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JEE Advanced levelOne or more correct optionsDifficulty 3 of 3df081

In aqueous solution, which of the following species are unstable towards disproportionation under the conditions stated (acidic or neutral medium as appropriate)?

A $Cu X^{+}$
B $MnO X_{4} X^{2 -}$ in acidic medium
C $Mn X^{3 +}$
D $Fe X^{3 +}$

Answer: A, B, C

Cu X^{+} \to Cu X^{2 +} + Cu

(

E_{cell}^{\circ} = + 0.36 V

);

3 MnO X_{4} X^{2 -} + 4 H X^{+} \to 2 MnO X_{4} X^{-} + MnO X_{2} + 2 H X_{2} O

;

2 Mn X^{3 +} + 2 H X_{2} O \to Mn X^{2 +} + MnO X_{2} + 4 H X^{+}

.

Fe X^{3 +}

is stable because there is no intermediate state to which it can both reduce and oxidise spontaneously (

Fe X^{4 +}

is not accessible in water).

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