answers Extra information Mark

Spec. Ref.

08.1 water vapour allow steam 1 AO1

allow gaseous water 4.1.1.1

4.2.2.2

AO /

Spec. Ref.

08.2 75 (cm3) 1 AO2

4.3.5

AO /

Spec. Ref.

08.3 product level below reactants ignore labelling of products 1 AO1

4.5.1.2

activation energy drawn and 1

labelled

overall energy change drawn if endothermic profile drawn 1

and labelled allow corresponding overall

energy change

– HEMISTRY – –

scores 3 marks

Question 8 continued

AO /

Spec. Ref.

08.4 (bonds broken = 4(364) + 3(498) 1 AO2

=) 2950 4.5.1.3

(bonds formed = 2950 + 1034 =) allow correct use of incorrectly 1

3984 calculated values of bonds

broken

4X + 4(464) = 3984 allow correct use of incorrectly 1

calculated values of bonds

formed

4X = (3984 – 1856 =) 2128 1

X = 532 (kJ/mol) 1

alternative approach:

(bonds broken = 4(364) + 3(498)

=) 2950 (1)

(bonds formed = 4(464) + 4X =)

1856 + 4X (1)

(1856 + 4X) – 2950 = 1034 (1) allow correct use of incorrectly

calculated values of bonds

broken and/or bonds formed

4X = (1034 + 2950 – 1856 =)

2128 (1)

X = 532 (kJ/mol) (1)

Total 10

How to answer it

Reaction Profiles, Gas Ratios & Bond Energy Calculations

📋 What this question tests

This question assesses fundamental concepts across quantitative chemistry and energy changes:

State Symbols & Formulas: Interpreting chemical notation, specifically state symbol (g) .
Reacting Gas Volumes: Using stoichiometric mole ratios from balanced equations to calculate reacting gas volumes (Avogadro's Law).
Reaction Profiles: Drawing and correctly labelling exothermic energy level profiles, activation energy ( Eₐ ), and overall energy change ( ΔH ).
Bond Energy Calculations: Counting bonds broken and made, and working backward to solve for an unknown single bond energy ( X ).

Part 08.1 • 1 Mark

Interpreting Chemical State Symbols

Question: What does H₂O(g) represent?

✅ Correct Answer

Water vapour (or steam / gaseous water)

Award [1 mark] for water vapour, steam, or water in gas state.

❌ Common Errors

Writing simply "water" without acknowledging the physical state (water alone implies a liquid at room temperature).
Writing "hydrogen oxide" without mentioning gas or vapour.

Part 08.2 • 1 Mark

Reacting Gas Volumes

Question: Calculate the volume of oxygen required to react with 50 cm³ of hydrogen sulfide.

2 H₂S(g) + 3 O₂(g) → 2 H₂O(g) + 2 SO₂(g)

📐 Step-by-Step Calculation

Step 1: Identify the molar ratio from the equation:
2 moles of H₂S react with 3 moles of O₂ (Ratio = 2 : 3).

Step 2: Apply ratio to gas volumes (equal volumes contain equal moles):
Volume of O₂ = 50 cm³ × (3 / 2) = 75 cm³

🧠 Exam Technique & Insight

At the same temperature and pressure, equal volumes of gases contain equal numbers of molecules (Avogadro's law). You do not need to convert to moles using 24 dm³; simply use the mole ratio directly with the given cm³.

Final Answer: 75 cm³ [1 mark]

Part 08.3 • 3 Marks

Exothermic Reaction Profile Diagram

Question: Complete Figure 6 for the exothermic reaction. Complete the profile line, label activation energy, and label overall energy change.

✅ Correct Answer & Drawing Details

Line completion: From the peak of the curve, draw the curve going down to a horizontal line for the products that is lower than the reactants level.
Activation Energy: A vertical arrow pointing upwards from the level of the reactants to the very peak of the curve, labelled Activation energy.
Overall Energy Change: A vertical arrow pointing downwards from the reactants level to the products level, labelled Overall energy change.

🧠 Diagram Layout Guide

What your completed sketch must look like:

Peak: Above reactants level (already partly drawn).
Product Line: Flat line to the right, clearly below the starting reactant level (because the reaction is exothermic — energy is lost to surroundings).
Arrow baselines: Extend a horizontal dashed line from the reactant level to clearly show where activation energy and overall energy change are measured from.

[1 mark] Product level drawn below reactant level
[1 mark] Activation energy drawn & labelled (reactant to peak)
[1 mark] Overall energy change drawn & labelled (reactant to products)

❌ Common Errors to Avoid

Drawing product line higher: This represents an endothermic reaction, losing the first mark.
Measuring from the bottom axis: Drawing arrows starting from the horizontal x-axis rather than from the reactants baseline.
Missing arrowheads: Always use clear vertical double-headed or properly oriented single-headed arrows for energy terms.

Part 08.4 • 5 Marks

Bond Energy Calculation: Finding an Unknown Bond (X)

Question: Calculate the bond energy X for the S=O bond using Figure 7 and Table 5.

Given: Energy released forming new bonds is 1034 kJ/mol greater than the energy needed to break existing bonds.

📐 Step-by-Step Calculation

Step 1 Calculate Energy to Break Bonds (Reactants):
Reactants: 2 H—S—H + 3 O=O
• H—S bonds broken = 2 molecules × 2 bonds = 4 × (H—S) = 4 × 364 = 1456 kJ/mol
• O=O bonds broken = 3 × (O=O) = 3 × 498 = 1494 kJ/mol
• Total energy needed to break bonds = 1456 + 1494 = 2950 kJ/mol [Mark 1]

Step 2 Determine Total Energy Released by Forming Bonds:
The question states energy released is 1034 kJ/mol greater than broken bonds:
• Total energy released = 2950 + 1034 = 3984 kJ/mol [Mark 2]

Step 3 Express Product Bonds in terms of X:
Products: 2 H—O—H + 2 O=S=O
• H—O bonds made = 2 molecules × 2 bonds = 4 × (H—O) = 4 × 464 = 1856 kJ/mol
• S=O bonds made = 2 molecules × 2 bonds = 4 × (S=O) = 4X
• Total bonds formed = 1856 + 4X = 3984 [Mark 3]

Step 4 Rearrange and Solve for 4X:
• 4X = 3984 − 1856
• 4X = 2128 kJ/mol [Mark 4]

Step 5 Divide by 4 to Find Single Bond Energy X:
• X = 2128 / 4 = 532 kJ/mol [Mark 5]

❌ Common Errors & Pitfalls

Counting bonds incorrectly: Forgetting that each SO₂ molecule contains two S=O double bonds. With 2 SO₂, there are 4 × S=O bonds, not 2.
Sign confusion: Because it says "energy released is 1034 greater", products formed = broken + 1034. Students often subtract 1034 instead.
Forgetting to divide by 4: Stopping at 2128 kJ/mol rather than solving for a single S=O bond.

🧠 Alternative Method (Using ΔH = −1034 kJ/mol)

Since the reaction is exothermic by 1034 kJ/mol:

ΔH = Σ(Bonds Broken) − Σ(Bonds Made)

−1034 = 2950 − (1856 + 4X)

−1034 = 1094 − 4X

4X = 1094 + 1034 = 2128 ⇒ X = 532 kJ/mol

Both methods are fully accepted on the mark scheme!

Topics

Chemistry · C2: Bonding, Structure and the Properties of Matter · C3: Quantitative Chemistry · C5: Energy Changes

Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 1 (Higher), November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.