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6.2.1Unit 6. Practical Skills in Chemistry II

Redox Titration – Iron(II) and Manganate(VII) — IAL Edexcel Linear (Chemistry) Revision Notes & Topical Questions

5 min read 6 parts Lesson 10 of 14 Unit quiz

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Part 1 of 6The Redox Reaction

6.2.1 Redox Titration - Iron(II) and Manganate(VII)

Manganate(VII) is a strong oxidising agent.

Half-equations
  • 1MnO₄⁻ + 8H⁺ + 5e⁻2Mn²⁺ + 4H₂O and Fe²⁺3Fe³⁺ + e⁻
Overall equation
  • MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O, so 1 mol of manganate(VII) reacts with 5 mol of iron(II).
Colour change
  • Purple manganate(VII) is reduced to almost colourless manganese(II), so the solution stays pale until the first excess appears.
Self-indicating
  • No separate indicator is needed because the first permanent pale pink colour marks the end point.

Part 2 of 6Practical Procedure

1
Pipette 25.0 cm³ of iron(II) solution into a conical flask and add an excess of dilute sulfuric acid.
2
Fill the burette with standard potassium manganate(VII) solution and read the initial volume at the top of the meniscus.
3
Add the manganate(VII) steadily while swirling, with the first titration as a rough run.
4
Add dropwise near the end point and stop at the first permanent pale pink colour.
5
Repeat until two or more titres agree within 0.10 cm³ and take the mean of the concordant results.

Part 3 of 6Why These Conditions

Why These Conditions
Dilute sulfuric acid
  • The reaction needs H⁺ ions, and sulfuric acid does not react with the manganate(VII) itself.
Not hydrochloric acid
  • Chloride ions would be oxidised to chlorine by manganate(VII), giving a titre that is too high.
Not nitric acid
  • Nitric acid is itself an oxidising agent and could oxidise the iron(II).
Meniscus reading
  • The manganate(VII) solution is so dark that the top of the meniscus is read instead of the bottom.

Part 4 of 6Reducing Errors

Iron(II) oxidation
  • Iron(II) is slowly oxidised by air, so solutions should be prepared fresh.
Insufficient acid
  • Too little acid gives brown manganese(IV) oxide instead of colourless Mn²⁺.
Rinsing
  • Rinse the burette with the manganate(VII) and the pipette with the iron(II) solution before use.
Concordant titres
  • Concordant titres show the technique is precise.

Part 5 of 6Worked Example

Question

  • 25.0 cm³ of acidified Fe²⁺(aq) needs 22.50 cm³ of 0.0200 mol dm⁻³ KMnO₄.
  • Find [Fe²⁺].

  1. Moles of MnO₄⁻ = 0.0200 × 0.02250 = 4.50 × 10⁻⁴ mol.
  2. Moles of Fe²⁺ = 5 × 4.50 × 10⁻⁴ = 2.25 × 10⁻³ mol.
  3. Concentration = 2.25 × 10⁻³ ÷ 0.0250 = 0.0900 mol dm⁻³.

Answer [Fe²⁺] = 0.0900 mol dm⁻³

Part 6 of 6Exam Checklist & Tips

6.2.1 – Key Points for the Exam

  • Oxidising agent A species that takes electrons from another species and is itself reduced.
  • Self-indicating Needing no extra indicator because a reactant gives the colour change.
  • Concordant titres Results that agree within 0.10 cm³.
  • End point The point where the reaction is just complete and the colour changes.

Common Mistakes

  • Using hydrochloric acid and obtaining a titre that is too large.
  • Using the 1 : 1 ratio instead of 1 MnO₄⁻ : 5 Fe²⁺.
  • Reading the bottom of the meniscus on a deeply coloured solution.
  • Adding the manganate(VII) too quickly near the end point and overshooting.

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