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

Short answer

When an aqueous solution of KCN is added to a solution containing Ni²⁺ ions, the initial reaction forms a precipitate of Ni(CN)₂: \(Ni^{2+} + 2CN^{-} \rightarrow Ni(CN)_2\downarrow\) Upon adding more KCN, the precipitate redissolves, forming a complex ion Ni(CN)₄²⁻: \(Ni(CN)_2 + 2CN^{-} \rightarrow Ni(CN)_4^{2-}\)

Step-by-step solution

Identify the ions in the solution

In the given exercise, we have an aqueous solution of \(\mathrm{KCN}\) and a solution containing \(\mathrm{Ni}^{2+}\) ions. We know that when \(\mathrm{KCN}\) dissolves in water, it forms \(\mathrm{K}^{+}\) and \(\mathrm{CN}^{-}\) ions. So, the ions in the solution are \(\mathrm{K}^{+}\), \(\mathrm{CN}^{-}\), and \(\mathrm{Ni}^{2+}\).

Write the equation for the initial reaction

The initial reaction is the formation of a precipitate. Since \(\mathrm{CN}^{-}\) is acting as a Brónsted-Lowry base (with \(K_\mathrm{b}=10^{-5}\)) and a Lewis base, it will form a coordination complex with \(\mathrm{Ni}^{2+}\). The reaction can be represented as: \[Ni^{2+} + 2CN^{-} \rightarrow Ni(CN)_2\downarrow\] Here, a precipitate of \(Ni(CN)_2\) is formed.

Write the equation for the redissolution of the precipitate

Upon adding more KCN to the solution, the precipitate redissolves. This occurs as more \(\mathrm{CN}^{-}\) ions are available to form a more stable complex with \(\mathrm{Ni}^{2+}\). In this case, \(\mathrm{CN}^{-}\) acts as a Lewis base, coordinating with \(\mathrm{Ni}^{2+}\). The redissolution reaction can be represented as: \[Ni(CN)_2 + 2CN^{-} \rightarrow Ni(CN)_4^{2-}\] In this step, \(Ni(CN)_4^{2-}\) complex ion is formed, and the precipitate redissolves. So, the complete set of reactions describing what happens in this solution are: 1. Initial reaction forming precipitate: \(Ni^{2+} + 2CN^{-} \rightarrow Ni(CN)_2\downarrow\) 2. Redissolution of the precipitate: \(Ni(CN)_2 + 2CN^{-} \rightarrow Ni(CN)_4^{2-}\)

Key concepts

Coordination Complex

A coordination complex forms when a central metal ion binds with surrounding molecules or ions, usually called ligands. These ligands form a kind of "shell" around the metal ion. In our example, the metal ion is nickel, denoted as \(\mathrm{Ni}^{2+}\). The ligands are the \(\mathrm{CN}^{-}\) ions from potassium cyanide (KCN).When \(\mathrm{Ni}^{2+}\) interacts with \(\mathrm{CN}^{-}\), they form a coordination compound. Initially, \(\mathrm{Ni(CN)_2}\) forms as a precipitate when only a small amount of \(\mathrm{CN}^{-}\) is present, meaning it is insoluble in water. However, when more \(\mathrm{CN}^{-}\) is added, a more stable soluble complex \(Ni(CN)_4^{2-}\) forms, causing the precipitate to dissolve again.Coordination complexes are crucial in understanding chemical reactivity and properties of solutions. They are used in many applications, from medical treatments to catalysis in industrial processes.

Bronsted-Lowry Base

The Bronsted-Lowry theory defines acids and bases in terms of proton (\(\mathrm{H}^+\) ion) transfer. According to this definition, a Bronsted-Lowry base is a substance that can accept a proton. In the context of the problem, \(\mathrm{CN}^{-}\) acts as a Bronsted-Lowry base.In water, \(\mathrm{CN}^{-}\) can accept a proton to form \(\mathrm{HCN}\), acting as a base because it wants to bind with the proton. Even though this behavior is not directly involved in the coordination reaction with nickel, understanding it helps explain how \(\mathrm{CN}^{-}\) can interact in different chemical environments.Being a Bronsted-Lowry base is essential for substances involved in acid-base reactions, affecting everything from biological systems to industrial processes.

Lewis Base

A Lewis base is defined as a substance that can donate an electron pair. Unlike the Bronsted-Lowry base, which centers around protons, the Lewis concept is all about electron pairs.In our problem, \(\mathrm{CN}^{-}\) ion acts as a Lewis base. It donates an electron pair to form bonds with \(\mathrm{Ni}^{2+}\), thus forming a coordination complex. The ability of \(\mathrm{CN}^{-}\) to form strong bonds using its electron pairs is what drives the formation of both the insoluble \(\mathrm{Ni(CN)_2}\) precipitate and the soluble \(Ni(CN)_4^{2-}\) complex.Understanding the role of Lewis bases in forming coordination complexes helps to predict and explain the behavior of different species in complex chemical reactions.

Precipitation Reaction

A precipitation reaction occurs when two soluble ions in a solution combine to form an insoluble solid, called a precipitate.In the given exercise, the precipitation reaction involves the formation of \(\mathrm{Ni(CN)_2}\). When \(\mathrm{Ni}^{2+}\) ions encounter \(\mathrm{CN}^{-}\) ions in the solution, they bond to form \(\mathrm{Ni(CN)_2}\), a solid that falls out of the solution. Precipitation is crucial for identifying the presence of specific ions in various solutions. It's frequently used in laboratories to isolate components in a mixture.

Redissolution Reaction

Redissolution refers to the soluble state achieved when a previously formed precipitate dissolves back into a solution.In our case, upon further addition of \(\mathrm{CN}^{-}\) from \(\mathrm{KCN}\), the \(\mathrm{Ni(CN)_2}\) precipitate redissolves as the \(Ni(CN)_4^{2-}\) complex. This occurs because the \(\mathrm{CN}^{-}\) ions provide more electron pairs to stabilize a new and more soluble coordination complex.Redissolution is an important concept in chemistry, often used to control and manipulate the solubility of compounds in various industrial and research settings. Understanding this process is key to advanced experimental design and product formulation.