AQA GCSE Chemistry Chemistry Paper 1 (Higher), 2024: Question 8

9 marks · Standard Demand difficulty · Extended Answer

Explain why propane has a low boiling point, identify the correct exothermic reaction profile, and calculate the C-H bond energy for the combustion of propane.

Practise this question

Question

The image contains three parts: 08.1 asks to explain propane's low boiling point. 08.2 provides four reaction profile graphs (A, B, C, D) and asks to identify the correct one for the exothermic combustion of propane. 08.3 provides a chemical equation for propane combustion and a table of bond energies, asking to calculate the bond energy of the C-H bond.
Question text

08 This question is about propane (C3H8).

Figure 7 shows the displayed structural formula of propane.

Figure 7

08.1 Explain why propane has a low boiling point.

[3 marks]

Propane reacts with oxygen to produce carbon dioxide and water.

The reaction is exothermic.

08.2 Figure 8 shows four reaction profiles.

Figure 8

Which is the correct reaction profile and labels for the reaction between propane and

oxygen?

Tick ( ) one box.

[1 mark]

A B 26 C D

08.3 Figure 9 shows the displayed formula equation for the reaction between propane and

oxygen.

Figure 9

The overall energy change of this exothermic reaction is 2219 kJ/mol.

Table 4 shows the bond energies of the bonds in the reaction.

Table 4

Calculate the bond energy of the C — H bond (X).

[5 marks]

Bond energy of the C — H bond (X) = kJ/mol

Mark scheme

Show the mark scheme The mark scheme provides the answers for 08.1 (simple molecule, weak intermolecular forces, little energy to overcome), 08.2 (identifying graph B), and 08.3 (step-by-step calculation showing bonds broken, bonds made, and the final value for X as 392 kJ/mol).

Question 8

AO /

Question Answers Extra information Mark

Spec. Ref.

08.1 propane is a small molecule allow propane is a simple 1 AO1

molecule 4.2.1.4

4.2.2.1

4.2.2.4

(so) the forces between 1

molecules are weak

or

(so) the intermolecular forces do not accept covalent bonds

are weak are weak

(which) require little energy to do not accept answers in terms 1

overcome of breaking covalent bonds

AO /

Spec. Ref.

08.2 B 1 AO1

– – – 4.5.1.2

AO /

Spec. Ref.

08.3 (bonds broken = AO2

2(347) + 8X + 5(498) =) 4.5.1.3

3184 + 8X 1

(bonds made =

6(805) + 8(464) =)

8542 1

(energy released = bonds made

– bonds broken =)

2219 = 8542 – (3184 + 8X) allow correct use of incorrectly 1

determined values of bonds

broken and/or bonds made

(8X =) 3139 (kJ/mol) allow correct evaluation of the 1

expression

energy released =

bonds broken – bonds made

(X =) 392 (kJ/mol) allow 392.375 correctly rounded 1

to at least 3 significant figures

allow correct use of an

incorrectly determined value for

8X

Total Question 8 9

How to answer it

Propane: Bonding, Energy Profiles, and Bond Calculations

What this question tests

This question assesses your understanding of simple molecular structures and their properties, the ability to identify exothermic reaction profiles, and the skill to perform multi-step bond energy calculations using algebraic substitution.

Part (a) / 08.1

Boiling Points of Simple Molecules

Explain why propane has a low boiling point. [3 marks]

💡 Key Knowledge

  • Propane (C₃H₈) is a small molecule.
  • Small molecules have intermolecular forces (forces between molecules).
  • These forces are much weaker than covalent bonds.

✅ Correct Answer

  1. Propane is a small/simple molecule.
  2. It has weak intermolecular forces (or weak forces between molecules).
  3. So, little energy is needed to overcome these forces.

❌ Common Errors

Never say that "covalent bonds are weak" or that "covalent bonds break" when a substance boils. Covalent bonds are very strong; it is only the weak forces between the molecules that break during boiling.

Part (b) / 08.2

Reaction Profiles

Identify the correct profile for the exothermic reaction of propane. [1 mark]

🧠 Exam Technique

For an exothermic reaction:

  • The products must be lower than the reactants (energy is lost to surroundings).
  • The "Overall energy change" arrow must point downwards.

✅ Correct Answer

Profile B

This shows the products at a lower energy level than the reactants, with the arrow correctly indicating a decrease in energy.

Part (c) / 08.3

Bond Energy Calculation

Calculate the bond energy of the C—H bond (X). [5 marks]

📐 Step-by-Step Calculation

The overall energy change is -2219 kJ/mol (it is negative because the reaction is exothermic).

Step 1: Calculate energy to break bonds (Reactants)

  • 2 × (C—C) = 2 × 347 = 694
  • 8 × (C—H) = 8 × X
  • 5 × (O=O) = 5 × 498 = 2490
  • Total Broken = 3184 + 8X

Step 2: Calculate energy released making bonds (Products)

  • 6 × (C=O) = 6 × 805 = 4830
  • 8 × (O—H) = 8 × 464 = 3712
  • Total Made = 8542

Step 3: Use the formula
Energy Change = Bonds Broken - Bonds Made
-2219 = (3184 + 8X) - 8542

Step 4: Solve for X
-2219 = 8X - 5358
3139 = 8X
X = 392.375

✅ Final Answer

392 kJ/mol

Marks awarded for: Correct sum of broken bonds (1), correct sum of made bonds (1), correct setup of equation (1), correct value for 8X (1), final answer (1).

❌ Calculation Traps

  • Counting Bonds: Look at the displayed formula carefully. Propane has 2 C—C bonds and 8 C—H bonds.
  • Coefficients: Don't forget the numbers in front of the molecules (e.g., 5 O₂ means 5 × 498).
  • Signs: If the question says "energy released is 2219", treat the change as -2219 in your equation.

Topics

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

Question and mark scheme from the AQA GCSE Chemistry examination, Chemistry Paper 1 (Higher), 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.