[23.2] manganate (VII) redox titrations

Question 1

what are the half equations for the reaction of manganate (VII) ions with iron (II) ions?

Answer 1

• reduction: MnO₄⁻ (aq) + 8H⁺ (aq) + 5e⁻ -> Mn²⁺ (aq) + 4H₂O (l)
• oxidation: Fe²⁺ (aq) -> Fe³⁺ (aq) + e⁻

Question 2

what is the overall equation for the oxidation of Fe²⁺ ions by manganate (VII)?

Answer 2

MnO₄⁻ (aq) + 8H⁺ (aq) + 5Fe²⁺ (aq) -> Mn²⁺ (aq) + Fe³⁺ (aq) + 4H₂O (l)

Question 3

what is the colour change for manganate (VII) redox titrations?

Answer 3

• MnO₄⁻ (aq) = purple
• Mn²⁺ (aq) = almost colourless
• end point is first permanent pink colour

Question 4

how are redox titrations different from acid-base titrations?

Answer 4

• redox titrations involve transfer of electrons, rather than neutralisation as in acid-base titrations
• redox titrations are self-indicating

Question 5

what are the problems using potassium manganate (VII) solution? (4)

Answer 5

1. can’t be used in titrations with chloride or bromide ions as it oxidises these. this is why the solution is acidified with sulfuric acid, not HCl
2. isn’t a primary standard ie. can’t be made up to give a stable solution of accurately known concentration. it is too strongly coloured to see when all of the crystals you have used have dissolved
3. over a period of time, it oxidises the water it is dissolved in to oxygen, leaving a brown suspension of manganese (IV) oxide, so the concentration is not guaranteed
4. difficult to read meniscus as the colour is so dark

Question 6

how can you analyse iron if it is in the element Fe (0)?

Answer 6

react it with sulfuric acid to oxidise it to Fe²⁺ ready for analysis

Question 7

how can you analyse iron if it is Fe (+3)?

Answer 7

• react with Zn to reduce it to Fe²⁺ ready for analysis
• remaining zinc must be removed first to stop it reducing Fe³⁺ formed in the titration back to Fe²⁺

Question 8

what are the half equations for the reaction of manganate (VII) ions with ethanedioic acid, (COOH)₂ (aq)?

Answer 8

• reduction: MnO₄⁻ (aq) + 8H⁺ (aq) + 5e⁻ -> Mn²⁺ (aq) + 4H₂O (l)
• oxidation: (COOH)₂ (aq) -> 2CO₂ (g) + 2H⁺ (aq) + 2e⁻

Question 9

what is the overall equation for the reaction of ethanedioic acid with acidified manganate (VII) ions?

Answer 9

2MnO₄⁻ (aq) + 6H⁺ (aq) + 5(COOH)₂ (aq) -> 2Mn²⁺ (aq) + 10CO₂ (g)+ 8H₂O (l)

Question 10

what is the overall equation for the reaction of ethandioate ions (C₂O₄²⁻) with acidified potassium manganate (VII)?

Answer 10

5C₂O₄²⁻ + 2MnO₄⁻ + 16H⁺ -> 10CO₂ + 2Mn²⁺ + 8H₂O