Problem 120 - Chemistry and Chemical Reactivity

Question

The nickel(II) ion exists as \(\left[\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\) in aqueous solution. Why is this solution acidic? As part of your answer, include a balanced equation depicting what happens when \(\left[\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+}\) interacts with water.

Step-by-Step Solution

Answer

The solution is acidic because the complex ion \( \left[\mathrm{Ni}\left(\mathrm{H}_{2}\mathrm{O}\right)_{6}\right]^{2+} \) donates protons to water, forming hydronium ions.

Step 1: Understanding the Complex Ion

The nickel(II) ion in aqueous solution is coordinated with six water molecules, forming the complex ion \( \left[\mathrm{Ni}\left(\mathrm{H}_{2}\mathrm{O}\right)_{6}\right]^{2+} \). This complex has nickel in the center surrounded by six water molecules bonded to it.

Step 2: Concept of Acidic Complexes

Complex ions like \( \left[\mathrm{Ni}\left(\mathrm{H}_{2}\mathrm{O}\right)_{6}\right]^{2+} \) can act as acids. The water ligands are slightly polarized, making the hydrogen atoms of \( \mathrm{H}_{2}\mathrm{O} \) more positive and the oxygen more negative.

Step 3: Reaction with Water

The \( \left[\mathrm{Ni}\left(\mathrm{H}_{2}\mathrm{O}\right)_{6}\right]^{2+} \) can donate a proton (\( \mathrm{H}^+ \)) from a water ligand to a water molecule in the surrounding environment. This forms hydronium ion \( \mathrm{H}_3\mathrm{O}^+ \) and a deprotonated complex \( \left[\mathrm{Ni}\left(\mathrm{H}_{2}\mathrm{O}\right)_{5}(\mathrm{OH})\right]^{+} \).

Step 4: Balanced Equation

The balanced equation for this proton donation process is: \[ \left[\mathrm{Ni}\left(\mathrm{H}_{2}\mathrm{O}\right)_{6}\right]^{2+} + \mathrm{H}_{2}\mathrm{O} \rightleftharpoons \left[\mathrm{Ni}\left(\mathrm{H}_{2}\mathrm{O}\right)_{5}(\mathrm{OH})\right]^{+} + \mathrm{H}_3\mathrm{O}^{+} \] This equilibrium illustrates how the complex ion can release protons into the solution, contributing to its acidity.

Key Concepts

Complex Ions

In coordination chemistry, a complex ion is a structure where a central metal ion, such as nickel(II), is bonded to surrounding molecules or ions, known as ligands. These ligands can be neutral molecules or negative ions that donate at least one pair of electrons to the metal ion. In the case of the nickel(II) ion in water, it forms the complex ion \( [\mathrm{Ni}(\mathrm{H}_2\mathrm{O})_6]^{2+} \). Here, nickel serves as the central metal ion, and it is surrounded by six water molecules acting as ligands.These water molecules are arranged around the nickel ion at equal distances, creating an octahedral geometry. The bonding between the water molecules and the nickel ion occurs through coordination covalent bonds, where the water molecules donate a lone pair of electrons from the oxygen to the nickel. This arrangement stabilizes the nickel ion in solution and allows it to exhibit different chemical properties, including engaging in reactions that exhibit its acidic nature.

Acid-Base Reactions

Acid-base reactions are a key concept in chemistry, and they involve the transfer of protons between substances. In the context of complex ions, such as \( [\mathrm{Ni}(\mathrm{H}_2\mathrm{O})_6]^{2+} \), the complex can act as an acid due to the presence of polarized bonds within the water ligands.When the complex ion is in solution, it can donate a proton to a nearby water molecule, which turns the water into a hydronium ion \( \mathrm{H}_3\mathrm{O}^{+} \). This proton donation results in the formation of a new complex: \( [\mathrm{Ni}(\mathrm{H}_2\mathrm{O})_5(\mathrm{OH})]^{+} \). This process reflects the basic principle of an acid, which is to increase the concentration of \( \mathrm{H}^+ \) ions in solution, thereby lowering the pH and making the solution acidic.The ability of these complex ions to undergo acid-base reactions is crucial in various chemical processes, as it modulates the behavior and properties of the solution in which they reside.

Ligand Exchange

Ligand exchange refers to the process where one or more ligands in a complex ion are replaced with other ligands. This is an essential aspect of coordination chemistry and influences the reactivity and properties of complex ions.In the context of the \( [\mathrm{Ni}(\mathrm{H}_2\mathrm{O})_6]^{2+} \) ion and its acidic behavior, the exchange often involves the transformation of one water ligand into a hydroxide ion (\( \mathrm{OH}^- \)). This occurs during the deprotonation process when the complex donates a proton to a surrounding water molecule.As the water ligand releases a proton to form \( \mathrm{H}_3\mathrm{O}^+ \), it turns into an \( \mathrm{OH}^- \), leading to the formation of \( [\mathrm{Ni}(\mathrm{H}_2\mathrm{O})_5(\mathrm{OH})]^{+} \). This change in the ligand composition of the complex can affect its solubility, color, and magnetic properties. Ligand exchange reactions are dynamic and allow complex ions to participate in a variety of chemical environments, showcasing their versatility in chemical processes.