Question

Predict whether aqueous solutions of the following compounds are acidic, basic, or neutral: \((\mathbf{a}) \mathrm{NH}_{4} \mathrm{Br},(\mathbf{b}) \mathrm{FeCl}_{3},\) (c) \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) (e) \(\mathrm{NaHC}_{2} \mathrm{O}_{4}\). (d) \(\mathrm{KClO}_{4}\),

Short answer

Based on the behavior of the cations and anions in each compound, we can predict the following nature of their aqueous solutions: (a) NH4Br: Acidic (b) FeCl3: Acidic (c) Na2CO3: Basic (d) KClO4: Neutral (e) NaHC2O4: Slightly Acidic

Step-by-step solution

Identify Cations and Anions

For each given compound, identify the cation and anion present. This will help us understand their behaviors when dissolved in water. (a) NH4Br: NH4+ (cation), Br- (anion) (b) FeCl3: Fe3+ (cation), Cl- (anion) (c) Na2CO3: Na+ (cation), CO3^2- (anion) (d) KClO4: K+ (cation), ClO4- (anion) (e) NaHC2O4: Na+ (cation), HC2O4- (anion)

Analyze the Behavior of Anions and Cations

Now we will analyze the behavior of the cations and anions when dissolved in water. (a) NH4Br: NH4+ behaves as an acid, donating a proton (H+) to water molecules, while Br- is a neutral spectator ion. The solution is acidic. (b) FeCl3: Fe3+ can hydrolyze water, producing H+ ions, while Cl- is a neutral spectator ion. The solution is acidic. (c) Na2CO3: Na+ is a neutral spectator ion, while CO3^2- behaves as a base, accepting protons (H+) from water molecules. The solution is basic. (d) KClO4: K+ is a neutral spectator ion, while ClO4- is a weak base. However, the basic character of ClO4- is negligible, so the solution is considered neutral. (e) NaHC2O4: Na+ is a neutral spectator ion, while HC2O4- can both accept a proton (acting as a base) and donate a proton (acting as an acid) from/to water molecules. However, the equilibrium lies more towards the acidic side. The solution is slightly acidic.

Predict the Aqueous Solutions of each Compound

Based on the behavior of the cations and anions, we can predict whether the aqueous solutions of the compounds are acidic, basic, or neutral. (a) NH4Br: Acidic (b) FeCl3: Acidic (c) Na2CO3: Basic (d) KClO4: Neutral (e) NaHC2O4: Slightly Acidic

Key concepts

Cation-Anion Analysis

When predicting the acidity, basicity, or neutrality of any aqueous solution, it is essential to start with a detailed analysis of the cations and anions in the compound.
Each ionic compound is made up of positive ions (cations) and negative ions (anions), and their behaviors in water dictate the overall nature of the solution.
For example:

• In \( ext{NH}_4 ext{Br}\), the cation is \( ext{NH}_4^+\), and the anion is \( ext{Br}^-\).
• In \( ext{FeCl}_3\), \( ext{Fe}^{3+}\) is the cation, and \( ext{Cl}^-\) is the anion.
• For \( ext{Na}_2 ext{CO}_3\), \( ext{Na}^+\) acts as the cation, while \( ext{CO}_3^{2-}\) is the anion.
• \( ext{KClO}_4\) contains a \( ext{K}^+\) cation and a \( ext{ClO}_4^-\) anion.
• And in \( ext{NaHC}_2 ext{O}_4\), \( ext{Na}^+\) is the cation, and \( ext{HC}_2 ext{O}_4^-\) serves as the anion.

Understanding these ions sets the stage for analyzing their individual behaviors in water and what that means for the solution's overall characteristics.

Hydrolysis

Hydrolysis, a critical process in the behavior of salts in solutions, occurs when cations or anions react with water, altering the pH of the solution.
- Certain cations, like \( ext{NH}_4^+\) in \( ext{NH}_4 ext{Br}\), release hydrogen ions by donating protons, making the solutions acidic. - Others, such as \( ext{Fe}^{3+}\) in \( ext{FeCl}_3\), can induce hydrolysis, releasing protons into the solution and also contributing to acidity.
- In contrast, some anions accept protons, which can make the solution basic. A good example is \( ext{CO}_3^{2-}\) in \( ext{Na}_2 ext{CO}_3\), where it acts to accept protons from water. - This process can either lead to a shift toward acidity or basicity, depending on the ions involved.

Aqueous Solution Behavior

The overall behavior of an aqueous solution is dictated by the sum of activities of its ions in water.

• Neutral spectator ions like \( ext{Na}^+\) and \( ext{Br}^-\) don't participate in pH change, allowing the solution to retain its inherent nature based on active ions.
• In some cases, as seen with \( ext{KClO}_4\), neither ion significantly reacts with water, resulting in a neutral solution.
• Aqueous solutions with ions that have strong tendencies for hydrolysis or proton exchange, like \( ext{NH}_4^+\) or \( ext{CO}_3^{2-}\), shape the solution to be either acidic or basic.

By considering how the individual ions engage with water, we predict the resulting nature of these solutions.

Acidic and Basic Solutions

An acidic or basic solution results from the interaction of its constituent ions with water, altering its hydrogen ion concentration. - Acidic solutions arise when anions or cations increase the concentration of \( ext{H}^+\) ions. For example, \( ext{NH}_4^+\) (from \( ext{NH}_4 ext{Br}\)) and \( ext{Fe}^{3+}\) (from \( ext{FeCl}_3\)) donate protons, making these solutions acidic. - Basic solutions occur when anions reduce \( ext{H}^+\) concentration by binding to protons, as \( ext{CO}_3^{2-}\) does in \( ext{Na}_2 ext{CO}_3\).These behaviors are fundamental in predicting and understanding the pH nature of different aqueous solutions. The nature and strength of cation and anion hydrolysis or ion interaction with water significantly influence whether a solution is acidic, neutral, or basic.