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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 Ni虏鈦� ions and CN鈦� ions, forming a solid precipitate: \[ \mathrm{Ni^{2+}(aq) + 2CN^-(aq) \rightarrow Ni(CN)_2(s)} \] Upon addition of more KCN, the CN鈦� ions act as Bronsted-Lowry and Lewis bases, generating OH鈦� 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 Ni虏鈦� ions and CN鈦� 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鈦� is a Bronsted-Lowry base, it can accept a proton from water molecules, forming OH鈦� ions. The generated OH鈦� ions can react with a water-soluble complex species formed by Ni(CN)鈧� and additional CN鈦� 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鈦� ions as both Bronsted-Lowry and Lewis bases.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Bronsted-Lowry base
A Bronsted-Lowry base is any substance that can accept a proton (H鈦�). In our scenario, the cyanide ion (\(\mathrm{CN}^-\)) acts as a Bronsted-Lowry base. When it encounters water, \(\mathrm{CN}^-\) can accept protons from water molecules (\(\mathrm{H_2O}\)). This reaction produces hydroxide ions (\(\mathrm{OH}^-\)) and hydrogen cyanide (\(\mathrm{HCN}\)).
  • The reaction: \[ \mathrm{CN^-(aq) + H_2O(l) \leftrightharpoons HCN(aq) + OH^-(aq)} \] is balanced and shows the base property of \(\mathrm{CN}^-\).
  • \(\mathrm{K_b}\) for \(\mathrm{CN^-}\) is approximately \(10^{-5}\), indicating it's a weak base. Although it's not as strong as typical bases like hydroxide itself, it effectively interacts with water.
This ability to accept protons and produce hydroxide ions is important. It helps change the conditions of the solution, playing a critical role in dissolving the precipitate formed during the reaction.
Lewis base
A Lewis base, unlike the Bronsted-Lowry base, is defined by its ability to donate a pair of electrons. The \(\mathrm{CN}^-\) ion also acts as a Lewis base. It donates its electron pair to \(\mathrm{Ni^{2+}}\) ions, thereby forming a complex. This electron pair donation leads to the formation of \(\mathrm{Ni(CN)_2}\), a solid precipitate.
  • In the initial reaction, \[ \mathrm{Ni^{2+}(aq) + 2CN^-(aq) \rightarrow Ni(CN)_2(s)} \] \(\mathrm{CN}^-\) provides an electron pair to \(\mathrm{Ni^{2+}}\), illustrating its nature as a Lewis base.
Though \(\mathrm{CN}^-\) starts as a precipitate forming agent, when further \(\mathrm{CN^-}\) ions are added, it can further coordinate with \(\mathrm{Ni^{2+}}\) to form a soluble complex.
This highlights the dual role of \(\mathrm{CN^-}\) as both a Bronsted-Lowry and Lewis base, enabling varying interactions for complexation reactions.
Precipitation reaction
A precipitation reaction occurs when ions in solution form an insoluble solid (the precipitate). In this exercise, \(\mathrm{Ni^{2+}}\) ions meet \(\mathrm{CN^-}\) ions, resulting in the precipitation reaction:
\[ \mathrm{Ni^{2+}(aq) + 2CN^-(aq) \rightarrow Ni(CN)_2(s)} \]
  • The product, \(\mathrm{Ni(CN)_2}\), is an insoluble solid that precipitates out of the solution.
The formation of a precipitate indicates a chemical change where specific conditions favor the formation of specific compounds.
In such reactions, solubility rules and the presence of a complexation agent like \(\mathrm{CN^-}\) can determine whether precipitation occurs or not.
Dissolution reaction
Dissolution reactions involve breaking down a precipitate back into its ionic components. With additional \(\mathrm{CN^-}\), \(\mathrm{Ni(CN)_2}\) redissolves by forming a complex, shown in:
\[ \mathrm{Ni(CN)_2(s) + 4CN^-(aq) \leftrightharpoons [Ni(CN)_6]^{4-}(aq)} \]
  • This equilibrium shows the breakdown of \(\mathrm{Ni(CN)_2}\) into a soluble species \(\mathrm{[Ni(CN)_6]^{4-}}\), suggesting complex formation.
  • The reaction reaches equilibrium as long as there are enough \(\mathrm{CN^-}\) ions.
The interacting \(\mathrm{CN^-}\) ions stabilize \(\mathrm{Ni}^{2+}\) in solution, shifting conditions from solid to liquid form. This dissolving action highlights how adding more reactant shifts equilibrium and underscores the reversible nature of many chemical reactions.

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Most popular questions from this chapter

Both \(\mathrm{Ni}\left(\mathrm{NH}_{3}\right)_{4}^{2+}\) and \(\mathrm{Ni}(\mathrm{SCN})_{4}^{2-}\) have four ligands. The first is paramagnetic, and the second is diamagnetic. Are the complex ions tetrahedral or square planar? Explain.

Will 0.10 mol of AgBr completely dissolve in 1.0 \(\mathrm{L}\) of 3.0 \(\mathrm{M} \mathrm{NH}_{3} ?\) The \(K_{\mathrm{sp}}\) value for \(\mathrm{AgBr}(s)\) is \(5.0 \times 10^{-13},\) and the overall formation constant for the complex ion \(\mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}+\) is \(1.7 \times 10^{7}\) , that is, $$\mathrm{Ag}^{+}(a q)+2 \mathrm{NH}_{3}(a q) \rightleftharpoons \mathrm{Ag}\left(\mathrm{NH}_{3}\right)_{2}^{+}(a q) \quad K=1.7 \times 10^{7}$$

Four different octahedral chromium coordination compounds exist that all have the same oxidation state for chromium and have \(\mathrm{H}_{2} \mathrm{O}\) and \(\mathrm{Cl}^{-}\) as the ligands and counterions. When 1 mole of each of the four compounds is dissolved in water, how many moles of silver chloride will precipitate upon addition of excess \(\mathrm{AgNO}_{3} ?\)

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