Problem 30 - Chemistry An Atoms First Approach

Question

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 \(\left.\left(K_{\mathrm{b}} \approx 10^{-5}\right) \text {and a Lewis base. }\right]\)

Step-by-Step Solution

Answer

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 1: 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: \(Ni^{2+}(aq) + 2CN^-(aq) \rightarrow Ni(CN)₂(s)\)

Step 2: 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.

Step 3: 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: \(Ni(CN)₂(s) + 2CN^-(aq) \rightarrow Ni(CN)_4^{2-}(aq)\)

Step 4: 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.

Key Concepts

Brönsted-Lowry Base

In coordination chemistry, the concept of a Brönsted-Lowry base is essential when analyzing chemical reactions. A Brönsted-Lowry base is a substance that can accept protons (H⁺ ions) in a reaction. In the context of the given exercise, cyanide ions (\(CN^-\)) play the role of a Brönsted-Lowry base.

This happens because the cyanide ion has an affinity for protons due to its negatively charged nature. When \(CN^-\) is added to water in solution, it can attract and accept a proton from a water molecule, resulting in the formation of hydrogen cyanide (HCN) and hydroxide ions (OH⁻). The reaction is defined by the following equation:

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

• This reaction demonstrates the basicity of \(CN^-\) as it increases the concentration of OH⁻ ions in the solution, hence enhancing the solution's basic characteristics.
• It’s important to note that this reaction is governed by an equilibrium constant, often indicated by \(K_b\), which in the case of cyanide is approximately \(10^{-5}\).

Lewis Base

The Lewis base concept extends from the idea of electron pairs. A Lewis base is any molecule or ion that can donate a pair of electrons to form a covalent bond. In this frame, \(CN^-\) acts as a Lewis base because it possesses a lone pair of electrons that can be donated to form bonds.

When \(CN^-\) is introduced to nickel ions (\(Ni^{2+}\)) in this exercise, it forms part of a coordination complex. The reaction involves \(CN^-\) using its extra electrons to coordinate with the metal cation, converting the initial solid precipitate of nickel (II) cyanide into a soluble nickel cyanide complex:

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

• This process involves the interaction of a Lewis base with a metal ion, showcasing the formation of a stable, soluble complex.
• The resulting complex, \(Ni(CN)_4^{2-}\), is highly soluble in water, effectively "redissolving" the original precipitate.

Precipitation Reaction

A precipitation reaction is one of the principal types of reactions in chemistry, where two soluble salts in aqueous solutions react to form an insoluble salt or solid, known as a precipitate. In the context of this exercise, the reaction begins when \(Ni^{2+}\) ions mix with cyanide ions (\(CN^-\)) to yield the solid nickel (II) cyanide, a classic example of a precipitation process.

The reaction is represented by:

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

• The formation of \(Ni(CN)_2\) as a precipitate is initially visible as a solid phase that separates from the solution.
• Precipitation reactions are generally governed by the solubility rules in chemistry, where the solubility product constant (\(K_{sp}\)) dictates whether a precipitate will form under given conditions.
• In this instance, the precipitation reaction is only a temporary state, as further addition of KCN leads the complex to dissolve once more, reverting to a soluble state.