Question

Does the pH of the solution increase, decrease, or stay the same when you (a) add solid ammonium chloride to a dilute aqueous solution of \(\mathrm{NH}_{3} ?\) (b) add solid sodium acetate to a dilute aqueous solution of acetic acid? (c) add solid NaCl to a dilute aqueous solution of \(\mathrm{NaOH}\)

Short answer

(a) pH decreases, (b) pH increases, (c) pH stays the same.

Step-by-step solution

Understand the Concept

The pH of a solution depends on its hydrogen ion concentration. Adding compounds to the solution can affect this concentration by introducing new ions or reacting with existing ones.

Step (a): Ammonium Chloride and Ammonia Solution

Ammonium chloride ( NH_4Cl ) dissociates into NH_4^+ and Cl^- ions in water. NH_4^+ can donate a proton (H+) to the solution, forming NH_3 and causing the pH to decrease (more acidic). NH3 being a weak base doesn’t completely neutralize this effect.

Step (b): Sodium Acetate and Acetic Acid Solution

Sodium acetate ( CH_3COONa ) dissolves into CH_3COO^- and Na^+ ions in water. The CH_3COO^- ion reacts with H^+ from acetic acid ( CH_3COOH ) to form more acetate and water, decreasing the concentration of H^+ and causing the pH to increase (less acidic).

Step (c): NaCl and NaOH Solution

Sodium chloride ( NaCl ) dissolves into Na^+ and Cl^- ions in water; it is a neutral salt and does not react with the ions in NaOH solution. Therefore, the pH of this strong base solution remains unchanged.

Key concepts

pH of Solutions

The pH of a solution is a measure of its acidity or basicity. It is determined by the concentration of hydrogen ions ( H^+ ) present in the solution. A high concentration of H^+ ions indicates an acidic solution, and it has a low pH value. On the other hand, a low concentration of H^+ ions signifies a basic solution with a higher pH value. The pH scale ranges from 0 to 14, where 7 is neutral, values less than 7 are acidic, and values greater than 7 are basic. When adding substances to a solution, it is essential to consider how they might alter the concentration of H^+ in the solution:

• Substances that increase H^+ concentration decrease the pH, making the solution more acidic.
• Substances that decrease H^+ concentration increase the pH, making the solution more basic.
• Some substances, like neutral salts, may not affect the pH at all.

By understanding these principles, you can predict how the pH will change when different compounds are added to a solution.

Acid-Base Reactions

Acid-base reactions involve the transfer of protons ( H^+ ions) between molecules. This transfer changes the acidity or basicity of the substances involved.

• An acid is a molecule that donates H^+ ions, increasing the acidity of the solution. Acids typically have a pH less than 7.
• A base accepts H^+ ions, reducing acidity and increasing the pH of the solution. Bases typically have a pH greater than 7.

In a neutralization reaction, an acid reacts with a base to form water and a salt. This reaction can significantly change the pH of the solution. For example, when ammonium chloride is added to an ammonia solution, the ammonium ions can donate protons to the ammonia, which affects the balance of the solution and decreases the pH. This is due to ammonia's inability to fully neutralize the added protons because it is a weak base. Understanding these interactions helps to predict whether a reaction will result in a more acidic or basic environment.

Buffer Solutions

Buffer solutions are vital in maintaining a stable pH level. These solutions are composed of a weak acid and its conjugate base, or a weak base and its conjugate acid. Buffers resist significant changes in pH when small amounts of acids or bases are added. This buffering action occurs because:

• The weak acid can neutralize added bases, minimizing pH increases.
• The conjugate base can neutralize added acids, minimizing pH decreases.

In the case of adding sodium acetate to acetic acid, the solution forms a buffer. The acetate ions ( CH_3COO^- ) react with any excess H^+ ions from acetic acid, reducing the overall H^+ concentration and slightly increasing the pH. Buffer solutions are crucial in various biological and chemical processes where pH stability is essential, such as in biological fluids and enzyme-catalyzed reactions. This buffering property ensures that processes can proceed efficiently without fluctuations caused by changing acidity or basicity.