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D and F Block Elements question

2016 · Shift 2 · Q11
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D and F Block Elements question

2016 · Shift 2 · Q11

JEE AdvancedChemistryD and F Block ElementsMCQ+3 / −1
In the following reaction, sequence in aqueous solution, the species X, Y and Z, respectively, are S2O32−→Ag+XClear solution→Ag+YWhite precipitate→With timeZBlack precipitate{S_2}O_3^{2 - }\xrightarrow{A{g^ + }} \mathop X\limits_{Clear\,solution} \xrightarrow{A{g^ + }} \mathop Y\limits_{White\,precipitate} \xrightarrow{With\,time} \mathop Z\limits_{Black\,precipitate}S2​O32−​Ag+​ClearsolutionX​Ag+​WhiteprecipitateY​Withtime​BlackprecipitateZ​
  1. A
    [Ag(S2O3)2]3−[Ag(S_2O_3)_2]^{3-}[Ag(S2​O3​)2​]3−, Ag2S2O3Ag_2S_2O_3Ag2​S2​O3​, Ag2SAg_2SAg2​S
  2. B
    [Ag(S2O3)2]5−[Ag(S_2O_3)_2]^{5-}[Ag(S2​O3​)2​]5−, Ag2SO3Ag_2SO_3Ag2​SO3​, Ag2SAg_2SAg2​S
  3. C
    [Ag(SO3)2]3−[Ag(SO_3)_2]^{3-}[Ag(SO3​)2​]3−, Ag2S2O3Ag_2S_2O_3Ag2​S2​O3​, AgAgAg
  4. D
    [Ag(SO3)3]3−[Ag(SO_3)_3]^{3-}[Ag(SO3​)3​]3−, Ag2SO4Ag_2SO_4Ag2​SO4​, AgAgAg
View written solutionFree

Correct answer: A

  1. Reaction of thiosulfate with silver ion

Thiosulfate ion, S2O32−S_2O_3^{2-}S2​O32−​, first forms a soluble complex with Ag+Ag^+Ag+ in aqueous solution.

The well-known complex is:

[Ag(S2O3)2]3−[Ag(S_2O_3)_2]^{3-}[Ag(S2​O3​)2​]3−

So,

X=[Ag(S2O3)2]3−X = [Ag(S_2O_3)_2]^{3-}X=[Ag(S2​O3​)2​]3−

This explains the clear solution.

  1. On adding more Ag+Ag^+Ag+

With further addition of Ag+Ag^+Ag+, the thiosulfate complex breaks and a white precipitate of silver thiosulfate forms:

Ag2S2O3Ag_2S_2O_3Ag2​S2​O3​

Thus,

Y=Ag2S2O3Y = Ag_2S_2O_3Y=Ag2​S2​O3​
  1. Transformation with time

Silver thiosulfate is unstable and decomposes with time to give black silver sulfide:

Ag2S2O3⟶Ag2S+other productsAg_2S_2O_3 \longrightarrow Ag_2S + \text{other products}Ag2​S2​O3​⟶Ag2​S+other products

Hence,

Z=Ag2SZ = Ag_2SZ=Ag2​S

Since Ag2SAg_2SAg2​S is black, this matches the observation.

  1. Check options
  • A: [Ag(S2O3)2]3−[Ag(S_2O_3)_2]^{3-}[Ag(S2​O3​)2​]3−, Ag2S2O3Ag_2S_2O_3Ag2​S2​O3​, Ag2SAg_2SAg2​S ✅
  • B: Wrong complex charge and wrong intermediate.
  • C: Uses sulfite complex instead of thiosulfate; final product incorrect.
  • D: Entirely inconsistent with thiosulfate chemistry.
  1. Final answer

Therefore,

X=[Ag(S2O3)2]3−,Y=Ag2S2O3,Z=Ag2SX = [Ag(S_2O_3)_2]^{3-}, \quad Y = Ag_2S_2O_3, \quad Z = Ag_2SX=[Ag(S2​O3​)2​]3−,Y=Ag2​S2​O3​,Z=Ag2​S

So the correct option is A.

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