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Preparation and Oxidising Properties of KMnO4 and K2Cr2O7

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Student-friendly explanation

How to write this in exams

1
Start with the exact idea
Potassium permanganate and potassium dichromate are important transition-metal compounds used as strong oxidising agents, especially in acidic medium. Their reactions involve reduction of Mn(VII) or Cr(VI) to lower oxidation states.
2
Then show how to use it
Identify whether the reagent is KMnO₄ or K₂Cr₂O₇, assign Mn or Cr oxidation state, check the medium, write the reduction half-reaction, then pair it with the species being oxidised.
3
Add one concrete example
In acidic medium, KMnO₄ oxidises Fe₂⁺ to Fe₃⁺ while MnO₄^- is reduced to Mn₂⁺. Acidified K₂Cr₂O₇ also oxidises Fe₂⁺ to Fe₃⁺ while Cr₂O₇ 2- is reduced to Cr₃⁺.
4
Avoid this incomplete answer
A common wrong answer is writing MnO₂ as the product for acidified KMnO₄. MnO₂ is associated with neutral or weakly alkaline conditions, not strongly acidic medium.

Definition

Example

In acidic medium, KMnO₄ oxidises Fe₂⁺ to Fe₃⁺ while MnO₄^- is reduced to Mn₂⁺. Acidified K₂Cr₂O₇ also oxidises Fe₂⁺ to Fe₃⁺ while Cr₂O₇ 2- is reduced to Cr₃⁺.

Rule to remember

Key acidic-medium reductions: MnO₄^- + 8H⁺ + 5e- → Mn₂⁺ + 4H₂O; Cr₂O₇ 2- + 14H⁺ + 6e- → 2Cr₃⁺ + 7H₂O. In both, the reagent acts as an oxidising agent.

Sequence to remember

Key acidic-medium reductions: MnO4- + 8H+ + 5e->

Mn2+ + 4H2O; Cr2O7 2- + 14H+ + 6e->

2Cr3+ + 7H2O. In both, the reagent acts as an oxidising agent. A process flow showing ore or starting material to final salt can help in preparation, and a redox-change table helps in reaction questions. Identify whether the reagent is KMnO4 or K2Cr2O7, assign Mn or Cr oxidation state, check the medium, write the reduction half-reaction, then pair it with the species being oxidised

Memory hook

Permanganate purple to Mn₂⁺ in acid; dichromate orange to Cr₃⁺ green in acid.

Examples and method

Worked example

For acidified KMnO₄ reacting with Fe₂⁺, Mn changes from +7 in MnO₄^- to +2 in Mn₂⁺, gaining 5 electrons. Fe₂⁺ changes to Fe₃⁺, losing 1 electron. Therefore, one MnO₄^- ion oxidises five Fe₂⁺ ions in acidic medium.

Method to apply

Identify whether the reagent is KMnO₄ or K₂Cr₂O₇, assign Mn or Cr oxidation state, check the medium, write the reduction half-reaction, then pair it with the species being oxidised.

Diagram support

A process flow showing ore or starting material to final salt can help in preparation, and a redox-change table helps in reaction questions.

How CBSE asks it

Questions often ask for preparation steps, oxidising action in acidic medium, colour change, oxidation states, balanced ionic equations, or uses in redox titrations.

Avoid common mistakes

Common confusion

Students often ignore the medium. KMnO₄ gives different reduction products in acidic, neutral, and alkaline media, so the medium must be read before writing the product.

Common wrong answer

A common wrong answer is writing MnO₂ as the product for acidified KMnO₄. MnO₂ is associated with neutral or weakly alkaline conditions, not strongly acidic medium.

Exam tip

For redox questions, mark the oxidation state of Mn or Cr first, then identify the medium and write the corresponding reduction product before balancing or explaining the observation.

Quick check

What happens to dichromate ion in acidic medium during oxidation of Fe₂⁺?

Cr₂O₇ 2- is reduced from Cr(VI) to Cr₃⁺, while Fe₂⁺ is oxidised to Fe₃⁺. The solution changes from orange towards green due to Cr₃⁺.

Answer writing and exam use

1-mark answer

2-mark answer

Potassium permanganate and potassium dichromate are important transition-metal compounds used as strong oxidising agents, especially in acidic medium. Their reactions involve reduction of Mn(VII) or Cr(VI) to lower oxidation states. Key acidic-medium reductions: MnO₄^- + 8H⁺ + 5e- → Mn₂⁺ + 4H₂O; Cr₂O₇ 2- + 14H⁺ + 6e- → 2Cr₃⁺ + 7H₂O. In both, the reagent acts as an oxidising agent. In acidic medium, KMnO₄ oxidises Fe₂⁺ to Fe₃⁺ while MnO₄^- is reduced to Mn₂⁺. Acidified K₂Cr₂O₇ also oxidises Fe₂⁺ to Fe₃⁺ while Cr₂O₇ 2- is reduced to Cr₃⁺.

3-mark answer

KMnO₄ contains Mn in +7 oxidation state and is purple. In acidic medium, MnO₄^- is reduced to Mn₂⁺, so it oxidises species such as Fe₂⁺ to Fe₃⁺. K₂Cr₂O₇ contains Cr in +6 oxidation state and is orange; in acidic medium, dichromate is reduced to Cr₃⁺, which gives green solution. Their oxidising action is central to titration and redox questions. Key acidic-medium reductions: MnO₄^- + 8H⁺ + 5e- → Mn₂⁺ + 4H₂O; Cr₂O₇ 2- + 14H⁺ + 6e- → 2Cr₃⁺ + 7H₂O. In both, the reagent acts as an oxidising agent. For acidified KMnO₄ reacting with Fe₂⁺, Mn changes from +7 in MnO₄^- to +2 in Mn₂⁺, gaining 5 electrons. Fe₂⁺ changes to Fe₃⁺, losing 1 electron. Therefore, one MnO₄^- ion oxidises five Fe₂⁺ ions in acidic medium. Questions often ask for preparation steps, oxidising action in acidic medium, colour change, oxidation states, balanced ionic equations, or uses in redox titrations. A common wrong answer is writing MnO₂ as the product for acidified KMnO₄. MnO₂ is associated with neutral or weakly alkaline conditions, not strongly acidic medium.

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