Question

List and distinguish between the four general categories of salts, based on their hydrolysis properties, and give an example of each.

Short answer

The four categories are salts of strong acids and strong bases (e.g., NaCl), salts of strong acids and weak bases (e.g., NH4Cl), salts of weak acids and strong bases (e.g., CH3COONa), and salts of weak acids and weak bases (e.g., NH4CH3COO).

Step-by-step solution

Understanding Salt Hydrolysis Categories

Salts can be classified into four general categories based on their hydrolysis properties: salts of strong acids and strong bases, salts of strong acids and weak bases, salts of weak acids and strong bases, and salts of weak acids and weak bases. Hydrolysis refers to the reaction of a salt with water to form an acidic or basic solution.

Salts of Strong Acids and Strong Bases

These salts do not hydrolyze in water and their solutions are neutral. An example is sodium chloride (NaCl), which comes from hydrochloric acid (a strong acid) and sodium hydroxide (a strong base).

Salts of Strong Acids and Weak Bases

These salts hydrolyze to form acidic solutions. An example is ammonium chloride (NH4Cl), which comes from hydrochloric acid (a strong acid) and ammonia (a weak base).

Salts of Weak Acids and Strong Bases

These salts hydrolyze to form basic solutions. An example is sodium acetate (CH3COONa), which comes from acetic acid (a weak acid) and sodium hydroxide (a strong base).

Salts of Weak Acids and Weak Bases

These salts can hydrolyze to form slightly acidic or slightly basic solutions, depending on the relative strengths of the weak acid and weak base. An example is ammonium acetate (NH4CH3COO), which comes from acetic acid (a weak acid) and ammonia (a weak base).

Key concepts

Salts of Strong Acids and Strong Bases

When a strong acid reacts with a strong base, the resulting salt does not interact with water to change the pH of the solution. This type of salt is known for its neutral behavior in aqueous solutions, meaning it neither raises nor lowers the pH significantly. The chemical equation for the dissolution and neutral behavior of sodium chloride (NaCl), a typical salt of this category, can be represented as:
NaCl → Na⁺ + Cl⁻
Since both ions are products of strong acid (HCl) and strong base (NaOH), they do not hydrolyze and thus keep the solution neutral.

• Make sure you understand that these salts result from neutralization reactions.
• Remember that no hydrolysis occurs because the ions formed are very stable.

Salts of Strong Acids and Weak Bases

Salts originating from strong acids and weak bases typically lead to acidic hydrolysis. A common example is ammonium chloride (NH4Cl). Here's how it behaves in water:
NH4Cl → NH₄⁺ + Cl⁻
The chloride ion (Cl⁻) doesn’t hydrolyze because HCl is a strong acid, forming a stable ion. However, the ammonium ion (NH₄⁺), originating from a weak base (NH3), reacts with water to produce H⁺ ions, lowering the solution's pH:
NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺

As a result, the hydronium ions (H₃O⁺) make the solution acidic.

• Always note which component, acid or base, contributes to the hydrolysis.
• Understand that the stronger acid's conjugate base will typically do nothing significant in the hydrolysis reaction.

Salts of Weak Acids and Strong Bases

When weak acids react with strong bases, the resulting salts tend to create basic solutions. Sodium acetate (CH3COONa) is a classic example. In water, it dissociates as follows:
CH3COONa → CH₃COO⁻ + Na⁺
The sodium ion (Na⁺) doesn’t hydrolyze due to its origin from a strong base (NaOH). However, the acetate ion (CH₃COO⁻), from acetic acid (a weak acid), reacts with water to produce OH⁻ ions, increasing the solution's pH:
CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻

The presence of hydroxide ions (OH⁻) turns the solution basic.

• Note that the conjugate base of a weak acid will engage in a hydrolysis reaction.
• Recognize that the overall solution becomes basic due to the release of OH⁻ ions.

Salts of Weak Acids and Weak Bases

Salts coming from both weak acids and weak bases exhibit unique hydrolysis characteristics. Ammonium acetate (NH4CH3COO) serves as an example. It dissolves in water as follows:
NH4CH3COO → NH₄⁺ + CH₃COO⁻
Here, both ions can hydrolyze because they are from weak acid (acetic acid) and weak base (ammonia), respectively. Thus,
NH₄⁺ + H₂O ⇌ NH₃ + H₃O⁺
and
CH₃COO⁻ + H₂O ⇌ CH₃COOH + OH⁻

The extent of acidity or basicity will depend on the relative strengths of the original weak acid and weak base.

• Balance between the ionization constants (Ka and Kb) will determine the final pH.
• Observe that either acidic or basic traits may dominate depending on the equilibrium position.