Problem 47 - Chemistry Matter and Change

Question

Write equations for the salt hydrolysis reactions occuring when the following salts dissolve in water. Classify each as acidic, basic, or neutral. \begin{equation} \begin{array}{l}{\text { a. ammonium nitrate } \quad \text { c. rubidium acetate }} \\ {\text { b. potassium sulfate}} \quad\quad {\text {d. calcium carbonate }}\end{array} \end{equation}

Step-by-Step Solution

Answer

a. Acidic; b. Neutral; c. Basic; d. Basic.

Step 1: Understand Salt Hydrolysis

Salt hydrolysis involves the reaction of a salt with water to form an acidic or basic solution. It's important to determine if the resulting solutions are acidic, basic, or neutral by considering the strengths of the parent acids and bases from which the salts are derived.

Step 2: Determine Hydrolysis and Solution Type for Ammonium Nitrate

Ammonium nitrate (NH₄NO₃) dissolves in water to form ammonium ions (NH₄⁺) and nitrate ions (NO₃⁻). The ammonium ion is the conjugate acid of a weak base (NH₃), leading to the following hydrolysis reaction: \[ NH_4^+ + H_2O \rightleftharpoons NH_3 + H_3O^+ \] This produces hydronium ions, making the solution acidic.

Step 3: Determine Hydrolysis and Solution Type for Potassium Sulfate

Potassium sulfate (K₂SO₄) dissociates completely into potassium ions (K⁺) and sulfate ions (SO₄²⁻). Potassium ion is derived from a strong base (KOH) and sulfate ion from a strong acid (H₂SO₄), so neither ion hydrolyzes significantly. Thus, the solution remains neutral.

Step 4: Determine Hydrolysis and Solution Type for Rubidium Acetate

Rubidium acetate (RbCH₃COO) dissociates into rubidium ions (Rb⁺) and acetate ions (CH₃COO⁻). The acetate ion, being the conjugate base of the weak acid acetic acid (CH₃COOH), reacts with water: \[ CH_3COO^- + H_2O \rightleftharpoons CH_3COOH + OH^- \] This increases hydroxide ions, making the solution basic.

Step 5: Determine Hydrolysis and Solution Type for Calcium Carbonate

Calcium carbonate (CaCO₃) dissociates into calcium ions (Ca²⁺) and carbonate ions (CO₃²⁻). The carbonate ion is the conjugate base of the weak acid carbonic acid (H₂CO₃), leading to hydrolysis: \[ CO_3^{2-} + H_2O \rightleftharpoons HCO_3^- + OH^- \] As this increases hydroxide ions, the solution is basic.

Key Concepts

Acidic Solutions

Acidic solutions result when a salt dissolves in water and yields hydronium ions (\(H_3O^+\)) as a product of hydrolysis. This typically happens when the salt contains a cation that is the conjugate acid of a weak base.

For example, ammonium nitrate (\(NH_4NO_3\)) dissolves in water to produce ammonium ions (\(NH_4^+\)). This ammonium ion reacts with water:

• The ammonium ion (\(NH_4^+\)) can donate a proton to water, forming ammonia (\(NH_3\)) and hydronium ion (\(H_3O^+\)).

The presence of the hydronium ions makes the solution acidic because:

• The increase in hydronium ions lowers the pH of the solution.
• An acidic solution has a pH less than 7.

Understanding how certain ions contribute to the acidity can help you predict the behavior of various salt solutions.

Basic Solutions

Basic solutions are formed when a salt yields hydroxide ions (\(OH^-\)) upon dissolving in water. These salts often consist of anions that are conjugate bases of weak acids.

Rubidium acetate (\(RbCH_3COO\)) is an example of a salt resulting in a basic solution. When dissolved, rubidium acetate dissociates into acetate ions (\(CH_3COO^-\)) which react:

• Acetate ions accept protons from water, forming acetic acid (\(CH_3COOH\)) and releasing hydroxide ions (\(OH^-\)).

This presence of hydroxide ions increases the pH, leading to a basic solution:

• Basic solutions have a pH greater than 7.
• The solution becomes less acidic and more alkaline as hydroxide ions increase.

Similarly, calcium carbonate (\(CaCO_3\)) can also result in a basic solution:

• The carbonate ion (\(CO_3^{2-}\)) interacts with water, producing bicarbonate (\(HCO_3^-\)) and hydroxide ions.

This reaction occurs with an eye towards reducing acidity and making the solution more basic.

Neutral Solutions

Neutral solutions occur when a salt dissolves in water without changing the concentration of hydronium (\(H_3O^+\)) or hydroxide (\(OH^-\)) ions. This is common for salts derived from strong acids and strong bases.

Potassium sulfate (\(K_2SO_4\)) is one such salt that results in a neutral solution. Here's why:

• Neither of its ions—potassium (\(K^+\)) from a strong base (KOH) and sulfate (\(SO_4^{2-}\)) from a strong acid (H₂SO₄)—undergo significant hydrolysis.

The neutrality of the solution is due to the fact that:

• Neither ion substantially alters the pH when dissolved.
• Neutral aqueous solutions typically have a pH of 7.

Identifying neutral salts is useful in predicting their behavior in chemical reactions and processes, as their solutions don't skew acidic or basic.

Conjugate Acid-Base Pairs

The concept of conjugate acid-base pairs is essential in understanding salt hydrolysis. When a salt dissolves in water, it may yield ions that are conjugate acids or bases, playing crucial roles in determining the solution's pH.

A conjugate acid is formed when a base gains a proton, and a conjugate base forms when an acid loses one:

• The strength of a conjugate acid or base affects whether a salt solution is acidic or basic.
• For instance, the ammonium ion (\(NH_4^+\)) is a conjugate acid of ammonia (\(NH_3\)), a weak base.

The acetate ion (\(CH_3COO^-\)) provides another example:

• It's a conjugate base of acetic acid (\(CH_3COOH\)), a weak acid, thereby affecting the solution's basicity.

Understanding these pairs offers insight into:

• Predicting the direction of reactions in aqueous solutions.
• Determining whether a solution will be acidic, basic, or neutral based on its components.

Administrating this knowledge aids in mastering the chemistry behind acid-base reactions and salt hydrolysis.