Edexcel A-Level Chemistry Paper 3, June 2019: Question 5

11 marks · Medium difficulty · Short Open Response

Answer questions on redox reactions, including oxidation numbers, redox definitions, standard electrode potentials, disproportionation, dot-and-cross diagrams, and organic reaction mechanisms.

Practise this question

Question

Exam question 5 about redox reactions, containing several parts: (a) naming PO3 3- with oxidation number, (b) explaining what happens to a reducing agent in terms of oxidation number and electrons, (c) identifying the strongest reducing agent from a data booklet, (d) balancing an ionic redox equation for MnO2, MnO4- and MnO4 2- and explaining why it is not disproportionation, (e)(i) drawing a dot-and-cross diagram for the BH4- ion, and (e)(ii) completing the mechanism for the reduction of propanone by BH4- to form propan-2-ol with curly arrows, lone pairs, and dipoles.
Question text

5 This question is about redox reactions.

(a) Name the ion with formula PO3–. Include the relevant oxidation number.

(1)

(b) State what happens to a reducing agent during a reaction, in terms of oxidation number

and electrons.

(1)

(c) Identify the species that is the strongest reducing agent from the list of standard

electrode potentials in the Data Booklet.

(1)

(d) Manganese(IV) oxide, MnO , and manganate(VII) ions, MnO– , react in alkaline

solution to form manganate(VI) ions, MnO2–.

(i) Write the ionic equation for this reaction.

State symbols are not required.

(2)

(ii) Give a reason why this reaction is not disproportionation.

(1)

(e) Sodium tetrahydridoborate(III), NaBH4, is used in organic chemistry.

It is an alternative reagent to lithium tetrahydridoaluminate(III) for the reduction

of carbonyl compounds.

(i) Draw a dot-and-cross diagram of the BH– ion.

Use crosses (×) for the boron electrons, dots (•) for the hydrogen electrons and

triangles ( V ) for the additional electron forming the negative ion.

*P58308A01032* (1)

(ii) The BH– ions reduce carbonyl compounds to alcohols in aqueous solution.

Complete the mechanism for the reduction of propanone to propan-2-ol by

adding curly arrows, and any relevant lone pairs and dipoles.

(4)

O O–

H3C C CH3 H3C C CH3 + BH3

– H

H H

B

H H

O

H H

O– OH

H C C CH H C C CH3 + OH–

33 3

H H

(Total for Question 5 = 11 marks)

Mark scheme

Show the mark scheme Mark scheme for question 5 showing acceptable answers and additional guidance for parts (a) through (e)(ii), including the expected name and oxidation number, definitions of reducing agent, sodium as the strongest reducing agent, the balanced ionic equation, explanations of why it is not disproportionation, the dot-and-cross diagram for BH4-, and the detailed steps for the nucleophilic addition mechanism involving curly arrows and dipoles.

How to answer it

Redox Reactions and Carbonyl Mechanisms Study Guide

Edexcel A-Level Chemistry — Exam Question Breakdown

What this question tests

This multi-topic question assesses your mastery of redox terminology, standard electrode potentials, balancing complex ionic redox equations, identifying disproportionation reactions, drawing dot-and-cross diagrams for complex ions, and writing nucleophilic addition mechanisms for the reduction of carbonyl compounds using hydride transfer agents.

Part (a) — Naming Ions and Oxidation Numbers

Name the ion with formula PO₃³⁻. Include the relevant oxidation number. (1 mark)

✅ Correct Answer

phosphate(III) (allow space between name and oxidation number: phosphate (+3))

💡 Key Knowledge

The Roman numeral indicates the oxidation state of the central phosphorus atom. Since each oxygen is -2 and the overall charge is -3, phosphorus must be +3 .

❌ Common Errors

Writing "phosphorus(III)" or "trioxide", or failing to include brackets/Roman numerals. Do not use old nomenclature like "phosphite(III)".

Mark Scheme Guidance: 1 mark for both name and correct oxidation number.

Part (b) — Defining Reducing Agents

State what happens to a reducing agent during a reaction, in terms of oxidation number and electrons. (1 mark)

✅ Correct Answer

Oxidation number increases (becomes more positive / gets larger) AND it loses electron(s) (donates electrons).

🧠 Exam Technique

You must include both criteria to secure the mark. Examiners strictly penalize answers that only mention electron loss without referencing the change in oxidation number.

Mark Scheme Guidance: 1 mark total for stating both an increase in oxidation number and loss of electrons.

Part (c) — Standard Electrode Potentials

Identify the species that is the strongest reducing agent from the list of standard electrode potentials in the Data Booklet. (1 mark)

✅ Correct Answer

Na or sodium (Do not award Na⁺)

💡 Key Knowledge

The strongest reducing agent is found at the most negative end of the electrochemical series (the bottom-left of the Data Booklet table), meaning it has the greatest tendency to lose electrons and undergo oxidation.

Mark Scheme Guidance: 1 mark for naming sodium or giving its correct chemical symbol.

Part (d) — Ionic Equations and Disproportionation

(i) Write the ionic equation for the reaction of MnO₂ and MnO₄⁻ in alkaline solution to form MnO₄²⁻. State symbols not required. (2 marks)

(ii) Give a reason why this reaction is not disproportionation. (1 mark)

✅ Correct Answer (i)

MnO₂ + 2MnO₄⁻ + 4OH⁻ → 3MnO₄²⁻ + 2H₂O

(Multiples and cancelled spectator ions are accepted)

✅ Correct Answer (ii)

Two different species are oxidized and reduced to form the same species (or: the same element is not simultaneously oxidized and reduced from a single oxidation state into two different states).

❌ Common Errors

Placing H⁺ ions on the left-hand side of the equation (remember the reaction occurs in an alkaline solution, so OH⁻ must be used).

🧠 Exam Technique

For part (ii), remember that disproportionation requires a single reactant species containing an element in one oxidation state to form two different oxidation states. Here, Mn starts in two states (+4 in MnO₂ and +7 in MnO₄⁻) and forms just one (+6 in MnO₄²⁻), which is the reverse of disproportionation (comproportionation).

Mark Scheme Guidance: (i) 1 mark for correct species and reactants, 1 mark for H₂O and balancing. (ii) 1 mark for explaining that two different starting species form a single oxidation state product.

Part (e) — Organic Mechanisms and Dot-and-Cross Diagrams

(i) Draw a dot-and-cross diagram of the BH₄⁻ ion using crosses for B electrons, dots for H electrons, and triangles for the extra electron. (1 mark)

(ii) Complete the mechanism for the reduction of propanone to propan-2-ol by adding curly arrows, lone pairs, and dipoles. (4 marks)

💡 Key Knowledge: BH₄⁻ Diagram

Boron has 3 outer shell electrons (shown as ×). Each of the 4 hydrogen atoms contributes 1 electron (shown as •). The negative charge is represented by an added electron (shown as ▲). Four covalent single bonds form around the central boron atom inside square brackets with an overall [-] charge.

🧠 Exam Technique: Mechanism Curly Arrows

  • Arrow 1: Curly arrow starting from the B–H bond of the borohydride ion pointing to the carbonyl carbon (C=O).
  • Arrow 2: Dipole on C=O (δ+ on carbon, δ- on oxygen) accompanied by a curly arrow from the C=O pi bond to the oxygen atom.
  • Arrow 3: A lone pair shown on the negatively charged oxygen atom O⁻ , with a curly arrow starting from this lone pair to a hydrogen atom on a water molecule ( H₂O ).
  • Arrow 4: A curly arrow from the H–O bond of the water molecule back to the oxygen atom, along with a dipole shown on the H–O bond.

❌ Common Errors

Drawing half-headed fishhook arrows instead of full double-headed curly arrows representing electron pair movement. Forgetting to draw dipoles or lone pairs where specifically demanded by the question prompt.

Mark Scheme Guidance: (i) 1 mark for correct symbols and electron sharing in BH₄⁻. (ii) 4 marks allocated per curly arrow / dipole / lone pair requirement specified above.

Topics

Physical Chemistry · Organic Chemistry · Topic 17: Organic Chemistry II · Topic 2: Bonding and Structure · Topic 3: Redox I · Topic 14: Redox II

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.