Chemistry - Complete d-block Question with Solution | TestHub

Explain Question :

Match the chemical transformations shown in the provided images (A, B, C, D) with the correct conditions ( $\mathrm{P},\mathrm{Q},\mathrm{R},\mathrm{S},\mathrm{T}$ ) required to carry out each reaction.

Concept :

This question is based on Redox chemistry of d-block elements, specifically the reactions of chromium and manganese compounds, and the conditions (acidic, basic medium, specific reagents) necessary for these transformations.

Solution:

(A) K₂Cr₂O₇ → Cr³⁺: This is the reduction of dichromate ion (Cr₂O₇²⁻) to chromium(III) ion (Cr³⁺). The oxidation state of Cr changes from +6 to +3. This reaction is a characteristic redox reaction of dichromate, which occurs in an acidic medium. Thus, it requires an acidic medium (Q). This is also a reduction, so a reducing agent is needed. SO₂ in an acidic medium is a common reducing agent for this conversion. The reaction is:

$$\mathrm{C}{\mathrm{r}}_2{\mathrm{O}}_7^{2-}+3\mathrm{S}{\mathrm{O}}_2+2{\mathrm{H}}^{+}\to 2\mathrm{C}{\mathrm{r}}^{3+}+3\mathrm{S}{\mathrm{O}}_4^{2-}+{\mathrm{H}}_2\mathrm{O}$$

Matches: (Q) Acidic medium is required, (S) SO₂/H⁺ is used for change.

(B) MnO₂ → MnO₄²⁻: This is the oxidation of manganese dioxide (MnO₂, Mn oxidation state +4) to the manganate ion (MnO₄²⁻, Mn oxidation state +6). This reaction requires a basic medium and an oxidizing agent (like oxygen from the air). This is a step in the industrial production of potassium permanganate.

Matches: (R) Basic medium is required. The change in oxidation state is from +4 to +6, which is a change of 2. This is not the maximum change.

(C) MnO₄⁻ → Mn²⁺: This is the reduction of permanganate ion (MnO₄⁻, Mn oxidation state +7) to manganese(II) ion (Mn²⁺, Mn oxidation state +2). This is a well-known redox reaction that occurs in an acidic medium. The change in oxidation state is from +7 to +2, which is a change of 5. This is the maximum change for manganese in this context.

Matches: (P) Maximum change in the oxidation state of metal, (Q) Acidic medium is required.

(D) MnO₄⁻ → MnO₂: This is the reduction of permanganate ion (MnO₄⁻, Mn oxidation state +7) to manganese dioxide (MnO₂, Mn oxidation state +4). This reaction occurs in a neutral or a weakly alkaline medium. Iodide ion (I⁻) is a good reducing agent that can be used for this conversion.

Matches: (T) I⁻/OH⁻ is used for change, (R) Basic medium is required.

Final answer:

(A) - (Q), (S)

(B) - (R)

(C) - (P), (Q)

(D) - (T), (R)