Edexcel A-Level Chemistry AS Paper 2, June 2024: Question 2

9 marks · Medium difficulty · Open Response

Explain equilibrium shifts for dichromate-chromate and nitrogen dioxide-dinitrogen tetroxide systems, and identify the equilibrium constant expression.

Practise this question

Question

Question 2 on chemical equilibria. Part (a) shows the equilibrium Cr2O72-(aq) [orange] + H2O(l) reversible reaction 2CrO42-(aq) [yellow] + 2H+(aq). (i) asks to explain the effect on the appearance of the solution of adding a small volume of sodium hydroxide solution (3 marks). (ii) is a multiple choice question asking for the correct expression for the equilibrium constant Kc among four options (1 mark). Part (b) describes a syringe containing 2NO2(g) [brown] reversible reaction N2O4(g) [colourless]. (i) asks to justify all colour changes when the plunger is pulled out to maximum volume at constant temperature until equilibrium is restored (3 marks). (ii) states the syringe is placed into hot water and becomes darker brown, asking what can be deduced about the reaction (2 marks). Total 9 marks.
Question text

2 This question is about chemical equilibria.

Potassium dichromate(VI), K2Cr2O7, is an orange solid which dissolves in water.

An equilibrium forms as shown.

Cr O72–(aq) + H O( ) 2CrO2–(aq) + 2H+(aq)

22 l 4

orange yellow

(a) (i) Explain the effect on the appearance of the solution of adding a small volume

of sodium hydroxide solution to the equilibrium mixture.

(3)

(ii) Which is the expression for the equilibrium constant, Kc, for this reaction?

(1)

[CrO2− ] [2H+ ]2

A Kc = 2−

[Cr O27 ][H O2]

2[CrO2− ] [2H+ ]

B Kc = 2−

[Cr O27 ][H O2]

[CrO2− ] [2H+ ]2

C Kc = 2−

[Cr O27 ]

2[CrO2− ] [2H+ ]

D Kc = 2−

[Cr O27 ]

(b) A gas syringe contains a mixture of the gases nitrogen dioxide (NO2) and

dinitrogen tetroxide (N2O4).

Its plunger is fixed at half the volume of the syringe.

The mixture is allowed to reach equilibrium at room temperature.

2NO2(g) N2O4(g)

brown colourless

The plunger is then pulled out to the maximum volume, while the temperature is*P76894A0632*

kept constant.

The mixture is left to stand until equilibrium is reached again.

(i) Justify all the colour changes of the contents of the syringe during

this process.

(3)

(ii) The syringe is placed into hot water.

The contents of the syringe become a darker brown.

Explain what can be deduced about the reaction from this change.

(2)

(Total for Question 2 = 9 marks)

Mark scheme

Show the mark scheme Mark scheme for Question 2. 2(a)(i): mixture goes yellow/less orange (1); equilibrium moves to the right (1); because sodium hydroxide reacts with/neutralises H+, lowering its concentration (1). 2(a)(ii): C is correct ([CrO42-]^2 [H+]^2 / [Cr2O72-]). 2(b)(i): initially gas becomes lighter in colour as NO2 spreads over a larger volume (1); gradually mixture becomes darker / more brown (1); as equilibrium shifts to the side with the larger number of moles of gas (1). 2(b)(ii): concentration of NO2 increases, equilibrium shifts to the left (1); equilibrium shifts in the endothermic direction / backward reaction is endothermic / forward reaction is exothermic (1).

Question

Answer Additional Guidance Mark

Number

2(a)(i) An explanation that makes reference to the following points: (3)

• the mixture goes yellow (1) Allow less orange

• as the equilibrium moves to the right (1)

• because sodium hydroxide reacts with / neutralises H+ Ignore water formed during neutralisation increases

lowering its concentration (1) the concentration of water moving the equilibrium to

the right

Do not award there are more reactants / the

concentration of the reactants is higher

Question

Answer Mark

Number

2(a)(ii) [CrO2‒]2 [H+]2 (1)

The only correct answer is C (K = 4 )

c 2‒

[Cr2O7 ]

A is not correct because water is not included as it is in the liquid state

B is not correct because water is not included and [CrO2‒] and [H+] should be squared not multiplied by 2

D is not correct because [CrO2‒]and [H+] should be squared not multiplied by 2

Question

Answer Additional Guidance Mark

Number

2(b)(i) An answer that makes reference to the following points: (3)

• initially the gas becomes lighter in colour as the

nitrogen dioxide is spread over a larger volume (1)

• gradually the mixture becomes darker in colour (1) Allow more brown

Allow goes from colourless to brown

• as the equilibrium shifts to the side with the larger

number of moles of gas (1)

Question

Answer Additional Guidance Mark

Number

2(b)(ii) An explanation that makes reference to the following points: (2)

• (the concentration / amount of NO2 increases with

increasing temperature), so the equilibrium is shifting to

the left (1)

• (because) the equilibrium shifts in the endothermic

direction

or

so the backward reaction is endothermic / forward

reaction is exothermic (1)

(Total for Question 2 = 9 marks)

How to answer it

Dynamic Equilibria: Le Chatelier's Principle & Kc

📌 What this question tests

This question assesses your understanding of dynamic equilibria in both aqueous solutions and gaseous systems, specifically:

  • Applying Le Chatelier's principle to predict shifts caused by removing ions (neutralisation).
  • Writing expressions for the equilibrium constant Kc and understanding why the solvent (H₂O) is omitted in dilute aqueous systems.
  • Distinguishing between immediate physical colour changes (dilution/expansion) versus subsequent equilibrium shifts in gaseous systems.
  • Deducing the enthalpy change (ΔH) of a reaction using temperature shifts.
Part 2(a)(i) • 3 Marks

Dichromate/Chromate Equilibrium & Adding Alkali

Explaining the effect on appearance when adding NaOH(aq)

Cr₂O₇²⁻(aq) [orange] + H₂O(l) ⇌ 2CrO₄²⁻(aq) [yellow] + 2H⁺(aq)

✅ Model Answer (Full Marks)

  • Observation: The solution turns yellow (or less orange). [1 mark]
  • Shift: The position of equilibrium shifts to the right. [1 mark]
  • Cause: Hydroxide ions (OH⁻) from NaOH react with/neutralise H⁺ ions, lowering the concentration of H⁺. [1 mark]

🧠 Exam Technique: 3-Step Le Chatelier

Always structure qualitative equilibrium questions in three clear steps:

  1. Identify the direct change: Added OH⁻ neutralises H⁺, so [H⁺] decreases.
  2. State the equilibrium response: System shifts right to replace lost H⁺ ions.
  3. State the visible outcome: More CrO₄²⁻ is formed, so the solution becomes yellow.

❌ Common Errors & Misconceptions

  • Water misconception: Arguing that the water produced by neutralisation increases [H₂O] and pushes the reaction right. (Water is the solvent in huge excess; its concentration change is negligible).
  • Vague answers: Stating simply "it reacts" without specifying that it is H⁺ that is being consumed/neutralised.
Mark Allocation:
• 1 mark for stating the colour change (yellow / less orange).
• 1 mark for equilibrium shifts to the right.
• 1 mark for NaOH reacts with / neutralises H⁺ (lowering its concentration).
Part 2(a)(ii) • 1 Mark

Equilibrium Constant Expression (Kc)

Selecting the correct mathematical expression

✅ Correct Answer: Option C

Kc = [CrO₄²⁻]² [H⁺]² / [Cr₂O₇²⁻]

C is the only correct answer.

💡 Why is H₂O(l) omitted?

  • Solvent in large excess: In aqueous equilibria, water acts as the solvent. Its concentration is extremely large (~55.5 mol dm⁻³) and remains virtually constant.
  • Its constant value is already incorporated into the equilibrium constant Kc.
  • Therefore, pure liquids and solvents are omitted from the Kc expression.

❌ Why Other Options Are Incorrect

  • Option A: Incorrectly includes [H₂O] in the denominator.
  • Option B: Includes [H₂O] and incorrectly multiplies the concentration by stoichiometric coefficients (2[X]) rather than raising them to the power ([X]²).
  • Option D: Correctly omits H₂O, but wrongly multiplies by 2 instead of squaring terms.
Part 2(b)(i) • 3 Marks

Gas Syringe Expansion: NO₂ / N₂O₄ System

Justifying ALL colour changes when pulling out the plunger

2NO₂(g) [brown] ⇌ N₂O₄(g) [colourless]

✅ Model Answer (Full Marks)

  • Stage 1 (Immediate physical change): Initially, the gas becomes lighter in colour / paler because the brown NO₂ molecules are spread over a larger volume (concentration decreases). [1 mark]
  • Stage 2 (Subsequent equilibrium shift): Gradually, the mixture becomes darker brown again. [1 mark]
  • Stage 3 (Reason for shift): The increase in volume causes a decrease in pressure, so the equilibrium shifts to the left / side with the larger number of gas moles (2 moles of NO₂ vs 1 mole of N₂O₄) to oppose the drop in pressure. [1 mark]

🧠 Examiner Trap: The "Two-Stage" Colour Change

Notice the question says: "Justify all the colour changes". Many students lose the first mark because they only describe what happens when the equilibrium shifts.

  • Instantaneous: Volume increases → instantaneous dilution → colour fades immediately.
  • Over time: Pressure is lower → system shifts to left (more moles of gas) → NO₂ forms → colour darkens.

❌ Common Student Errors

  • Stating only that the mixture "goes brown" without mentioning that it initially went pale/lighter first.
  • Failing to mention moles of gas when explaining the pressure effect (you must state "shifts to the side with more moles of gas").
Part 2(b)(ii) • 2 Marks

Temperature & Enthalpy of Reaction

Deducing the thermodynamics from a colour change in hot water

✅ Model Answer (Full Marks)

  • Equilibrium direction: Increasing temperature shifts the equilibrium to the left (towards the darker brown NO₂). [1 mark]
  • Thermodynamic deduction: An increase in temperature favours the endothermic direction. Therefore, the backward reaction is endothermic (or the forward reaction is exothermic). [1 mark]

💡 Temperature vs Equilibrium

  • Increase in temperature: Always shifts equilibrium in the endothermic (+ΔH) direction to absorb added heat.
  • Decrease in temperature: Always shifts equilibrium in the exothermic (-ΔH) direction to release heat.
  • Since heating made the mixture darker brown, more NO₂ was produced, meaning the reverse reaction absorbs heat:
    2NO₂(g) ← N₂O₄(g) is endothermic
    2NO₂(g) → N₂O₄(g) is exothermic

❌ Common Errors

  • Saying "the reaction is endothermic" without specifying whether you mean the forward or backward reaction. Always specify!
  • Confusing the shift: hotter = darker brown = more NO₂ = shifted left.

Topics

Physical Chemistry · Topic 10: Equilibrium I · Topic 11: Equilibrium II

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 2, June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.