Edexcel A-Level Chemistry Paper 1, June 2019: Question 10

11 marks · Medium difficulty · Calculations

Calculate the equilibrium constant Kp, the number of sulfur dioxide molecules, explain the effect of adding oxygen or sodium hydroxide on equilibria, and identify the correct Kc expression for heterogeneous equilibria.

Practise this question

Question

Exam question with three main parts about chemical equilibria. Part (a) provides data for the equilibrium 2SO2(g) + O2(g) rightleftharpoons 2SO3(g) with mole values, asking to calculate Kp with units, the number of SO2 molecules, and the effect of adding oxygen. Part (b) asks to explain the effect of sodium hydroxide on the chromate(VI)-dichromate(VI) equilibrium. Part (c) is a multiple-choice question asking to select the correct Kc expression for the reaction Fe3O4(s) + 4H2(g) rightleftharpoons 3Fe(s) + 4H2O(g).
Question text

10 This question is about equilibrium systems.

(a) Sulfur dioxide and oxygen form an equilibrium with sulfur trioxide.

2SO2(g) + O2(g) `_ 2SO3(g)

The composition of an equilibrium mixture at 698 K and a total pressure of

2.40 atm is shown in the table.

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

Number of moles /mol 0.0160 0.0120 0.772

(i) Calculate the value of Kp at this temperature.

Include units, if appropriate.

(5)

(ii) Calculate the number of sulfur dioxide molecules present in this equilibrium mixture.

(1)

(iii) Deduce, by referring to Kp, how the number of sulfur dioxide molecules will

change if more oxygen is added to the equilibrium mixture.

(2)

… 26

… *P58306A02628*

(b) An equilibrium exists in aqueous solution between the chromate(VI) ions and the

dichromate(VI) ions.

2CrO2–(aq) + 2H+(aq) `_ Cr O2–(aq) + H O(l)

42 7 2

Explain any change in the position of equilibrium if a few drops of

sodium hydroxide solution are added to this equilibrium system.

(2)

(c) The equilibrium for the reaction between hydrogen gas and an oxide of iron is

Fe3O4(s) + 4H2(g) `_ 3Fe(s) + 4H2O(g)

The Kc expression for this equilibrium is

(1)

[Fe] × [H O2]

A Kc =

[Fe O34] × [H2]

[Fe]3 × [H O]4

B K = 2

c

[Fe O ] × [H ]4

34 2

[H O2]

C Kc =

[H2]

[H O]4

D K = 2

c

[H ]4

(Total for Question 10 = 11 marks)

Mark scheme

Show the mark scheme Mark scheme detailing the step-by-step calculation marks for Kp including mole fractions, partial pressures, expression, value, and units. It provides marking points for explaining the shift in equilibrium when oxygen or sodium hydroxide is added, and identifies option D as the correct multiple-choice answer with reasoning for why solids are omitted from equilibrium expressions.

How to answer it

Equilibrium Systems Masterclass

📌 What this question tests

This question assesses your mastery of chemical equilibria, specifically calculating equilibrium constant Kp using mole fractions and partial pressures, manipulating Avogadro's constant, applying Le Chatelier's Principle to homogeneous and heterogeneous systems, and constructing correct equilibrium constant expressions (including knowing when to omit pure solids/liquids).

Question 10(a)(i) — Calculating Kp

5 Marks | Multi-step Calculation

📐 Step-by-Step Calculation

  1. Total Moles: Add together all equilibrium moles: 0.0160 + 0.0120 + 0.772 = 0.800 mol
  2. Mole Fractions (X): Divide individual moles by total moles.
    X(SO₂) = 0.0160 / 0.800 = 0.020
    X(O₂) = 0.0120 / 0.800 = 0.015
    X(SO₃) = 0.772 / 0.800 = 0.965
  3. Partial Pressures (P): Multiply mole fraction by total pressure (2.40 atm).
    P(SO₂) = 0.020 × 2.40 = 0.048 atm
    P(O₂) = 0.015 × 2.40 = 0.036 atm
    P(SO₃) = 0.965 × 2.40 = 2.316 atm
  4. Kp Expression & Substitution:
    Kp = P(SO₃)² / (P(SO₂)² × P(O₂))
    Kp = (2.316)² / ((0.048)² × 0.036) = 64668...
  5. Units: atm⁻¹ (derived from atm² / (atm² × atm))

❌ Common Errors & Examiner Traps

  • Square Brackets: Using square brackets [ ] instead of round brackets ( ) in the Kp expression. Square brackets strictly denote concentration (C), whereas Kp requires partial pressures.
  • Unit Omission or Errors: Forgetting units or incorrectly simplifying atm² / atm³ to get positive indices.
  • Rounding too early: Rounding intermediate partial pressures heavily skews the final Kp value. Keep full calculator values until the end.
Mark Scheme Breakdown: (M1) Mole fractions calculated; (M2) Partial pressures calculated; (M3) Correct Kp expression; (M4) Correct numerical value (~64700 or 6.47 × 10⁴); (M5) Correct units (atm⁻¹).

Question 10(a)(ii) — Number of Molecules Calculation

1 Mark | Mole-Particle Conversion

✅ Correct Answer

Number of molecules = Moles × Avogadro constant (L)

0.0160 × 6.02 × 10²³ = 9.63 × 10²¹ molecules

🧠 Exam Technique

This is a quick 1-mark recall and application. Make sure you use the specific number of moles given for sulfur dioxide in the table (0.0160 mol), not the total moles.

Question 10(a)(iii) — Effect of Adding Oxygen on SO₂ Molecules

2 Marks | Le Chatelier & Equilibrium Constants

💡 Key Knowledge

  • Kp is constant at a constant temperature. Adding more oxygen temporarily changes the reaction quotient, causing the position of equilibrium to shift to the right to keep Kp constant.
  • As the system shifts right, SO₂ is consumed, meaning the number of sulfur dioxide molecules decreases.

❌ Common Misconceptions

Students often incorrectly believe that adding a reactant changes the value of Kp. Remember: only temperature changes the value of an equilibrium constant!

Mark Scheme Breakdown: (1 mark) Reference that Kp stays the same (or quotient stays constant / only temperature changes Kp); (1 mark) SO₂ molecules decrease because equilibrium shifts to the right (or denominator adjustment).

Question 10(b) — Aqueous Equilibrium & NaOH Addition

2 Marks | Qualitative Equilibrium Shift

✅ Correct Answer

  • Position of equilibrium shifts to the left.
  • Reason: Hydroxide ions (OH⁻) from NaOH react with/neutralise the hydrogen ions (H⁺) present in the equilibrium mixture, removing them from the system.

🧠 Exam Technique

Avoid vague statements like "equilibrium moves to the right/left to oppose the change" without specifying what happened to the species. You must explicitly state that OH⁻ ions react with H⁺ ions to form water, decreasing [H⁺].

Mark Scheme Breakdown: (1 mark) Equilibrium shifts to the left; (1 mark) Hydroxide ions combine/neutralise/remove H⁺ ions. Note: "Equilibrium moves to the right" scores 0.

Question 10(c) — Heterogeneous Equilibrium Expression

1 Mark | Multiple Choice Question

✅ Correct Answer: Option D

Kc = [H₂O]⁴ / [H₂]⁴

💡 Why the other options are wrong:

  • Solids omitted: Fe₃O₄(s) and Fe(s) are pure solids. In heterogeneous equilibria, pure solids and liquids have a constant concentration and are omitted from Kc expressions. This rules out options A and B.
  • Stoichiometric Powers: The coefficients in the balanced equation ( 4H₂O and 4H₂ ) must appear as powers, ruling out option C.

Topics

Physical Chemistry · Topic 5: Formulae, Equations and Amounts of Substance · Topic 10: Equilibrium I · Topic 11: Equilibrium II

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 1, June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.