Chemistry - Salt Analysis Question with Solution | TestHub

Question Explanation :

Identify species A and D in the sequence.

Concept:

Redox and precipitation of sulfur.

Solution :

The reaction sequence involves the reduction and subsequent reactions of sulfur oxyanions:

Step 1: (A → B): When an aqueous solution of sulfite (SO₃²⁻, A) reacts with Zn and NaOH, it is reduced to sulfide (S²⁻, B).

$$S{O}_3^{2-}+3Zn+2{OH}^{-}+3H_{2}O\to S^{2-}+3{Zn(OH{)}_2}^{2-}$$

Step 2: (B → C): The sulfide ion (B) reacts with acidified dichromate $C{r}_2{O}_7^{2-}$/$H^{+}$ . Dichromate is a strong oxidizing agent that oxidizes S²⁻ to elemental sulfur (S, C), which appears as white turbidity.

$$3S^{2-}+C{r}_2{O}_7^{2-}+14H^{+}\to 3S\downarrow (white)+2C{r}^{3+}+7H_{2}O$$

Step 3: : When the original sulfite (A) is boiled with the sulfur (C) produced, they combine to form thiosulfate (S₂O₃²⁻, D).

$$S{O}_3^{2-}\left(aq\right)+S\left(s\right)\to S_2{O}_3^{2-}\left(aq\right)$$