Chemistry · D And F Block Elements · NEET
It is a redox reaction where ONE element, sitting in ONE middle oxidation state, is at the same time oxidised (goes up) and reduced (goes down). So a single species turns into two products: one with a higher oxidation state and one with a lower oxidation state. The element must be able to exist in at least three oxidation states for this to happen. Example: 2Cu+ -> Cu2+ + Cu. Here Cu(+1) becomes Cu(+2) and Cu(0) at once.
In water, Cu+ (the +1 state) is unstable. The reaction 2Cu+(aq) -> Cu2+(aq) + Cu(s) has a favourable (positive) E value, so Cu+ splits into Cu2+ and Cu metal. The large hydration energy of the small, highly charged Cu2+ ion releases a lot of energy, which more than pays for the second ionisation. So in aqueous solution Cu2+ is the stable form, not Cu+.
In manganate MnO4^2-, manganese is in the +6 state (a d1 middle state). In neutral or acidic solution the +6 state is unstable compared with +7 and +4. So it disproportionates: 3MnO4^2- + 4H+ -> 2MnO4^- + MnO2 + 2H2O. Here Mn(+6) goes up to Mn(+7) in MnO4^- and down to Mn(+4) in MnO2. This is exactly why the green manganate turns into purple permanganate in acid.
NO. It looks like one because Mn(+7) becomes Mn(+6) and Mn(+4). But look at oxygen too: O(-2) is oxidised to O2 (0). Two different elements change oxidation state here, so it is a thermal decomposition, NOT a disproportionation. Disproportionation needs the SAME single element to be both oxidised and reduced. NEET 2019 used this exact trap.
They are opposites. Disproportionation: ONE middle state splits into a higher and a lower state (one to two). Comproportionation: a higher state and a lower state combine into one middle state (two to one). Example of comproportionation: 2MnO4^- + 3Mn2+ + 2H2O -> 5MnO2 + 4H+ (Mn+7 and Mn+2 meet to give Mn+4). This is NOT disproportionation - a common NEET trap option.
No. To disproportionate, the element must go both UP and DOWN. If it is already in its highest oxidation state it cannot go up. That is why MnO4^- (Mn +7, highest) does not disproportionate, and ClO4^- (Cl +7) does not either. Only a MIDDLE oxidation state that has room above and below can disproportionate.
Which of the following reactions are disproportionation reactions? (a) 2Cu+ -> Cu2+ + Cu (b) 3MnO4^2- + 4H+ -> 2MnO4^- + MnO2 + 2H2O (c) 2KMnO4 -> K2MnO4 + MnO2 + O2 (d) 2MnO4^- + 3Mn2+ + 2H2O -> 5MnO2 + 4H+
Which one of the following ions exhibits d-d transition and paramagnetism as well?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
1) 2Cu+(aq) -> Cu2+(aq) + Cu(s): copper(+1) splits into copper(+2) and copper metal. 2) 3MnO4^2- + 4H+ -> 2MnO4^- + MnO2 + 2H2O: manganese(+6) splits into +7 and +4. Both are NCERT examples used in NEET.
Find the element that repeats in the products. Check its oxidation state in the single reactant, then in each product. If ONE element goes both UP and DOWN (and no other element changes), it is disproportionation. If a second element also changes, it is decomposition. If two states combine into one, it is comproportionation.
Transition metals show many oxidation states, so unstable middle states like Cu+ (+1), Mn+6 (manganate) and Mn+3 disproportionate. NEET repeatedly asks you to identify these reactions and to separate them from decomposition and comproportionation traps, so it is a high-yield, direct-marks topic.
Yes. Mn3+ (+3) is unstable in solution: 2Mn3+ + 2H2O -> Mn2+ + MnO2 + 4H+. Mn(+3) goes down to Mn(+2) and up to Mn(+4). It is a middle state with room above and below, so it disproportionates.
