Problem 112 - 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 donates protons via hydrolysis, increasing H₃O⁺ ions.

Step 1: Understand the Nickel (II) Complex

The nickel(II) ion forms a complex with six water molecules, represented as \( \left[\mathrm{Ni}\left(\mathrm{H}_{2} \mathrm{O}\right)_{6}\right]^{2+} \). The water molecules act as ligands, stabilizing the nickel ion through coordination bonds. This complex can influence the pH of the solution.

Step 2: Recognize the Role of Water in Acidity

Water molecules that are coordinated to a transition metal, like nickel(II), can become more acidic due to the withdrawal of electron density towards the metal ion, weakening the O-H bonds in the water.

Step 3: Write the Chemical Equation for Hydrolysis

The coordinated water molecules can undergo hydrolysis, donating a proton (H⁺) to the surrounding water, thereby increasing the concentration of hydronium ions (H₃O⁺) in the solution.The balanced chemical equation for this process is:\[ \left[\mathrm{Ni}\left(\mathrm{H}_{2}\mathrm{O}\right)_{6}\right]^{2+} + \mathrm{H_{2}O} \rightarrow \left[\mathrm{Ni}\left(\mathrm{H}_{2}\mathrm{O}\right)_{5}\mathrm{OH}\right]^{+} + \mathrm{H}_{3}\mathrm{O}^{+} \]

Step 4: Conclude Why the Solution is Acidic

The release of hydronium ions (H₃O⁺) into the solution due to the hydrolysis reaction increases the acidity of the solution.

Key Concepts

Nickel(II) Ion

The nickel(II) ion is a positively charged ion, specifically a divalent cation with the notation Ni²⁺. In the context of aqueous solutions, nickel(II) often forms a complex with water molecules, indicating a strong interaction between the nickel ion and surrounding molecules. The nickel(II) ion has the capability to attract and coordinate with water molecules to stabilize itself in solution.
This stabilization is due to the formation of coordination bonds, which are special bonds where both electrons used in the bond come from the same atom—in this case, the oxygen in water.

• Nickel(II) exhibits a preference for forming six-part water complexes.
• These complexes can alter certain chemical properties of the solution, such as acidity.
• The positively charged ion enhances the attraction with polar molecules like water.

Understanding the nickel(II) ion's structure helps explain subsequent chemical behaviors in solution, like acidity changes.

Aqueous Solution

An aqueous solution is one in which water is the solvent—meaning water is the primary medium in which substances dissolve or interact. The abundance of water in such solutions allows for a variety of interactions, especially with ions or other polar substances.
When nickel(II) ions form complexes in an aqueous solution, the water molecules act as ligands. Ligands are ions or molecules that donate a pair of electrons to a metal atom or ion, forming a complex.

• In an aqueous nickel(II) solution, water ligands can change their electric charge distribution due to the nickel ion.
• This interaction results in changes in the chemical behavior of the solution.
• The capacity of water to act as both solvent and ligand is crucial for hydrolysis reactions.

The nature of an aqueous solution allows for the hydration and the subsequent chemical activity of dissolved ions, exemplifying the dynamic interactions at play.

Acidic Solution

An acidic solution is a solution with an excess of hydrogen ions ( H⁺ ). In simpler terms, it means the solution has a low pH, typically less than 7. This acidity is essential for many chemical reactions, including those involving metal complexes.
When nickel(II) complexes are present, they can lead to an acidic solution due to their unique interaction with water molecules.

• Acidity in such solutions often arises from reactions, like the release of hydrogen ions.
• Nickel(II) ions, through their complexes, can alter water molecules, making them more prone to release H⁺ ions.
• The excess hydrogen ions increase the solution's acidity, contributing to a pH decrease.

Understanding why a solution becomes acidic from a chemical standpoint involves breaking down these complex interactions.

Hydrolysis Reaction

A hydrolysis reaction is one where a water molecule is split, often resulting in the release of a proton (H⁺). In the case of nickel(II) ions in aqueous solution, these ions initiate hydrolysis, causing acidity by producing hydronium ions (H₃O⁺). This specific process occurs when a water molecule within the nickel complex donates a proton to free water in the solution, effectively increasing the concentration of hydronium ions.

• The stoichiometry of the reaction affects the overall balance and acidity.
• For nickel(II) complexes, the hydrolysis reaction can be represented as:

\[ \left[\mathrm{Ni}\left(\mathrm{H}_{2}\mathrm{O}\right)_{6}\right]^{2+} + \mathrm{H_{2}O} \rightarrow \left[\mathrm{Ni}\left(\mathrm{H}_{2}\mathrm{O}\right)_{5}\mathrm{OH}\right]^{+} + \mathrm{H}_{3}\mathrm{O}^{+} \] This equation clearly illustrates that through hydrolysis, an increase in hydrogen ion concentration occurs, which is responsible for the acidic nature of the solution.