Problem 98 - Chemistry The Central Science In Si Units

Question

Which member of each pair produces the more acidic aqueous solution: (a) \(\mathrm{ZnBr}_{2}\) or \(\mathrm{CdCl}_{2},\) (b) \(\mathrm{CuCl}\) or \(\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}\), (c) \(\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}\) or \(\mathrm{NiBr}_{2} ?\)

Step-by-Step Solution

Answer

(a) \\(\mathrm{ZnBr}_{2}\\), (b) \\(\mathrm{Cu(NO}_{3})_{2}\\), (c) \\(\mathrm{NiBr}_{2}\\) produce more acidic solutions.

Step 1: Understanding Acidity in Solutions

When a salt dissolves in water, it can undergo hydrolysis where it reacts with water to form an acidic or basic solution. The cations from a transition metal often have acidic properties because they can hydrolyze and produce \( ext{H}^+\) ions, making the solution acidic.

Step 1: Compare Metal Cations for (a) \\(\mathrm{ZnBr}_{2}\\) vs. \\(\mathrm{CdCl}_{2}\\)

Both \(\mathrm{Zn^{2+}}\) and \(\mathrm{Cd^{2+}}\) can hydrolyze in water due to their transition metal properties. Comparing their positions on the periodic table, \(\mathrm{Zn^{2+}}\) is generally known to be slightly more acidic in aqueous solutions than \(\mathrm{Cd^{2+}}\) due to its smaller ionic radius and higher charge density.

Step 2: Analyze Acidity for (b) \\(\mathrm{CuCl}\\) vs. \\(\mathrm{Cu(\mathrm{NO}_{3})_{2}}\\)

In this pair, \(\mathrm{Cu^{+}}\) in \(\mathrm{CuCl}\) undergoes negligible hydrolysis and is less acidic compared to \(\mathrm{Cu^{2+}}\). The \(\mathrm{Cu^{2+}}\) ion in \(\mathrm{Cu(NO_3)_2}\) will significantly increase the acidity due to its higher positive charge and ability to attract water molecules, releasing \(\text{H}^+\) ions.

Step 3: Determine Acidity for (c) \\(\mathrm{Ca(NO}_{3})_{2}\\) vs. \\(\mathrm{NiBr}_{2}\\)

Calcium ion, \(\mathrm{Ca^{2+}}\), has minimal effect on the acidity since it is a group 2 metal with low hydrolysis potential. Nickel ion, \(\mathrm{Ni^{2+}}\), like other transition metals, can hydrolyze and make the solution more acidic compared to \(\mathrm{Ca^{2+}}\).

Key Concepts

Transition Metal Cations

Transition metal cations, such as those found in compounds like \(\mathrm{Zn^{2+}}\), \(\mathrm{Cd^{2+}}\), and \(\mathrm{Cu^{2+}}\), have unique properties that contribute to their behavior in aqueous solutions. These cations come from the d-block of the periodic table, known for partially filled d-orbitals. When these cations dissolve in water, they can exert significant influence due to their electronic configurations:

• They often have high charge densities because they carry a positive charge while possessing a relatively small ionic radius.
• Their ability to attract electron pairs from water molecules is stronger due to the positive charge exerted over a smaller area (high charge density).

One result of these properties is the tendency of transition metal cations to participate in hydrolysis reactions, which often leads to increased acidity in solution.

Hydrolysis in Chemistry

Hydrolysis is a chemical process where a chemical compound reacts with water. In the context of transition metal cations, hydrolysis involves the donation of protons from water molecules to the metal cations. This process occurs because water molecules induce the release of \(\text{H}^+\) ions, resulting in an acidic solution. A few key points about hydrolysis when it involves metal cations:

• Metal cations with a higher charge and smaller size are more prone to hydrolysis, leading to more acidic solutions.
• During hydrolysis, water molecules coordinate to the metal ion, releasing \(\text{H}^+\) ions as a consequence.

Hydrolysis not only influences the acidity of a solution but can also impact the solubility and reactivity of various compounds in aqueous environments.

Acidic Properties of Metal Ions

The acidic properties of metal ions, especially transition metals, stem largely from their ability to engage in interactions with water. When these ions dissolve in water, several factors influence their potential to increase acidity:

• The charge on the metal ion: Higher positive charges result in stronger interactions with water, leading to the higher release of \(\text{H}^+\) ions.
• The size of the ion: Smaller ions tend to create more acidic solutions as their compact size increases their charge density, strengthening the attraction to water molecules.

These ions can significantly lower the pH of a solution, making it more acidic. This explains why, for instance, \(\mathrm{Cu^{2+}}\) is more acidic than \(\mathrm{Cu^{+}}\) or why \(\mathrm{Ni^{2+}}\) is more acidic than \(\mathrm{Ca^{2+}}\). Understanding these acidic properties is crucial in predicting the behavior of metal ions in various chemical contexts.