medium difficulty8 min

Preparation and Oxidising Properties of KMnO4 and K2Cr2O7

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.

In one line: Permanganate purple to Mn²⁺ in acid; dichromate orange to Cr³⁺ green in acid.

Practice This Concept

Understand it

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.

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.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

For acidified KMnO₄ reacting with Fe²⁺, Mn changes from +7 in MnO₄^- to +2 in Mn²⁺, gaining 5 electrons.

1Fe²⁺ changes to Fe³⁺, losing 1 electron.
2Therefore, one MnO₄^- ion oxidises five Fe²⁺ ions in acidic medium.

Method you can reuse

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.

Visual tip

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

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

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

Think first, then show answer

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⁺.

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.

Don't mix this up

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.

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.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

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.

1
Enough for 1 mark
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
Add for 2 marks
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⁺.
3
Add for 3 marks
KMnO₄ contains Mn in +7 oxidation state and is purple.

MCQ Quiz

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