PYQ + Trends
1. TITLE
NEET 2019 Q9: Disproportionation: Who Can and Who Can't? — Full solution + Why they asked this
2. THE ACTUAL NEET QUESTION
NEET 2019 — Question
Which of the following reactions are disproportionation reactions?
(i) $2Cu^+ \rightarrow Cu^{2+} + Cu^0$
(ii) $3MnO_4^{2-} + 4H^+ \rightarrow 2MnO_4^- + MnO_2 + 2H_2O$
(iii) $2KMnO_4 \rightarrow K_2MnO_4 + MnO_2 + O_2$
(iv) $2MnO_4^- + 3Mn^{2+} + 2H_2O \rightarrow 5MnO_2 + 4H^+$
Select the correct option from the following:
A) (i) and (iv) only
B) (i) and (ii) only
C) (i), (ii) and (iii)
D) (i), (iii) and (iv)
3. OFFICIAL ANSWER
NEET Answer Key: (B)
4. COMPLETE SOLUTION
Concept Explanation:
Disproportionation reactions are a special type of redox reaction where a single element in one specific oxidation state is simultaneously oxidised (oxidation number increases) and reduced (oxidation number decreases).
For a species to be capable of undergoing disproportionation, the element must be present in an intermediate oxidation state. If it is at its highest or lowest possible oxidation state, it simply lacks the "room" to either lose or gain electrons, respectively.
Analysing the options:
* (i) $2Cu^+ \rightarrow Cu^{2+} + Cu^0$: Copper starts in the +1 state and simultaneously oxidises to +2 ($Cu^{2+}$) and reduces to 0 ($Cu^0$). (Correct)
* (ii) $3MnO_4^{2-} \rightarrow 2MnO_4^- + MnO_2$: Manganese starts in an intermediate +6 state. It oxidises to +7 in $MnO_4^-$ and reduces to +4 in $MnO_2$. (Correct)
* (iii) $2KMnO_4 \rightarrow K_2MnO_4 + MnO_2 + O_2$: Here, Mn is already in its highest possible oxidation state of +7. It can only be reduced (to +6 and +4), so this is a standard thermal decomposition, not disproportionation. (Incorrect)
* (iv) $2MnO_4^- + 3Mn^{2+} \rightarrow 5MnO_2$: Manganese in the +7 and +2 states reacts to form an intermediate +4 state. This is two different oxidation states combining into one, a process known as comproportionation (the exact reverse of disproportionation). (Incorrect)
Therefore, only reactions (i) and (ii) are disproportionation reactions.
5. WHY NEET ASKED THIS
Explain the examiner's intent:
* What concept were they testing? They were testing whether students conceptually understand that disproportionation requires an intermediate oxidation state, rather than just blindly looking for complex redox equations.
* What's the common mistake they expected students to make? Examiners expected students to confuse thermal decomposition or comproportionation (reaction iv) with disproportionation.
* Which option was the "trap"? Statement (iii) featuring $KMnO_4$. It is a classic trap because students often memorise $KMnO_4$ reactions without checking the oxidation state. Since Mn is at its maximum (+7) state, it is mathematically impossible for it to be oxidised further!
6. TREND ANALYSIS
PATTERN ALERT:
| Year | How they asked | Twist |
|---|---|---|
| 2024 (Re) | $3MnO_4^{2-} \rightarrow 2MnO_4^- + MnO_2$ | Instead of asking to identify the reaction type, they asked to identify which oxidation states were not shown by Mn in this specific disproportionation reaction (+2 and +3). |
| 2019 | Identify disproportionation from four given reactions | Mixed disproportionation with its exact opposite (comproportionation) and thermal decomposition. |
| 2018 | Identify the species undergoing disproportionation using an EMF diagram | Required students to calculate $E^\circ_{cell}$ (SRP cathode - SRP anode) to prove that the disproportionation of $HBrO$ to $Br_2$ and $BrO_3^-$ is spontaneous. |
| 2012 | $Cl_2$ gas reacting with hot and conc. NaOH | Tested the classic halogen disproportionation, asking for the exact oxidation number changes (0 to -1 and +5). |
Prediction for NEET 2026:
Expect a variation directly testing the $ClO_4^-$ vs $ClO^-$ concept. The examiner will likely present oxoanions of chlorine ($ClO^-$, $ClO_2^-$, $ClO_3^-$, $ClO_4^-$) and ask which one cannot disproportionate. The answer will be $ClO_4^-$ because chlorine is already in its maximum +7 oxidation state and cannot be further oxidised.
7. SIMILAR QUESTIONS TO PRACTICE
"If you understood this, you can solve:"
* NEET 2024 Re Q2: The oxidation states not shown by Mn in the disproportionation of $MnO_4^{2-}$.
* NEET 2018 Q11: Disproportionation of $HBrO$ calculated via EMF/Latimer diagram values.
* NEET 2012 Q19: Oxidation number changes when $Cl_2$ undergoes disproportionation in hot, concentrated NaOH.
8. QUICK REVISION FOR THIS TYPE
For Disproportionation questions, always:
- First check the oxidation state of the key element in the reactant.
- Then apply the "Max/Min Rule": If the element is already in its absolute highest oxidation state (e.g., +7 for Mn in $KMnO_4$ or +7 for Cl in $ClO_4^-$), it cannot disproportionate. It must be in an intermediate state.
- Watch out for comproportionation traps! If two different reactant oxidation states merge into a single product oxidation state, it is the reverse of disproportionation.
9. NCERT BASE
This question tests: NCERT Class 11, Part 2, Chapter 7 (Redox Reactions), Page 244-245 (Section 7.3.1 - Types of Redox Reactions, Disproportionation Reactions, and Problem 7.5).
10. CTA
All PYQs from this chapter → Redox Reactions & Electrochemistry Guide
Practice similar questions → Mock exams