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 \(\mathrm{NiBr}_{2}\) ?

Short answer

The more acidic aqueous solutions are produced by \(\mathrm{CdCl}_{2}\), \(\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}\), and \(\mathrm{NiBr}_{2}\) among each pair.

Step-by-step solution

Identify the position of Zn and Cd in the periodic table.

Zinc (Zn) and Cadmium (Cd) are both elements in Group 12 of the periodic table, but Cadmium is lower in the group than Zinc.

Determine the acidic properties of the salts.

Lower Group 12 elements, such as Cadmium, generally have a higher tendency to form acidic salts (compounds that dissolve in water to produce acidic solutions) due to their increased metallic character. Given that CdCl2 is a salt of Cadmium, it is more likely to produce an acidic aqueous solution when compared to ZnBr2. Answer (a): \(\mathrm{CdCl}_{2}\) (b) Comparing \(\mathrm{CuCl}\) and \(\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}\):

Identify the type of anions.

In both compounds, the cation is the same - Copper (Cu). The difference comes in the anions - Chloride (Cl) and Nitrate (NO3) in CuCl and Cu(NO3)2, respectively.

Determine the acidic properties of the anions.

Nitrate ions (NO3-) are known to produce acidic solutions due to their ability to react with water to form nitric acid (HNO3). On the other hand, chloride ions (Cl-) do not react with water to form hydrochloric acid (HCl). Answer (b): \(\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}\) (c) Comparing \(\mathrm{Ca}\left(\mathrm{NO}_{3}\right)_{2}\) and \(\mathrm{NiBr}_{2}\):

Identify the position and type of metals in the periodic table.

Calcium (Ca) is an alkaline earth metal in Group 2 of the periodic table, while Nickel (Ni) is a transition metal in Group 10.

Determine the acidic properties of the salts.

Alkaline earth metals, like Calcium, are known to form basic salts (compounds that dissolve in water to produce basic solutions). On the other hand, transition metals, like Nickel, are more likely to form acidic salts due to their increased metallic character. Answer (c): \(\mathrm{NiBr}_{2}\) In summary, the more acidic aqueous solutions are produced by \(\mathrm{CdCl}_{2}\), \(\mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}\), and \(\mathrm{NiBr}_{2}\) among each pair.

Key concepts

Periodic Table

The periodic table is a fundamental tool in chemistry used to organize all known chemical elements. It arranges elements based on increasing atomic number, grouping them into periods (rows) and groups (columns) that share similar properties. For example, in our exercise, we looked at Zinc (Zn) and Cadmium (Cd), which both belong to Group 12.
Elements in the same group have similar chemical properties due to their valence electron configuration. This similarity helps predict an element's chemical behavior, such as its ability to form acidic or basic solutions.
For instance, elements from the bottom of a group, like Cadmium compared to Zinc, are typically more metallic and can influence the acidity of compounds in aqueous solutions.

Acid-Base Chemistry

Acid-base chemistry deals with the properties and reactions of acids and bases. An acid is a substance that can donate a proton (H⁺) in a chemical reaction, while a base can accept a proton. When certain metal salts dissolve in water, they can produce acidic or basic solutions depending on the ions present.
For instance, nitrate ions \((NO_3^-)\) can react with water to form nitric acid \(HNO_3\), making the resulting solution more acidic. In contrast, chloride ions \(Cl^-\) do not form hydrochloric acid in water, thus not contributing to acidity like nitrate ions do.
This enables us to determine which compounds will produce more acidic aqueous solutions, such as identifying that \(Cu(NO_3)_2\) is more acidic than \(CuCl\) because of the presence of acid-producing nitrate ions.

Metallic Character

The metallic character of an element indicates how readily it can lose electrons to form positive ions, which is a characteristic of metals. Metallic character increases as you move down a group in the periodic table.
For example, within the same group, Cadmium exhibits more metallic character than Zinc. This quality affects the acidity of their compounds when dissolved in water.
Compounds of metals with higher metallic character, such as \(CdCl_2\), are more likely to produce acidic solutions than those with lower metallic character, such as \(ZnBr_2\). Having a greater metallic character means the metal's salt can more easily influence the hydrogen ion concentration, leading to increased acidity.

Anions and Cations

Anions and cations are ions with opposite charges. Cations are positively charged ions, while anions carry a negative charge. Together, they create the ion makeup of a compound. In evaluating acidity, the type of anion and cation is crucial. Cations like Cu²⁺ in both \(CuCl\) and \(Cu(NO_3)_2\) do not largely affect the acidity, but anions can make a noticeable difference.
Nitrate ions \(NO_3^-\) are capable of making solutions acidic, whereas chloride ions \(Cl^-\) do not contribute significantly to acidity when dissolved in water. As a result, \(Cu(NO_3)_2\) is more acidic than \(CuCl\).
The interaction between anions, cations, and water determines whether a solution becomes more acidic or remains neutral/buffered.