Extra stability to half filled electronic configuration is observed than incompletely filled electronic configuration.

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Solution

Q.
Given below are two statements: one is labelled as Assertion (A) and the other is labelled as Reason (R)

Assertion (A): In aqueous solutions ${Cr}^{2 +}$ is reducing while ${Mn}^{3 +}$ is oxidising in nature.

Reason (R): Extra stability to half filled electronic configuration is observed than incompletely filled electronic configuration.

In the light of the above statements, choose the most appropriate answer from the options given below: [2024]

1 (A) is true but (R) is false

2 Both (A) and (R) are true and (R) is the correct explanation of (A)

3 Both (A) and (R) are true but (R) is not the correct explanation of (A)

4 (A) is false but (R) is true

Ans.
(2)

Assertion:

Metal Ti V Cr Mn Fe Co

$M^{3 +} / M^{2 +}$ potential in V $- 0.37 V$ $- 0.26$ $- 0.41$ $+ 1.57$ $+ 0.77$ $+ 1.97$

As ${Cr}^{3 +} / {Cr}^{2 +}$ potential is negative, ${Cr}^{2 +} / {Cr}^{3 +}$ potential is positive, i.e. ${Cr}^{2 +}$ has a tendency to get oxidized to ${Cr}^{3 +}$ . Thus aqueous solution of ${Cr}^{2 +}$ is reducing.

As ${Mn}^{3 +} / {Mn}^{2 +}$ potential is positive, ${Mn}^{3 +}$ has a tendency to get reduced to ${Mn}^{2 +}$ . Thus aqueous solution of ${Mn}^{3 +}$ is oxidizing.

Reason:

${Mn}^{2 +}$ $({}_{18}{[Ar]} 3 d^{5})$ has stable half filled subshell. Thus ${Mn}^{2 +}$ has a tendency to be formed. This makes ${Mn}^{3 +} / {Mn}^{2 +}$ positive. ${Cr}^{3 +}$ has half filled $t_{2 g}$ orbital. Thus ${Cr}^{3 +}$ has a tendency to be formed. This makes ${Cr}^{2 +} / {Cr}^{3 +}$ potential positive.

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