Question

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

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

- 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). \( Ni^{2+}(aq) + 2CN^-(aq) \rightarrow Ni(CN)_2(s) \)

- 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: \( Ni(CN)_2(s) + 2CN^-(aq) \rightarrow [Ni(CN)_4]^{2-}(aq) \)

- 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^-): \( CN^-(aq) + H_2O(l) \rightleftharpoons HCN(aq) + OH^-(aq) \)

Key concepts

Brønsted-Lowry base

In the realm of acid-base chemistry, a Brønsted-Lowry base is any species capable of accepting a proton (H\(^+\)) from another species. This definition broadens our understanding of bases beyond just their behavior in water.
Cyanide ion (CN\(^-\)) is a great example of a Brønsted-Lowry base. When dissolved in water, it has the ability to accept a proton from water molecules, resulting in the formation of hydrogen cyanide (HCN) and hydroxide ions (OH\(^-\)).
This process is shown in an equilibrium reaction:

• \( CN^-(aq) + H_2O(l) \rightleftharpoons HCN(aq) + OH^-(aq) \)

This reaction illustrates the basicity of the cyanide ion, emphasizing how it can influence the pH of a solution by generating an increased concentration of hydroxide ions. This specific reaction demonstrates the dual nature of CN\(^-\) as it can act as a base in different environments.
Understanding how Brønsted-Lowry bases interact in solution is paramount for comprehending the broader scope of chemical reactions.

Lewis base

Lewis bases are chemical compounds that can donate an electron pair to a Lewis acid to form a coordinate covalent bond. This definition focuses on the electron-pair nature of the interaction, differentiating it from Brønsted-Lowry's proton exchange concept.
The cyanide ion (CN\(^-\)) functions as a Lewis base thanks to its lone pair of electrons, which can form bonds with metal ions like Ni\(^{2+}\).
When CN\(^-\) ions are added to a solution containing Ni\(^{2+}\), a reaction forms an initially insoluble compound, nickel cyanide (Ni(CN)\(_2\)). Continuing to add CN\(^-\) results in the formation of a soluble complex ion, [Ni(CN)\(_4\)]\(^{2-}\), since CN\(^-\) donates electron pairs to the metal ion:

• \( Ni^{2+}(aq) + 4CN^-(aq) \rightarrow [Ni(CN)_4]^{2-}(aq) \)

Thus, we can observe how Lewis base characteristics of CN\(^-\) facilitate complex ion formation, important in many aspects of coordination chemistry and industrial applications.

Precipitation and dissolution reactions

Precipitation and dissolution are key reactions in chemistry, often linked with the formation of insoluble and soluble compounds in a solution. When ions in solution combine to form an insoluble product, a precipitate is formed. Dissolution is the process where this solid returns to an aqueous state.
In the case of Ni\(^{2+}\) and CN\(^-\), an initial reaction forms the precipitate nickel cyanide (Ni(CN)\(_2\)):

• \( Ni^{2+}(aq) + 2CN^-(aq) \rightarrow Ni(CN)_2(s) \)

Larger quantities of CN\(^-\) ions added to this mixture result in a secondary reaction, where the precipitate dissolves, forming the soluble complex ion [Ni(CN)\(_4\)]\(^{2-}\):

• \( Ni(CN)_2(s) + 2CN^-(aq) \rightarrow [Ni(CN)_4]^{2-}(aq) \)

The dynamics of precipitation and dissolution reactions often rely on the concentrations of the reactants and knowledge of solubility characteristics. These reactions are vital in processes like purification and extraction of materials.

Coordination chemistry

Coordination chemistry focuses on the structure and behavior of coordination compounds formed between metal ions and ligands. These ligands, often Lewis bases, donate electron pairs to the metal center, creating diverse structures with varying properties.
In the cyanide-nickel reaction, CN\(^-\) acts as a ligand, forming a coordination complex with Ni\(^{2+}\). Initially, CN\(^-\) binds to Nickel, resulting in an insoluble coordination compound, Ni(CN)\(_2\). Adding more CN\(^-\), as the ligand, leads to the soluble complex [Ni(CN)\(_4\)]\(^{2-}\):

• \( Ni^{2+}(aq) + 4CN^-(aq) \rightarrow [Ni(CN)_4]^{2-}(aq) \)

Coordination compounds exhibit unique properties like color variability and magnetic behavior, adapting to numerous applications from catalysis to medicine. Mastery of coordination chemistry not only enriches the understanding of chemical complexes but also opens paths to innovative technological advancements.