Transition and InnerTransition Elements - Result Question 44
44. Which of the following halides react(s) with $\mathrm{AgNO}_3(a q)$ to give a precipitate that dissolves in $\mathrm{Na}_2 \mathrm{~S}_2 \mathrm{O}_3(a q)$ ?
(a) HCl
(b) HF
(c) HBr
(d) HI
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Answer:
Correct Answer: 44. (a, c, d)
Solution: Solubilities of silver halides in water decreases from fluoride $(\mathrm{AgF})$ to iodide $(\mathrm{AgI})$. Silver fluoride is readialy soluble in water, hence when $\mathrm{AgNO}_3$ solution is added to HF solution (HF being weak acid, its solution maintain very low concentration of $\mathrm{F}^{-}$) no precipitate of AgF is formed.
$\mathrm{HCl}, \mathrm{HBr}$ and HI being all strong acid, forms precipitates of $\mathrm{AgCl}, \mathrm{AgBr}$ and AgI when $\mathrm{AgNO}_3$ solution is added to their aqueous solution.
$$ \begin{aligned} & \mathrm{HCl}(a q)+\mathrm{AgNO}_3(a q) \longrightarrow \mathrm{AgCl}(\mathrm{s})+\mathrm{HNO}_3(a q) \\ & \hspace {50mm} \text { Curdy white } \\ & \mathrm{HBr}(a q)+\mathrm{AgNO}_3(a q) \longrightarrow \underset{\text { Palc yellow }}{\mathrm{AgBr}(s)}+\mathrm{HNO}_3(a q) \\ & \mathrm{Hl}(a q)+\mathrm{AgNO}_3(a q) \longrightarrow \mathrm{AgI}(s)+\mathrm{HNO}_3(a q) \\ & \hspace {50mm} \text { Yellow } \\ & \end{aligned} $$
The solubilities decreases from AgCl to $\mathrm{AgI}, \mathrm{AgCl}$ dissolves in aqueous ammonia, AgBr dissolves only slightly in concentrated ammonia while AgI does not dissolve in ammonia solution.
$\mathrm{Na}_2 \mathrm{~S}_2 \mathrm{O}_3$ solution dissolve all three, $\mathrm{AgCl}, \mathrm{AgBr}, \mathrm{AgI}$ by forming complex $\left[\mathrm{Ag}\left(\mathrm{S}_2 \mathrm{O}_3\right)_2\right]^{3-}$ as $\mathrm{S}_2 \mathrm{O}_3^{2-}$ is a stronger complexing agent than ammonia.
BETA
