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Chapter 21: Problem 30

When an aqueous solution of \(\mathrm{KCN}\) 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 Br?nsted-Lowry base \(\left(K_{\mathrm{b}} \approx 10^{-5}\right)\) and a Lewis base.]

Short Answer

Expert verified
Initially, KCN reacts with Ni^2+ ions to form a precipitate (Ni(CN)_2): \( Ni^{2+}(aq) + 2CN^-(aq) \rightarrow Ni(CN)_2(s) \). Upon further addition of KCN, excess CN^- ions react with the precipitate to form a soluble complex ion [Ni(CN)_4]^2-: \( Ni(CN)_2(s) + 2CN^-(aq) \rightarrow [Ni(CN)_4]^{2-}(aq) \). Additionally, CN^- as a Brønsted-Lowry base reacts with water: \( CN^-(aq) + H_2O(l) \rightleftharpoons HCN(aq) + OH^-(aq) \).

Step by step solution

01

- Formation of precipitate

Initially, when KCN is added to the Ni^2+ ion solution, the CN^- ions from KCN react with the Ni^2+ ions to form a precipitate (Ni(CN)_2). \( Ni^{2+}(aq) + 2CN^-(aq) \rightarrow Ni(CN)_2(s) \)
02

- Dissolution of precipitate

Upon further addition of KCN solution to the mixture, the excess CN^- ions now react with the precipitate Ni(CN)_2 and form a complex ion [Ni(CN)_4]^2-. The formation of this complex ion causes the precipitate to dissolve as it is now in a soluble form: \( Ni(CN)_2(s) + 2CN^-(aq) \rightarrow [Ni(CN)_4]^{2-}(aq) \)
03

- Acid-base reaction

Since CN^- is a Brønsted-Lowry base, it can also react with water molecules by accepting a hydrogen ion (H^+), forming the weak acid hydrogen cyanide (HCN) and the hydroxide ion (OH^-): \( CN^-(aq) + H_2O(l) \rightleftharpoons HCN(aq) + OH^-(aq) \)

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Most popular questions from this chapter

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

Give formulas for the following. a. potassium tetrachlorocobaltate(II) b. aquatricarbonylplatinum(II) bromide c. sodium dicyanobis(oxalato)ferrate(III) d. triamminechloroethylenediaminechromium(III) iodide

a. In the absorption spectrum of the complex ion \(\mathrm{Cr}(\mathrm{NCS})_{6}{ }^{3-}\), there is a band corresponding to the absorption of a photon of light with an energy of \(1.75 \times 10^{4} \mathrm{~cm}^{-1}\). Given \(1 \mathrm{~cm}^{-1}=\) \(1.986 \times 10^{-23} \mathrm{~J}\), what is the wavelength of this photon? b. The \(\mathrm{Cr}-\mathrm{N}-\mathrm{C}\) bond angle in \(\mathrm{Cr}(\mathrm{NCS})_{6}{ }^{3-}\) is predicted to be \(180^{\circ}\). What is the hybridization of the \(\mathrm{N}\) atom in the \(\mathrm{NCS}^{-}\) ligand when a Lewis acid-base reaction occurs between \(\mathrm{Cr}^{3+}\) and \(\mathrm{NCS}^{-}\) that would give a \(180^{\circ}\) \(\mathrm{Cr}-\mathrm{N}-\mathrm{C}\) bond angle? \(\mathrm{Cr}(\mathrm{NCS})_{6}{ }^{3-}\) undergoes sub- stitution by ethylenediamine (en) according to the equation $$ \mathrm{Cr}(\mathrm{NCS})_{6}^{3-}+2 \mathrm{en} \longrightarrow \mathrm{Cr}(\mathrm{NCS})_{2}(\mathrm{en})_{2}^{+}+4 \mathrm{NCS}^{-} $$ Does \(\mathrm{Cr}(\mathrm{NCS})_{2}(\mathrm{en})_{2}^{+}\) exhibit geometric isomerism? Does \(\mathrm{Cr}(\mathrm{NCS})_{2}(\mathrm{en})_{2}^{+}\) exhibit optical isomerism?

Would it be better to use octahedral \(\mathrm{Ni}^{2+}\) complexes or octahedral \(\mathrm{Cr}^{2+}\) complexes to determine whether a given ligand is a strong-field or weak-field ligand by measuring the number of unpaired electrons? How else could the relative ligand field strengths be determined?

Which of the following molecules exhibit(s) optical isomerism? a. \(c i s-\mathrm{Pt}\left(\mathrm{NH}_{3}\right)_{2} \mathrm{Cl}_{2}\) b. trans \(-\mathrm{Ni}(\mathrm{en})_{2} \mathrm{Br}_{2}\) (en is ethylenediamine) c. cis-Ni(en) \(_{2} \mathrm{Br}_{2}\) (en is ethylenediamine) d.
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