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Chapter 21: Problem 32

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 \(\mathrm{KCN}\) solution. Write reactions describing what happens in this solution. [Hint: \(\mathrm{CN}^{-}\) is a Bronsted-Lowry base \(\left(K_{\mathrm{b}} \approx 10^{-5}\right)\) and a Lewis base.]

Short Answer

Expert verified
First, the precipitation reaction occurs between Ni²⁺ ions and CN⁻ ions, forming a solid precipitate: \[ \mathrm{Ni^{2+}(aq) + 2CN^-(aq) \rightarrow Ni(CN)_2(s)} \] Upon addition of more KCN, the CN⁻ ions act as Bronsted-Lowry and Lewis bases, generating OH⁻ ions and water-soluble complex species, causing the precipitate to redissolve: \[ \mathrm{Ni(CN)_2(s) + 4CN^-(aq) + H_2O(l) \leftrightharpoons [Ni(CN)_6]^{4-}(aq) + HCN(aq) + OH^-(aq)} \]

Step by step solution

01

Formation of precipitate

First, we have the precipitation reaction between Ni²⁺ ions and CN⁻ ions, which results in the formation of a solid precipitate. The reaction can be represented as follows: \[ \mathrm{Ni^{2+}(aq) + 2CN^-(aq) \rightarrow Ni(CN)_2(s)} \]
02

Dissolution of precipitate

Since CN⁻ is a Bronsted-Lowry base, it can accept a proton from water molecules, forming OH⁻ ions. The generated OH⁻ ions can react with a water-soluble complex species formed by Ni(CN)₂ and additional CN⁻ ions. The reactions in this step are: First, the base reaction: \[ \mathrm{CN^-(aq) + H_2O(l) \leftrightharpoons HCN(aq) + OH^-(aq)} \] Then, the dissolution reaction: \[ \mathrm{Ni(CN)_2(s) + 4CN^-(aq) \leftrightharpoons [Ni(CN)_6]^{4-}(aq)} \]
03

Combined reaction

We can now combine these reactions to represent the overall process when more KCN solution is added: \[ \mathrm{Ni(CN)_2(s) + 4CN^-(aq) + H_2O(l) \leftrightharpoons [Ni(CN)_6]^{4-}(aq) + HCN(aq) + OH^-(aq)} \] These are the reactions describing what happens in the solution during the process. The precipitate forms at first and then redissolves when more KCN is added, showing the behavior of CN⁻ ions as both Bronsted-Lowry and Lewis bases.

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Most popular questions from this chapter

Nickel can be purified by producing the volatile compound nickel tetracarbonyl. Nickel is the only metal that reacts with carbon monoxide at room temperature. Assuming this compound is overall neutral, what is the oxidation state of Ni in the compound? Deduce the formula of the compound.

Draw the \(d\) -orbital splitting diagrams for the octahedral complex ions of each of the following. a. \(\mathrm{Fe}^{2+}\) (high and low spin) b. \(\mathrm{Fe}^{3+}(\text { high spin })\) c. \(\mathrm{Ni}^{2+}\)

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

The compound $\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6} \mathrm{Cl}_{2}$ is green, whereas \(\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{6} \mathrm{Cl}_{2}\) is violet. Predict the predominant color of light absorbed by each compound. Which compound absorbs light with the shorter wavelength? Predict in which compound \(\Delta\) is greater and whether \(\mathrm{H}_{2} \mathrm{O}\) or \(\mathrm{NH}_{3}\) is the stronger field ligand. Do your conclusions agree with the spectrochemical series?

Name the following coordination compounds. a. $\operatorname{Na_t}_{4}\left[\operatorname{Ni}\left(\mathrm{C}_{2} \mathrm{O}_{4}\right)_{3}\right]$ b. \(\mathrm{K}_{2}\left[\mathrm{CoCl}_{4}\right]\) c. \(\left[\mathrm{Cu}\left(\mathrm{NH}_{3}\right)_{4}\right] \mathrm{SO}_{4}\) d. $\left[\mathrm{Co}(\mathrm{en})_{2}(\mathrm{SCN}) \mathrm{Cl}\right] \mathrm{Cl}$
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