OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2018: Question 11

1 mark · Easy difficulty · Multiple Choice

Calculate the standard entropy change for the formation of methanol from carbon monoxide and hydrogen using standard entropy data.

Practise this question

Question

Multiple choice question 11. A table provides standard entropies S-theta for CO(g) at 197.6 J mol-1 K-1, H2(g) at 130.6 J mol-1 K-1, and CH3OH(l) at 239.7 J mol-1 K-1. The reaction given is CO(g) + 2H2(g) -> CH3OH(l). Four options A (-219.1), B (-88.5), C (+88.5), and D (+219.1) are listed with a box for the answer.
Question text

11 The table below shows standard entropies, Sө.

Substance CO(g) H2(g) CH3OH(l)

Sө / J mol–1 K–1 197.6 130.6 239.7

What is the entropy change, ∆Sө, in J mol–1 K–1, for the following reaction?

CO(g) + 2H2(g) CH3OH(l)

A –219.1

B –88.5

C +88.5

D +219.1

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer for question 11 is A.

11 A 1 AO1.2

How to answer it

Calculating Standard Entropy Changes

What this question tests

This question assesses your understanding of energetic stability and disorder by testing your ability to calculate standard entropy changes (ΔS_e) using standard molar entropy values (S_e) of reactants and products, factoring in balancing stoichiometry.

Question 11 (Multiple Choice)

Full Worked Solution & Breakdown

✅ Correct Answer

A ( −219.1 J mol⁻¹ K⁻¹)

Awarded 1 mark for selecting option A.

💡 Key Knowledge

  • Entropy Definition: A measure of the dispersal of energy within the chemicals, where higher disorder means higher entropy.
  • The Formula: ΔS_e = ΣS_e(products) − ΣS_e(reactants)
  • Physical States Matter: Gases have significantly higher standard entropies than liquids or solids due to greater freedom of movement.

🧠 Exam Technique

  • Always check the stoichiometric balancing numbers in the chemical equation before performing your calculation.
  • Double-check your subtraction order: it is always Products minus Reactants. Reversing this is a very common trap yielding the wrong sign.

❌ Common Errors

  • Stoichiometry Omission: Forgetting to multiply the entropy value of hydrogen ( H₂ ) by its balancing coefficient of 2.
  • Sign Reversal: Calculating +219.1 (Option D) by mistakenly doing Reactants minus Products.

📐 Step-by-Step Calculation

  1. Identify the equation and lookup values:
    Reaction: CO(g) + 2H₂(g) → CH₃OH(l)
    S_e(CO) = 197.6 J mol⁻¹ K⁻¹
    S_e(H₂) = 130.6 J mol⁻¹ K⁻¹
    S_e(CH₃OH) = 239.7 J mol⁻¹ K⁻¹
  2. Sum the entropies of the products:
    ΣS_e(products) = 1 × 239.7 = 239.7
  3. Sum the entropies of the reactants (accounting for moles):
    ΣS_e(reactants) = (1 × 197.6) + (2 × 130.6)
    ΣS_e(reactants) = 197.6 + 261.2 = 458.8
  4. Subtract reactants from products:
    ΔS_e = 239.7 − 458.8 = −219.1 J mol⁻¹ K⁻¹

Topics

Module 5: Physical chemistry and transition elements · 5.2 Energy

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.