Balance and calculate permanganate redox reactions
Balance acidified permanganate reactions with iron(II) or ethanedioate and use the stoichiometric ratios in calculations.
A Level; Cambridge International Chemistry 9701, 2028–2030 syllabus, Topic 28. Use the first-row transition elements titanium to copper and the species explicitly named in the syllabus. Where redox potentials or stability constants are needed, use the values supplied in the question. Colour descriptions can depend on conditions and concentration. Written practice is self-checked
Before you begin
- Use electron configurations, oxidation numbers, ionic equations, equilibrium expressions and the AS redox and bonding models.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
In acidified permanganate/Fe²⁺, what is the mole ratio MnO₄⁻:Fe²⁺?
What you will learn
- Balance MnO₄⁻/Fe²⁺ in acid
- Balance MnO₄⁻/C₂O₄²⁻ in acid
- Use mole ratios and supplied concentrations or volumes in calculations
Permanganate oxidises iron(II) in acid
In acidic solution, MnO₄⁻ is reduced to Mn²⁺. Fe²⁺ is oxidised to Fe³⁺. Combining the half-equations gives MnO₄⁻ + 8H⁺ + 5Fe²⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺.
The mole ratio is 1 mol MnO₄⁻ to 5 mol Fe²⁺. Use the balanced equation only for the specified acidic conditions; changing the medium may change the manganese product.
Permanganate oxidises ethanedioate to carbon dioxide
In acid, C₂O₄²⁻ is oxidised to CO₂ and MnO₄⁻ is reduced to Mn²⁺: 2MnO₄⁻ + 16H⁺ + 5C₂O₄²⁻ → 2Mn²⁺ + 8H₂O + 10CO₂.
The mole ratio is 2 mol MnO₄⁻ to 5 mol C₂O₄²⁻. In a calculation, convert each measured concentration and volume to moles, use the ratio, then calculate the requested amount or concentration. These are written-equation exercises, not instructions to carry out a reaction.
Pause and explain
What is the carbon product when ethanedioate is oxidised by acidified permanganate?
Worked example
25.0 cm³ of 0.0200 mol dm⁻³ Fe²⁺ reacts exactly with acidified MnO₄⁻. Calculate the volume of 0.0100 mol dm⁻³ MnO₄⁻ required.
Show the worked solution
- Find n(Fe²⁺) = 0.0200 × 0.0250.
- Use 1 MnO₄⁻ : 5 Fe²⁺.
- Divide permanganate moles by its concentration.
Answer n(Fe²⁺) = 5.00 × 10⁻⁴ mol, so n(MnO₄⁻) = 1.00 × 10⁻⁴ mol. V = n/c = 0.0100 dm³ = 10.0 cm³.
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Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Calculate moles of Fe²⁺ in 20.0 cm³ of 0.150 mol dm⁻³ solution.
Give me a hint
Convert the volume to dm³ before multiplying by concentration.
Compare my reasoning
- 20.0 cm³ = 0.0200 dm³.
- n = cV.
- Keep three significant figures.
n(Fe²⁺) = 0.150 × 0.0200 = 3.00 × 10⁻³ mol.
Look for these in your work
- Volume converted
- Concentration relation used
- Precision appropriate
Use the balanced Fe²⁺ equation to find moles of MnO₄⁻ needed for 3.00 × 10⁻³ mol Fe²⁺.
Give me a hint
Apply the 1:5 mole ratio.
Compare my reasoning
- Divide Fe²⁺ moles by five.
- State the permanganate amount.
- Check the result is smaller than the Fe²⁺ amount.
n(MnO₄⁻) = 6.00 × 10⁻⁴ mol.
Look for these in your work
- Ratio used in correct direction
- Moles calculated
- Magnitude check
For the ethanedioate equation, 10.0 cm³ of 0.0250 mol dm⁻³ C₂O₄²⁻ reacts with excess acidified MnO₄⁻. Find the required moles of MnO₄⁻.
Give me a hint
Find oxalate moles, then use 2:5 MnO₄⁻:C₂O₄²⁻.
Compare my reasoning
- Convert 10.0 cm³ to 0.0100 dm³.
- Find n(C₂O₄²⁻) = 2.50 × 10⁻⁴ mol.
- Multiply by 2/5.
n(MnO₄⁻) = 1.00 × 10⁻⁴ mol.
Look for these in your work
- Volume conversion correct
- Oxalate moles correct
- Stoichiometric ratio applied
Common mistakes
- Forgetting the eight H⁺ in the iron(II) reaction
- Using a 1:1 ratio for MnO₄⁻ and Fe²⁺
- Confusing the oxalate and iron(II) mole ratios
- Using cm³ as dm³ without converting
What can you explain now?
In acidified permanganate/Fe²⁺, what is the mole ratio MnO₄⁻:Fe²⁺?
Compare with the explanation
1:5
One permanganate ion accepts five electrons supplied by five Fe²⁺ ions.
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Make it stick
What are the products and mole ratios for acidic permanganate with Fe²⁺ and ethanedioate?
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