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
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 6The Redox Reaction
Manganate(VII) is a strong oxidising agent.
Half-equations
- Half-equations MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O and Fe²⁺ → Fe³⁺ + e⁻.
Overall equation
- Overall equation MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 5Fe³⁺ + 4H₂O, so 1 mol of manganate(VII) reacts with 5 mol of iron(II).
Colour change
- Colour change Purple manganate(VII) is reduced to almost colourless manganese(II), so the solution stays pale until the first excess appears.
Self-indicating
- Self-indicating No separate indicator is needed because the first permanent pale pink colour marks the end point.
Section 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.
Section 3 of 6Why These Conditions
Why These Conditions
Dilute sulfuric acid
- Dilute sulfuric acid The reaction needs H⁺ ions, and sulfuric acid does not react with the manganate(VII) itself.
Not hydrochloric acid
- Not hydrochloric acid Chloride ions would be oxidised to chlorine by manganate(VII), giving a titre that is too high.
Not nitric acid
- Not nitric acid Nitric acid is itself an oxidising agent and could oxidise the iron(II).
Meniscus reading
- Meniscus reading The manganate(VII) solution is so dark that the top of the meniscus is read instead of the bottom.
Section 4 of 6Reducing Errors
Iron(II) oxidation
- Iron(II) oxidation Iron(II) is slowly oxidised by air, so solutions should be prepared fresh.
Insufficient acid
- Insufficient acid Too little acid gives brown manganese(IV) oxide instead of colourless Mn²⁺.
Rinsing
- Rinsing Rinse the burette with the manganate(VII) and the pipette with the iron(II) solution before use.
Concordant titres
- Concordant titres Concordant titres show the technique is precise.
Section 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⁺].
- Moles of MnO₄⁻ = 0.0200 × 0.02250 = 4.50 × 10⁻⁴ mol.
- Moles of Fe²⁺ = 5 × 4.50 × 10⁻⁴ = 2.25 × 10⁻³ mol.
- Concentration = 2.25 × 10⁻³ ÷ 0.0250 = 0.0900 mol dm⁻³.
Answer [Fe²⁺] = 0.0900 mol dm⁻³
Section 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.
Last updated:
\n