Question
Write the reactions of KMnO₄ as an oxidising agent in (i) acidic medium, (ii) neutral medium, and (iii) basic medium.
Solution — Step by Step
The key is what product manganese forms — that depends entirely on the medium. The oxidation state of Mn changes differently in each case, and the number of electrons transferred changes too.
| Medium | Mn oxidation state | Product | Colour change |
|---|---|---|---|
| Acidic | +7 → +2 | Mn²⁺ (MnSO₄) | Purple → colourless |
| Neutral | +7 → +4 | MnO₂ | Purple → brown ppt |
| Basic | +7 → +6 | MnO₄²⁻ | Purple → green |
This is the memory anchor for the entire question.
In the presence of dilute , is the strongest oxidising agent of the three cases, gaining 5 electrons per Mn atom.
Half-reaction:
Reaction with FeSO₄ (NCERT example):
Reaction with oxalic acid (PYQ favourite):
The solution turns from purple to colourless — the visual confirmation that Mn²⁺ has formed.
With no acid or alkali, only gains 3 electrons. The product is , which precipitates as a brown solid.
Half-reaction:
Reaction with Na₂SO₃:
You can also write the simpler NCERT form showing nascent oxygen release:
In the presence of alkali, is the weakest oxidising agent — it only gains 1 electron, converting to green manganate ().
Half-reaction:
Reaction with Na₂SO₃:
The solution turns from purple to green — the signature of .
Why This Works
The medium controls the availability of ions, which directly affects how many electrons can accept. More available → more electrons transferred → stronger oxidising power.
In acidic medium, the 8 ions stabilise the product by pulling the equilibrium hard to the right. In neutral medium, there’s only enough proton availability to reach (+4 state). In basic medium, the OH⁻ actually competes with the reduction, so only a 1-electron change to (+6 state) occurs.
This is why the titration with oxalic acid in Mohr’s method requires acidification — without , the reaction stops at and the endpoint is obscured by the brown precipitate.
Alternative Method — Electron Counting Shortcut
Instead of balancing by inspection, use the electron transfer method to verify your equation quickly.
For acidic medium with FeSO₄:
- Mn gains 5 electrons (per formula unit):
- Fe loses 1 electron:
- To balance: 2 KMnO₄ (10e⁻ gained) needs 10 FeSO₄ (10e⁻ lost) ✓
For board exams, always write the colour change alongside each reaction. Examiners specifically award marks for: acidic = colourless, neutral = brown precipitate, basic = green. A correct equation with no colour mention often loses half a mark.
Common Mistake
Students write (manganate) as the product in neutral medium instead of . Remember: manganate is the basic medium product. In neutral medium, Mn goes to the +4 state (), not the +6 state. The oxidation states in order are: acidic → +2, neutral → +4, basic → +6. They increase as the medium becomes less acidic. That’s your anchor.
This question carries 3 marks in CBSE Class 12 boards and appears almost every alternate year. Write all three reactions with the correct balancing and colour changes — don’t skip the neutral medium one, which students often forget under exam pressure.