Identification of Species in Thiosulfate and Silver Ion Reaction

To determine the identities of the species $\mathbf{X}$, $\mathbf{Y}$, and $\mathbf{Z}$ in the given reaction sequence involving thiosulfate ($\text{S}_2\text{O}_3^{2-}$) and silver ions ($\text{Ag}^+$), we analyze each step sequentially:

1. Formation of Species $\mathbf{X}$ (Clear Solution): When silver ions ($\text{Ag}^+$) are initially added to an excess of sodium thiosulfate solution, a soluble, stable bis(thiosulfato)argentate(I) complex is formed: $\text{Ag}^+ (\text{aq}) + 2\text{S}_2\text{O}_3^{2-} (\text{aq}) \rightarrow [\text{Ag}(\text{S}_2\text{O}_3)_2]^{3-} (\text{aq})$ Hence, the clear solution $\mathbf{X}$ is the complex ion $[\text{Ag}(\text{S}_2\text{O}_3)_2]^{3-}$.

2. Formation of Species $\mathbf{Y}$ (White Precipitate): When additional $\text{Ag}^+$ ions are added to the solution containing $[\text{Ag}(\text{S}_2\text{O}_3)_2]^{3-}$, a white precipitate of silver thiosulfate is formed: $[\text{Ag}(\text{S}_2\text{O}_3)_2]^{3-} (\text{aq}) + 3\text{Ag}^+ (\text{aq}) \rightarrow 2\text{Ag}_2\text{S}_2\text{O}_3 (\text{s}) \downarrow$ Hence, the white precipitate $\mathbf{Y}$ is $\text{Ag}_2\text{S}_2\text{O}_3$.

3. Formation of Species $\mathbf{Z}$ (Black Precipitate): Silver thiosulfate is unstable in aqueous solution and decomposes over time via hydrolysis/disproportionation to form black silver sulfide ($\text{Ag}_2\text{S}$) and sulfuric acid: $\text{Ag}_2\text{S}_2\text{O}_3 (\text{s}) + \text{H}_2\text{O} (\text{l}) \xrightarrow{\text{with time}} \text{Ag}_2\text{S} (\text{s}) \downarrow + \text{H}_2\text{SO}_4 (\text{aq})$ Hence, the black precipitate $\mathbf{Z}$ is $\text{Ag}_2\text{S}$.

Thus, the species $\mathbf{X}$, $\mathbf{Y}$, and $\mathbf{Z}$ are $[\text{Ag}(\text{S}_2\text{O}_3)_2]^{3-}$, $\text{Ag}_2\text{S}_2\text{O}_3$, and $\text{Ag}_2\text{S}$, respectively.

Correct Answer: A