A student mistakenly added concentrated (12M) HCI to a sample of unknown, instea
ID: 971703 • Letter: A
Question
A student mistakenly added concentrated (12M) HCI to a sample of unknown, instead of adding 6M HCI, as directed. (1) At first, a large amount of light-colored precipitate formed, but some dissolved on further addition of concentrated HVI. Write an equation(s) for the reaction(s) that might have occurred. (2) The precipitate was separated from the supernaltant liquid in (1) and was found to be inslouble in hot water. The precipitate turned grey-black upon addition of 6M NH_3. The student concluded that the unknown contained Hg2^2+ ion. The student received a alow mark on the experiment, because the unknown actually contained all three Group I cations. Explain what happened. 5. Many of the reactions utilized in this laboratory experiment involve substances with small solubility product constants. AgCl is one such substance. Nevertheless, a precipitate of AgCl will dissolved in either 6 M HCl or 6M NH_3. (1) Briefly explain why each of these reagents dissolves AgCl(s). Write appropriate equations to support your answer. (2) Briefly explaind why it is possible to reprecipiate AgCl(s) by adding HCl to [Ag(NH_3)_2]^+ ion but not by adding ammonia to [AgCl_2]^- ion. Write appropriate equations to support your answer.Explanation / Answer
4. If a student has added 12M HCl instead of 6 M HCl
(a) The initial formed chloride salt dissolves when it forms complex ion,
M^n+ + nCl- ---> MCln
MCln + nCl- ---> [M(Cl)n]^n+
(b) the chlorides were found to be insoluble in water but turned gray-black upon addition of NH3(aq).
All Group -I metal ions form insoluble chlorides which upon addition of NH3 forms compounds as,
[Ag(NH)3]2^+ for Ag+, HgNH2Cl for Hg2^2+ and Pb(NH3)2 with Pb2+. Thus all three gives dark grayish compound with NH3.
5. AgCl precipitate readily dissolves in 6 M HCl or 6 M NH3 solution
(1) AgCl reacts with HCl and NH3 and forms soluble complexes
AgCl + 2NH3 ---> [Ag(NH3)2]^+ + Cl-
formation of stable amine complex favors the reaction to proceed in forward direction
AgCl + H+ ---> Ag+ + HCl
As more HCl is formed from the Cl- in solution from AgCl. The concentration of Cl- is maintained by dissolution of more of solid AgCl.
(2) [Ag(NH3)2]^+ reacts with HCl to generate NH4+ species which favors the formation and forward reaction,
[Ag(NH3)2]^+ + Cl- + H+ ----> AgCl + 2NH4+
However, in case of [AgCl2]- the formation of AgCl does not occur upon addition of NH3. This is due to the fact that no stable [AgCl2]- is more stable than the amine complex and hence favors the reverse reaction.
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