OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2023: Question 9

1 mark · Medium difficulty · Multiple Choice

Calculate the standard entropy change for the formation of 1 mol of SO3(l) from SO2(g) and O2(g) using provided standard entropy values.

Practise this question

Question

Multiple choice question showing a table of standard entropies for SO2(g) (248), O2(g) (204), and SO3(l) (96). Students must determine the standard entropy change for forming 1 mol of SO3(l) from SO2(g) and O2(g) among four options A through D.
Question text

9 The table below shows standard entropies, Sө.

Substance SO2(g) O2(g) SO3(l)

Sө/J K–1 mol–1 248 204 96

What is the standard entropy change, ∆Sө, in J K mol–1, for the formation of 1 mol of SO (l) from

SO2(g) and O2(g)?

A –508

B –254

C +254

D +508

Your answer [1]

Mark scheme

Show the mark scheme The mark scheme indicates that the correct answer for question 9 is B.

9 B 1 AO2.2

How to answer it

Calculating Standard Entropy Change (Delta S)

What this question tests

This question assesses your ability to construct a balanced equation for a specific enthalpy/entropy change, recall and apply the entropy formula using standard molar entropies (Delta S = sum(S products) - sum(S reactants)), and correctly account for stoichiometric reacting ratios.

Question 9 Multiple-Choice Solution

Correct Answer: B (-254)

✅ Correct Answer Breakdown

The correct option is B (-254 J K⁻¹ mol⁻¹). Applying the standard entropy change equation with the correct stoichiometric coefficients yields a negative value, reflecting a decrease in disorder as gases convert into a liquid.

💡 Key Knowledge

  • Equation: Delta S = sum(S(products)) - sum(S(reactants))
  • Units: J K⁻¹ mol⁻¹ (Note that entropy values are given in Joules, unlike enthalpy values which are usually in kJ).
  • State changes: Gases have significantly higher entropies than liquids or solids due to greater random dispersal of molecules.

🧠 Exam Technique

  • Always write out a balanced symbol equation first before extracting entropy values.
  • Double-check the balancing numbers required by the wording (e.g., formation of 1 mol of product).
  • Check the sign (+ or -) carefully: going from 1.5 moles of gas to 1 mole of liquid must result in a decrease in entropy (negative Delta S).

❌ Common Errors & Traps

  • Forgetting stoichiometry: Forgetting to divide the oxygen entropy term by 2 (using 1 full mole of O₂ instead of 0.5) leads to an incorrect total.
  • Wrong reactant/product subtraction: Subtracting products from reactants instead of reactants from products reverses the final sign (giving +254 instead of -254).

📐 Step-by-Step Calculation

  1. Write the balanced equation for the formation of 1 mol of SO₃(l):

    SO₂(g) + 0.5 O₂(g) → SO₃(l)

  2. Identify individual standard molar entropies (S values) from the table:
    • S(SO₂(g)) = 248 J K⁻¹ mol⁻¹
    • S(O₂(g)) = 204 J K⁻¹ mol⁻¹
    • S(SO₃(l)) = 96 J K⁻¹ mol⁻¹
  3. Set up the formula:

    Delta S = S(SO₃) - [ S(SO₂) + 0.5 × S(O₂) ]

  4. Substitute the values:

    Delta S = 96 - [ 248 + (0.5 × 204) ]

    Delta S = 96 - [ 248 + 102 ]

    Delta S = 96 - 350 = -254 J K⁻¹ mol⁻¹

Mark Scheme Allocation: 1 mark for correct selection of option B (AO2.2 - Applying chemical knowledge and mathematical skills to a chemical context).

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 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.