WJEC A-Level Chemistry AS Unit 2, June 2025: Question 6

2 marks · Easy difficulty · Short Answer

Deduce standard enthalpy changes for reverse and coupled reactions using given thermochemical equations.

Practise this question

Question

Question 6 provides two thermochemical equations with unknown substances: V + W -> X + Y with standard enthalpy change of -275 kJ/mol, and X + Y -> Z with standard enthalpy change of 160 kJ/mol. Part (a) asks for the enthalpy change of the reverse reaction X + Y -> V + W for 1 mark. Part (b) asks for the enthalpy change of the overall reaction V + W -> Z for 1 mark.
Question text

6. Use the information in the following equations.

∆ θ _ –1

V + W X + Y H = 275kJmol

X + Y Z ∆H θ = 160 kJ mol–1

Give ΔH θ for the reactions shown below.

(a) X + Y V + W [1]

ΔH θ = kJ mol–1

(b) V + W Z [1]

ΔH θ = kJ mol–1

Mark scheme

Show the mark scheme Mark scheme for Question 6: (a) gives 275 kJ/mol for 1 mark. (b) gives -115 kJ/mol for 1 mark.

6 (a) 275

(b) –115

11 1

Section A total 5 5 0 10 1 0

SECTION B

How to answer it

Hess's Law & Reaction Enthalpy Combinations

WHAT THIS QUESTION TESTS

Core thermochemical manipulation skills from WJEC AS Chemistry:

  • Reversing reactions: Knowing that reversing a chemical reaction inverts the sign of its standard enthalpy change (ΔHᶿ).
  • Combining reactions (Hess's Law): Summing multiple step reactions and their corresponding enthalpy changes to deduce the overall enthalpy change for a target reaction.
  • Directed numbers: Accurate handling of positive and negative integer arithmetic under exam conditions.

Given Information

(1) V + W → X + Y    ΔHᶿ = -275 kJ mol⁻¹
(2) X + Y → Z       ΔHᶿ = +160 kJ mol⁻¹
PART (a)

Target Reaction: X + Y → V + W

Determining ΔHᶿ for the reverse reaction [1 Mark]

✅ Correct Answer

+275 kJ mol⁻¹

(Mark scheme also accepts 275 without the '+' sign, though including the sign is best practice.)

Mark allocation: [1] mark for 275 or +275.

💡 Key Knowledge

If an exothermic reaction releases energy in the forward direction, the reverse reaction must absorb the exact same amount of energy (endothermic):

ΔHᶿ(reverse) = -ΔHᶿ(forward)

📐 Step-by-Step Working

  1. Compare target equation with Equation (1):
    Target: X + Y → V + W
    Eq (1): V + W → X + Y
  2. The target is the exact reverse of (1).
  3. Multiply ΔHᶿ by -1:
    -(-275 kJ mol⁻¹) = +275 kJ mol⁻¹

❌ Common Errors

  • Forgetting to change the sign: Writing -275 because students spot the same chemicals and copy the value without checking direction.
  • Arithmetic overcomplication: Attempting to subtract equation (2) unnecessarily.
PART (b)

Target Reaction: V + W → Z

Applying Hess's Law to combine steps [1 Mark]

✅ Correct Answer

-115 kJ mol⁻¹

Mark allocation: [1] mark for -115. The negative sign is mandatory.

💡 Key Knowledge

Hess's Law: The total enthalpy change for a chemical reaction is independent of the route by which the reaction takes place.

Intermediates that appear on both the product side of one step and the reactant side of the next cancel out when the equations are added.

📐 Step-by-Step Working

  1. Step 1: Convert V + W into intermediates (X + Y):
    ΔH₁ = -275 kJ mol⁻¹
  2. Step 2: Convert intermediates (X + Y) into final product Z:
    ΔH₂ = +160 kJ mol⁻¹
  3. Step 3: Sum the two equations:
    (V + W) + (X + Y) → (X + Y) + Z
    Cancelling (X + Y) leaves: V + W → Z
  4. Step 4: Sum the enthalpies:
    ΔHᶿ = ΔH₁ + ΔH₂ = (-275) + 160 = -115 kJ mol⁻¹

🧠 Exam Technique & Trap Prevention

  • Double-check signs on your calculator: Enter -275 + 160 , NOT 275 - 160 (which gives +115) and NOT -275 - 160 (which gives -435).
  • Sanity check: The reaction starts with a very exothermic step (-275) followed by a moderately endothermic step (+160). The overall process must remain exothermic (negative sign).

Topics

Physical Chemistry · 2.1 Thermochemistry

Question and mark scheme from the WJEC A-Level Chemistry examination, AS Unit 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.