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Chapter 20: Problem 30

When an aqueous solution of KCN is added to a solution containing \(\mathrm{Ni}^{2+}\) ions, a precipitate forms, which redissolves on addition of more KCN solution. Write reactions describing what happens in this solution. [Hint: \(\mathrm{CN}^{-}\) is a Brönsted-Lowry base \(\left.\left(K_{\mathrm{b}} \approx 10^{-5}\right) \text {and a Lewis base. }\right]\)

Short Answer

Expert verified
To summarize the chemical reactions that occur when an aqueous solution of KCN is added to a solution containing Ni²⁺ ions: 1. Initial precipitation reaction: \(Ni^{2+}(aq) + 2CN^-(aq) \rightarrow Ni(CN)₂(s)\) 2. Brönsted-Lowry base reaction of CN⁻: \(CN^-(aq) + H_2O(l) \rightleftharpoons HCN(aq) + OH^-(aq)\) 3. Lewis base reaction - formation of soluble complex: \(Ni(CN)₂(s) + 2CN^-(aq) \rightarrow Ni(CN)_4^{2-}(aq)\) These reactions explain that the precipitate (Ni(CN)₂) initially formed redissolves on addition of more KCN solution due to the formation of a soluble coordination complex (Ni(CN)₄²⁻).

Step by step solution

01

Identify the initial precipitate reaction

When an aqueous solution of KCN is added to the solution containing Ni^2+ ions, a precipitation reaction occurs as KCN reacts with Ni^2+ to form nickel (II) cyanide (Ni(CN)₂). This reaction can be written as: \(Ni^{2+}(aq) + 2CN^-(aq) \rightarrow Ni(CN)₂(s)\)
02

Brönsted-Lowry base reaction of CN^-

CN^- acts as a Brönsted-Lowry base in aqueous solutions. We know that Brönsted-Lowry bases are proton (H^+) acceptors. The given Kb value is approximately \(10^{-5}\). CN^- can react with water molecules in the solution to form the conjugate acid (HCN) and hydroxide ions (OH^-). This reaction can be written as: \(CN^-(aq) + H_2O(l) \rightleftharpoons HCN(aq) + OH^-(aq)\) This reaction generates hydroxide ions, which can increase the basicity of the solution.
03

Lewis base reaction of CN^-

Now, let's analyze the Lewis base property of CN^-. We know that a Lewis base is an electron-pair donor. CN^- has a lone pair of electrons, and Ni^2+ can accept an electron pair to form a coordination complex. When we add excess KCN, the CN^- ions react with the formed precipitate of Ni(CN)₂ to form a soluble coordination complex, which can be written as: \(Ni(CN)₂(s) + 2CN^-(aq) \rightarrow Ni(CN)_4^{2-}(aq)\)
04

Final chemical reactions of the process

Now, we can summarize the chemical reactions that describe what happens in the solution after the addition of KCN: 1. Initial precipitation reaction: \(Ni^{2+}(aq) + 2CN^-(aq) \rightarrow Ni(CN)₂(s)\) 2. Brönsted-Lowry base reaction of CN^-: \(CN^-(aq) + H_2O(l) \rightleftharpoons HCN(aq) + OH^-(aq)\) 3. Lewis base reaction - formation of soluble complex: \(Ni(CN)₂(s) + 2CN^-(aq) \rightarrow Ni(CN)_4^{2-}(aq)\) By completing these reactions, the precipitate initially formed is redissolved as more KCN solution is added to form a soluble coordination complex.

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Most popular questions from this chapter

Name the following coordination compounds. a. \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{2}\) b. \(\left[\mathrm{Co}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right] \mathrm{I}_{3}\) c. \(\mathrm{K}_{2}\left[\mathrm{PtCl}_{4}\right]\) d. \(K_{4}\left[P_{t} C l_{6}\right]\) e. \(\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{5} \mathrm{Cl}\right] \mathrm{Cl}_{2}\) f. \(\left[\operatorname{Co}\left(\mathrm{NH}_{3}\right)_{3}\left(\mathrm{NO}_{2}\right)_{3}\right]\)

Figure \(20-17\) shows that the \(c i s\) isomer of \(\mathrm{Co}(\mathrm{en})_{2} \mathrm{Cl}_{2}^{+}\) is optically active while the \(t r a n s\) isomer is not optically active. Is the same true for \(\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{Cl}_{2}^{+} ?\) Explain.

The wavelength of absorbed electromagnetic radiation for \(\mathrm{CoBr}_{4}^{2-}\) is \(3.4 \times 10^{-6} \mathrm{m} .\) Will the complex ion \(\mathrm{CoBr}_{6}^{4-} \mathrm{ab}\) sorb electromagnetic radiation having a wavelength longer or shorter than \(3.4 \times 10^{-6} \mathrm{m} ?\) Explain.

Draw geometrical isomers of each of the following complex ions. a. \(\mathrm{Co}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{2}\left(\mathrm{H}_{2} \mathrm{O}\right)_{2}^{-}\) b. \(\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{4} \mathrm{I}_{2}^{2+}\) c. \(\operatorname{Ir}\left(\mathrm{NH}_{3}\right)_{3} \mathrm{Cl}_{3}\) d. \(\operatorname{Cr}(\mathrm{en})\left(\mathrm{NH}_{3}\right)_{2} \mathrm{I}_{2}^{+}\)

Draw all geometrical and linkage isomers of \(\operatorname{Co}\left(\mathrm{NH}_{3}\right)_{4}\left(\mathrm{NO}_{2}\right)_{2}\)
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