Edexcel A-Level Chemistry Paper 3, November 2020: Question 8

15 marks · Hard difficulty · Open Response

Investigate chromium chemistry including qualitative reactions of chromium(III), oxidation and reduction half-equations, interconversion of chromate(VI) and dichromate(VI), and drawing an electrochemical cell apparatus.

Practise this question

Question

Question 8 about chromium(III) and chromium(VI) compounds. Part (a) asks to describe observations when aqueous sodium hydroxide is added drop by drop until excess to chromium(III) ions (2 marks). Part (b)(i) asks to deduce the oxidation half-equation for oxidizing [Cr(OH)6]3- to CrO42- using hydrogen peroxide in alkaline conditions (2 marks). Part (b)(ii) asks for the equation when chromate(VI) is acidified to form orange dichromate(VI) (1 mark). Part (b)(iii) gives a cell diagram and asks for oxidation and reduction half-equations and overall equation (3 marks). Part (c) asks to draw a labelled diagram of the apparatus to measure standard emf of a zinc-zinc(II) and chromium(III)-dichromate(VI) electrode system, including formulae and concentrations (7 marks).
Question text

8 This is a question about chromium(III) and chromium(VI) compounds.

(a) Describe the observations when aqueous sodium hydroxide is added drop by

drop until in excess to a solution of chromium(III) ions.

(2)

(b) The chromium(III) complex, [Cr(OH) ]3−, can be oxidised to

chromate(VI) ions, CrO2−, by hydrogen peroxide solution.

(i) Deduce the oxidation half-equation for this reaction, which takes place in

alkaline conditions. State symbols are not required.

(2)

(ii) If the solution of chromate(VI) ions is then acidified, the colour of the solution

changes to orange as dichromate(VI) ions form.

Write the equation for this change. State symbols are not required.

(1)

(iii) In acidic conditions, dichromate(VI) ions can also be reduced to chromium(III) ions

using hydrogen peroxide.

The value of E—O = + 0.65 V for which the cell diagram is

cell

Pt(s)│H O (aq), [2H+(aq)+O (g)] [Cr O 2−(aq)+14H+(aq)], [2Cr3+(aq)+7H O(l)]│Pt(s)

22 2 2 7 2

Deduce from the cell diagram the oxidation and the reduction half-equations,

and thus the overall equation for this reaction.

State symbols are not required.

(3)

*P62670A02432*

(c) Draw a labelled diagram of the apparatus that you would use to measure the

standard emf of a cell with a zinc-zinc(II) electrode system and

a chromium(III)-dichromate(VI) electrode system.

Include the formulae of all the compounds required and the concentrations of

the solutions.

(7)

(Total for Question 8 = 15 marks)

Mark scheme

Show the mark scheme Mark scheme for Question 8 showing detailed marking points for observations of green precipitate dissolving to a green solution, half-equations and overall equations for chromium oxidation and reduction, and an example diagram of an electrochemical cell with a high-resistance voltmeter, salt bridge, zinc electrode in 1 mol dm-3 zinc solution, platinum electrode, and suitable chromium salts at 1 mol dm-3 concentration.

How to answer it

Chromium Chemistry & Electrochemical Cells Study Guide

What this question tests

  • Transition metal chemistry: ligand substitution, precipitation, and amphoteric behavior of chromium(III) hydroxide.
  • Redox reactions in aqueous systems: constructing oxidation and reduction half-equations and combining them into overall equations in acidic/alkaline media.
  • Electrochemical cells: interpreting cell diagrams, predicting spontaneous reactions, and drawing/labelling standard electrochemical cells with proper state conditions (1.00 mol dm⁻³, platinum black electrodes, salt bridges).
Question Part (a)

Aqueous Sodium Hydroxide with Chromium(III) Ions

✅ Correct Observations

  • First observation: A green precipitate forms. (Accept 'green solid' or 'grey-green precipitate').
  • Second observation: The precipitate dissolves in excess sodium hydroxide to give a green solution.

❌ Common Errors

  • Describing the initial precipitate as blue-green (this is not accepted by examiners).
  • Failing to link the second mark to the first—mark 2 is strictly dependent on getting mark 1 correct or making a near-miss observation.
Marks: 2 marks total. 1 mark for green ppt, 1 mark for ppt dissolving to form a green solution in excess NaOH.
Question Part (b)(i)

Oxidation Half-Equation for Chromium(III) to Chromate(VI) in Alkaline Conditions

✅ Correct Answer

[Cr(OH)₆]³⁻ + 2OH⁻ → CrO₄²⁻ + 4H₂O + 3e⁻

(Accept multiples)

💡 Key Knowledge

  • In alkaline conditions, balance oxygen atoms using water and hydrogen atoms/charge using hydroxide ions ( OH⁻ ) or electrons.
  • Chromium changes oxidation state from +3 in [Cr(OH)₆]³⁻ to +6 in CrO₄²⁻ , representing a loss of 3 electrons.
Marks: 2 marks total. 1 mark for four correct species, 1 mark for correct balancing and inclusion of 3 electrons.
Question Part (b)(ii)

Acidification of Chromate(VI) to Form Dichromate(VI)

✅ Correct Answer

2CrO₄²⁻ + 2H⁺ → Cr₂O₇²⁻ + H₂O

(Accept reversible arrows ⇌ or multiples)

🧠 Exam Technique

Remember that chromate( VI ) ( CrO₄²⁻ , yellow) and dichromate( VI ) ( Cr₂O₇²⁻ , orange) exist in an equilibrium that is sensitive to pH. Adding acid shifts the equilibrium towards the orange dichromate ion.

Marks: 1 mark for the balanced equation. State symbols were not required.
Question Part (b)(iii)

Deducing Half-Equations and Overall Redox Equation

✅ Correct Equations

  • Oxidation half-equation: H₂O₂ → 2H⁺ + O₂ + 2e⁻
  • Reduction half-equation: Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O
  • Overall equation: Cr₂O₇²⁻ + 8H⁺ + 3H₂O₂ → 2Cr³⁺ + 7H₂O + 3O₂

❌ Common Errors & Traps

  • Leaving excess H⁺ or electrons on both sides of the final overall equation after cancelling.
  • Swapping the oxidation and reduction half-equations relative to the cell diagram order without checking electron transfers.
Marks: 3 marks total (1 for oxidation half-equation, 1 for reduction half-equation, 1 for overall equation with cancelled ions). No error carried forward (TE) if initial half-equations are wrong.
Question Part (c)

Apparatus Diagram for Measuring Standard Cell EMF

💡 Key Labelling Requirements (7 Marks)

  • M1: High resistance voltmeter connected across the two half-cells.
  • M2: Salt bridge connecting both half-cells to complete the circuit (must dip into both solutions).
  • M3: Filter paper (or absorbent material) soaked in saturated potassium nitrate ( KNO₃ ) or potassium chloride ( KCl ) solution.
  • M4: Zinc electrode immersed in a suitable zinc salt solution (e.g., ZnSO₄ ).
  • M5: Platinum (black) electrode for the chromium half-cell.
  • M6: Suitable chromium salts containing both Cr³⁺ and Cr₂O₇²⁻ species (e.g., CrCl₃ and K₂Cr₂O₇ ).
  • M7: All ionic solutions at standard concentration: exactly 1.00 mol dm⁻³ (note: if Cr₂(SO₄)₃ is used, chromium ion concentration must account for stoichiometry, i.e. 0.5 mol dm⁻³ ).

🧠 Examiner's Drawing Guidance

When drawing this apparatus:

  • Draw two separate beakers containing the respective aqueous solutions.
  • Clearly show metal strips / inert platinum dipping into the liquids with wires leading to a voltmeter marked V.
  • Draw an inverted U-tube or filter paper bridge spanning the two beakers, making physical contact with both solutions.
  • State formulas clearly (avoid using vague names like "zinc solution"). Temperature (298 K) can be omitted or stated.
Marks: 7 marks total corresponding directly to requirements M1 through M7 above.

Topics

Inorganic Chemistry · Physical Chemistry · Core Practicals · Topic 15: Transition Metals · Topic 14: Redox II · Core Practical 10: Construct electrochemical cells and measure electrode potentials

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.