OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2024: Question 21

15 marks · Hard difficulty · Structured Questions

Write equations and identify reaction types for barium compounds, calculate the mass of potassium iodate in a tablet via titration, draw a standard electrode potential measurement diagram for an Fe3+/Fe2+ half-cell, and construct cell equations for lithium-ion and hydrogen-oxygen fuel cells.

Practise this question

Question

A four-part structured chemistry question about redox reactions and electrode potentials. Part (a) features a flowchart converting barium to barium oxide, hydroxide, and chloride with spaces for balanced equations and reaction type. Part (b) provides an ionic equation and titration data for potassium iodate tablets, asking for the mass in mg in one tablet. Part (c) asks to draw a labelled diagram for measuring the standard electrode potential of the Fe3+/Fe2+ redox system and state standard conditions. Part (d) provides a table of six redox systems with half-equations and E-values, followed by sub-questions to construct a lithium-ion cell equation and show that acidic and alkaline hydrogen-oxygen fuel cells have the same overall cell potentials and equations.
Question text

21 This question is about redox reactions and electrode potentials.

(a) The flowchart shows some reactions of barium and its compounds.

Barium

Reaction 1

Barium oxide

Reaction 2

Barium hydroxide

Reaction 3

Barium chloride

• Write balanced equations for Reaction 1 and Reaction 2.

• Identify the type of reaction in Reaction 3.

Reaction 1: equation …

Reaction 2: equation …

Reaction 3: type of reaction …

[3]

(b) Potassium iodate tablets prevent the uptake of radioactive iodine in the human body following a

nuclear accident.

The mass of potassium iodate(V), KIO3, in a tablet can be determined by reaction with an

aqueous solution of potassium iodide, KI, in the presence of acid.

IO – + 5I– + 6H+ 3I + 3H O

32 2

A chemist finds that two KIO tablets react with exactly 26.2 cm3 0.150 mol dm–3 KI.

Calculate the mass, in mg, of KIO3 in one tablet.

Give your answer to the nearest whole number.

mass KIO3 = … mg [4]

(c) Standard electrode potentials are measured by comparison with a reference half-cell.

Draw a labelled diagram to show how the standard electrode potential could be measured for the

redox system below.

Fe3+(aq) + e– Fe2+(aq)

Include details of the apparatus, solutions and the standard conditions needed when measuring

this standard electrode potential.

Standard conditions …

26 [4]

(d) Many electric vehicles are powered by lithium-ion cells.

Hydrogen-oxygen fuel cells can also be used to power vehicles.

Six redox systems are shown in the table. State symbols have been omitted.

Redox system Half-equation E ө/ V

1 Li+ + e– Li –3.04

2 2H O + 2e– H + 2OH– –0.83

3 2H+ + 2e– H 0.00

4 O + 2H O + 4e– 4OH– +0.40

5 Li+ + CoO + e– LiCoO +1.16

6 O + 4H+ + 4e– 2H O +1.23

(i) A lithium-ion cell involves redox systems 1 and 5.

Construct the overall cell equation for a lithium-ion cell.

… [1]

(ii) Hydrogen-oxygen fuel cells can operate in acidic or in alkaline conditions.

Show that for acidic and alkaline hydrogen–oxygen fuel cells, the standard cell potentials, and

the overall cell equations, are the same.

Acidic …

Alkaline …

[3]

Mark scheme

Show the mark scheme The mark scheme provides answers and guidance for all parts of question 21. Part (a) lists the barium equations and neutralization. Part (b) shows step-by-step mole calculations leading to 84 mg. Part (c) outlines labeling requirements for the Fe3+/Fe2+ and hydrogen half-cell apparatus and standard conditions. Part (d)(i) gives the lithium-ion equation and (d)(ii) shows the standard cell potential calculations and both acidic and alkaline half-equations and overall equations.

Question Answer Marks Guidance

21 (a) 3 ALLOW multiples

IGNORE state symbols, even if incorrect

ALLOW Ba + H2O → BaO + H2 (reaction with steam)

2 Ba + O2 → 2 BaO

ALLOW other correct equations e.g. with less reactive metal

oxide

BaO + H2O → Ba(OH)2

Neutralisation

OR acid-base

(b) FIRST CHECK ANSWER ON ANSWER LINE 4

If answer = 84 award 4 marks ALLOW 3 SF or more throughout

------------------------------------------------------------------------ ALLOW ECF throughout

– - 26.2 0.150 –3

n(I ) = 1000 = 3.93 10 Care – other sequence of calculations can be valid.

3.93 10–3 Alternative route

n(IO –) = = 7.86 10–4 -4

35 7.86 x 10

M3 mol (IO -) in one tablet = = 3.93 10-4

mass KIO in 2 tablets = 7.86 10–4 214 = 0.168204 g

M4 Mass (KIO ) in one tablet = 3.93 x 10-4 214 = 84

mass KIO3 in 1 tablet = 0.084102 g = 84 mg (nearest Final answer must be a whole number

whole number)

Common Errors

3 marks:

69 mg (using M of IO - )

r 3

421mg (not divided by 5)

(c) Complete circuit AND voltmeter 4 Electrodes / salt bridge must at least touch the surface of

AND labelled salt bridge linking two half-cells solutions

ALLOW small gaps in circuit wires

Pt AND Fe2+ AND Fe3+ ALLOW half-cells drawn on either side

Pt AND H AND H+ AND delivery system for H gas ALLOW a formula of a strong acid for H+

For standard conditions:

Standard conditions

1 mol dm–3 Can be awarded if all quoted on standard condition line or in

AND labelled diagram.

Temperature: 298 K / 25 ºC

AND ALLOW 1M

Pressure: 1 atm / 100 kPa/101 kPa

ALLOW equimolar solutions for Fe2+ AND Fe3+ only.

i.e. need 1 mol dm–3 for [H+]

IGNORE H2SO4 in diagram unless concentration is stated with

a value other than 0.5 moldm-3

DO NOT ALLOW if any concentration is incorrect

(d) (i) Li + CoO2 → LiCoO2 1 ALLOW ⇌

DO NOT ALLOW uncancelled species

(ii) Cell potentials: 3 IGNORE state symbols throughout

(Eo) = 1.23 - 0.00 OR 1.23 (V)

OR (redox system 6 – redox system 3) = 1.23 (V)

AND

(Eo) = 0.40 – (–0.83) = 1.23 (V)

OR (redox system 4 – redox system 2) = 1.23 (V)

ALLOW multiples

ALLOW ⇌

Acidic Overall equation

Cell equation

+ – AND

(2 ) H2 ⇌ 2H + 2e

with evidence of working:

AND

O2 + 4H+ + 4e– ⇌ 2H2O e.g. half-equations written out / combined but not cancelled /

AND with crossings out OR System 6 goes forward / reduced OR

2 H2 + O2 → 2H2O system 3 goes backwards / oxidised

ALLOW multiples

Overall equation

Alkaline

Cell equation AND

(2 ) H2 + 2OH– ⇌ 2H2O + 2e– with evidence of working:

AND e.g. half-equations written

O + 2H O + 4e– ⇌ 4OH– out / combined but not cancelled / with crossings out OR

AND System 4 goes forward / reduced OR system 2 goes

2 H2 + O2 → 2 H2O backwards oxidised

ALLOW 1 mark for both equations with uncancelled species.

ALLOW 1 mark for System 6 / reduced goes forward and

system 3 goes backwards oxidised AND System 4 / goes

forward / reduced and system 2 / goes backwards / oxidised

How to answer it

Redox Reactions and Electrode Potentials Study Guide

What this question tests

This comprehensive OCR A-Level question tests your mastery across multiple core physical and inorganic chemistry topics: group 2 chemistry and oxide/hydroxide reactions, complex multi-step stoichiometry/titration calculations, drawing and defining standard electrode potential apparatus, combining half-equations for lithium-ion cells, and analysing hydrogen-oxygen fuel cell systems in both acidic and alkaline environments.

Part (a) — Barium Chemistry & Reaction Types

Reactions of Barium and its Compounds

✅ Correct Answers

  • Reaction 1: 2Ba + O₂ → 2BaO
  • Reaction 2: BaO + H₂O → Ba(OH)₂
  • Reaction 3: Neutralisation (or acid-base )

💡 Key Knowledge

Group 2 metals react with oxygen upon heating to form solid metal oxides. These basic oxides react vigorously with water to form alkaline metal hydroxides, which then undergo neutralisation reactions with hydrochloric acid to form metal chlorides and water.

🎯 Marks: 3 marks total (1 mark per valid equation/classification). State symbols are ignored, so balancing correctly is the primary focus.
Part (b) — Stoichiometry & Titration Calculations

Iodate Tablet Calculation

📐 Step-by-Step Calculation

  1. Find moles of KI used:
    n(I⁻) = (26.2 × 0.150) / 1000 = 3.93 × 10⁻³ mol
  2. Use stoichiometry from equation ( IO₃⁻ : 5I⁻ = 1 : 5):
    n(IO₃⁻) = (3.93 × 10⁻³ ) / 5 = 7.86 × 10⁻⁴ mol (total in 2 tablets)
  3. Scale down to find moles in ONE tablet:
    n in 1 tablet = (7.86 × 10⁻⁴) / 2 = 3.93 × 10⁻⁴ mol
  4. Calculate mass of KIO₃ in one tablet (Mr of KIO₃ = 214.0):
    Mass = 3.93 × 10⁻⁴ × 214 = 0.084102 g = 84 mg

❌ Common Calculation Traps

  • Forgetting to divide by 5: Failing to apply the mole ratio from the balanced equation.
  • Forgetting the "2 tablets" scale: Calculating the mass for both tablets and failing to divide by 2 at the end.
  • Incorrect Mr: Using the wrong molar mass for iodate (e.g., using I⁻ instead of KIO₃).
  • Rounding errors: Rounding intermediate steps too aggressively; always keep full calculator precision until the final step.
🎯 Marks: 4 marks. Final answer MUST be given to the nearest whole number ( 84 mg ) as requested by the stem. ECF applies throughout.
Part (c) — Electrochemical Cells & Standard Conditions

Measuring Standard Electrode Potentials

🧠 Exam Technique & Apparatus Setup

  • Fe³⁺/Fe²⁺ Half-Cell: Requires an inert platinum ( Pt ) electrode dipping into a solution containing both Fe²⁺(aq) and Fe³⁺(aq) ions.
  • Standard Hydrogen Electrode (SHE): Platinum electrode in contact with H₂(g) and H⁺(aq) ions via a gas delivery tube.
  • Salt Bridge: Filter paper soaked in an inert electrolyte (e.g., KNO₃ ) connecting the two solutions to complete the electrical circuit.
  • Meter: High-resistance voltmeter connected between the two metal/inert electrodes.

💡 Standard Conditions Checklist

To score the final mark, explicitly state all three mandatory standard conditions:

  • Temperature: 298 K (or 25 °C)
  • Pressure: 100 kPa (or 1 atm)
  • Concentration: 1.0 mol dm⁻³ for all aqueous ions
🎯 Marks: 4 marks. Examiners heavily penalise missing platinum electrodes, open circuits (salt bridge not touching solutions), or incorrect ion concentrations.
Part (d) — Cell Equations & Fuel Cells

Lithium-Ion Cells & Hydrogen-Oxygen Fuel Cells

✅ (i) Lithium-Ion Cell Overall Equation

Combining redox system 1 ( Li⁺ + e⁻ ⇌ Li reversed) and system 5 ( Li⁺ + CoO₂ + e⁻ ⇌ LiCoO₂ ):

Li + CoO₂ → LiCoO₂

✅ (ii) Hydrogen-Oxygen Fuel Cell Proof & Equations

Cell Potential Proof: Both acidic and alkaline calculations yield the identical standard cell potential: E°cell = 1.23 V .

  • Acidic Half-Equations & Overall:
    Oxidation: H₂ → 2H⁺ + 2e⁻
    Reduction: O₂ + 4H⁺ + 4e⁻ → 2H₂O
    Overall: 2H₂ + O₂ → 2H₂O
  • Alkaline Half-Equations & Overall:
    Oxidation: H₂ + 2OH⁻ → 2H₂O + 2e⁻
    Reduction: O₂ + 2H₂O + 4e⁻ → 4OH⁻
    Overall: 2H₂ + O₂ → 2H₂O
🎯 Marks: 1 mark for (d)(i) and 3 marks for (d)(ii). Ensure half-equations are properly scaled so electrons balance out completely in the overall equations.

Topics

Module 2: Foundations in chemistry · Module 3: Periodic table and energy · Module 5: Physical chemistry and transition elements · 2.1 Atoms and reactions · 3.1 The periodic table · 5.2 Energy

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.