18.3 Equilibria of Acids, Bases and Salts

Summary

This video explains the equilibria of acids, bases, and salts, focusing on the ionization constant of weak acids like acetic acid. It covers buffer solutions that resist pH changes, hydrolysis of salts, and how cation and anion hydrolysis affect pH levels. The video also explores how different types of salts—formed from strong or weak acids and bases—can impact the pH of a solution, with specific examples showing how weak acids and bases create acidic, basic, or neutral solutions depending on their strengths. The importance of equilibrium constants in these processes is emphasized throughout.

Takeaways

• 😀 Acetic acid only dissolves 1.3% of its molecules in water, forming hydronium and acetate ions.
• 😀 The equilibrium constant for acetic acid ionization is abbreviated as Ka, which excludes water since it's a constant in the reaction.
• 😀 Buffers are solutions that resist pH changes, often made from a weak acid and its salt, such as acetic acid and sodium acetate.
• 😀 Buffers work by neutralizing added hydronium ions with acetate ions or by neutralizing hydroxide ions with sodium or hydronium.
• 😀 Salts are formed from the neutralization of acids and bases, and their hydrolysis in water can affect the pH of the solution.
• 😀 In salt hydrolysis, cation hydrolysis produces an acidic solution, while anion hydrolysis produces a basic solution.
• 😀 When an anion undergoes hydrolysis, it reacts with water to form hydronium ions and increase the solution's acidity.
• 😀 Cation hydrolysis occurs when a cation reacts with water to produce excess hydronium, resulting in an acidic solution.
• 😀 The pH of a solution from salt hydrolysis depends on the strength of the ions in the salt and their concentrations.
• 😀 Neutralization of strong acids and bases results in a neutral pH (7), but neutralization involving weak acids or bases can produce pH values below or above 7, depending on the strength of the constituents.

Q & A

What is the ionization constant (KA) of acetic acid, and how is it derived?

The ionization constant (KA) of acetic acid represents the equilibrium constant for its dissociation in water, where acetic acid (CH3COOH) ionizes into hydronium (H3O+) and acetate ions (CH3COO-). KA is derived by removing the water (H2O) from the equilibrium expression because it is a pure liquid and doesn't affect the constant. The KA is the abbreviated form of the acid dissociation constant, calculated by the ratio of the concentrations of hydronium and acetate over the concentration of undissolved acetic acid.

What is a buffer, and how does it resist changes in pH?

A buffer is a solution that resists changes in pH when small amounts of acid or base are added. It typically consists of a weak acid and its corresponding salt (or a weak base and its salt). For example, acetic acid and sodium acetate form a buffer solution, where acetic acid can neutralize added base, and acetate can neutralize added acid, thus maintaining a stable pH.

How does the concentration of hydronium (H3O+) affect the pH of a buffer solution?

The concentration of hydronium (H3O+) determines the pH of a solution, and in the case of a buffer, this concentration remains relatively constant even when small amounts of acid or base are added. The buffer's ability to neutralize excess acid or base helps maintain this constant concentration of hydronium, preventing significant changes in pH.

What is the difference between anion hydrolysis and cation hydrolysis?

Anion hydrolysis occurs when an anion (such as acetate from a salt like sodium acetate) reacts with water to produce hydronium (H3O+) and increase the concentration of hydroxide (OH-), resulting in a basic solution. Cation hydrolysis, on the other hand, occurs when a cation (such as sodium from sodium chloride) interacts with water to form hydronium ions, leading to an acidic solution.

Why do salts formed from a strong acid and a strong base not significantly affect the pH of a solution?

Salts formed from a strong acid and a strong base, such as sodium chloride (NaCl), do not significantly affect the pH of a solution because they dissociate completely in water into their respective ions (e.g., Na+ and Cl-). These ions do not react with water to produce significant amounts of hydronium or hydroxide ions, so the pH remains neutral (around 7).

How do weak acids and weak bases in salts influence the pH of a solution?

When salts form from weak acids and weak bases, they can either increase or decrease the pH depending on the strength of the acid and base involved. For example, if a salt is formed from a weak acid and a strong base, the resulting solution will be basic because the anion (from the acid) hydrolyzes to form excess hydroxide ions. Conversely, a salt formed from a strong acid and a weak base will create an acidic solution due to cation hydrolysis.

What happens during cation hydrolysis and how does it affect pH?

Cation hydrolysis occurs when a cation (from a salt formed by a strong base and weak acid) reacts with water to form hydronium ions (H3O+), leading to an acidic solution. The extent of pH change depends on the strength of the cation and its ability to interact with water to donate protons.

How do the relative strengths of acids and bases determine the pH of a salt solution?

The relative strengths of the acids and bases from which a salt is formed influence the pH of its solution. A salt formed from a strong acid and a strong base will generally result in a neutral solution, while salts from weak acids or weak bases may result in either acidic or basic solutions depending on the hydrolysis reactions of the ions in solution.

What are the four types of salts that can form based on the acid-base pair involved?

The four types of salts that can form based on the acid-base pair involved are: 1) salts from a strong acid and a strong base, resulting in a neutral solution; 2) salts from a strong acid and a weak base, leading to an acidic solution; 3) salts from a weak acid and a strong base, producing a basic solution; and 4) salts from a weak acid and a weak base, where the pH can vary depending on the relative strengths of the acid and base.

How does the ionization constant of acetic acid (KA) relate to the concentration of hydronium and acetate ions in solution?

The ionization constant (KA) for acetic acid is calculated by the ratio of the concentrations of hydronium (H3O+) and acetate (CH3COO-) ions over the concentration of undissolved acetic acid (CH3COOH). This equilibrium expression reflects the degree of dissociation of acetic acid in water, which is relatively low (only about 1.3% of acetic acid molecules dissociate).