Question

Explain why a mixture of \(\mathrm{CH}_{3} \mathrm{COOH}\) and \(\mathrm{CH}_{3} \mathrm{COONa}\) can act as a buffer while a mixture of \(\mathrm{HCl}\) and \(\mathrm{NaCl}\) cannot.

Short answer

A mixture of CH₃COOH (acetic acid) and CH₃COONa (sodium acetate) can act as a buffer because it contains a weak acid and its conjugate base, allowing it to resist significant changes in pH when small amounts of strong acids or bases are added. In contrast, a mixture of HCl (hydrochloric acid) and NaCl (sodium chloride) cannot act as a buffer because it involves a strong acid and its conjugate base, which lacks buffering capacity, leading to significant changes in pH upon the addition of strong acids or bases.

Step-by-step solution

1. Description of the mixtures

First, let's describe the two mixtures: - A mixture of CH₃COOH (acetic acid) and CH₃COONa (sodium acetate): This mixture contains a weak acid (acetic acid) and its sodium salt, sodium acetate, which behaves as a conjugate base. - A mixture of HCl (hydrochloric acid) and NaCl (sodium chloride): Hydrochloric acid is a strong acid, and sodium chloride is the sodium salt of its conjugate base, chloride ion (Cl⁻).

2. Buffer characteristics

For a mixture to act as a buffer, it must contain a weak acid along with its conjugate base, or a weak base along with its conjugate acid. This is because weak acids and bases do not completely ionize in a solution; hence, they can react with added strong acids or bases, neutralizing their effect and resisting a significant change in pH.

3. Analyzing CH₃COOH and CH₃COONa mixture

Since acetic acid (CH₃COOH) is a weak acid, it does not completely dissociate into ions in the solution. When sodium acetate (CH₃COONa) is added to the mixture, it dissociates completely, yielding CH₃COO⁻ ions. These ions are the conjugate base of the weak acid CH₃COOH, and they will react with additional strong acids or bases to maintain pH. For example, when a strong acid is added to the mixture, the added H⁺ ions will react with the CH₃COO⁻ ions, forming CH₃COOH and effectively neutralizing the added acid: \(CH₃COO^⁻ + H^+ \rightarrow CH₃COOH\) Similarly, when a strong base is added, the OH⁻ ions will react with CH₃COOH, forming CH₃COO⁻ and H₂O: \(CH₃COOH + OH^⁻ \rightarrow CH₃COO^⁻ + H₂O\) In both cases, the acetic acid-sodium acetate mixture is able to resist significant changes in pH, making it a buffer.

4. Analyzing HCl and NaCl mixture

In contrast to the first mixture, HCl is a strong acid. It completely ionizes in a solution: \(HCl \rightarrow H^+ + Cl^⁻\) Sodium chloride (NaCl) is the sodium salt of the conjugate base (Cl⁻) of strong acid HCl. It dissociates completely in a solution: \(NaCl \rightarrow Na^+ + Cl^⁻\) When a strong base is added to the mixture, the additional OH⁻ ions will react with the H⁺ ions to form water: \(H^+ + OH^⁻ \rightarrow H₂O\) However, the reaction will not be buffered, since Cl⁻ is not the conjugate base of a weak acid; it does not have the capacity to consume additional H⁺ ions released by the strong acid HCl. The pH of the solution will change significantly upon the addition of either a strong acid or a base, meaning that this mixture cannot act as a buffer. In conclusion, a mixture of acetic acid and sodium acetate (CH₃COOH and CH₃COONa) can act as a buffer because it contains a weak acid and its conjugate base, while a mixture of hydrochloric acid and sodium chloride (HCl and NaCl) cannot act as a buffer because it contains a strong acid and its conjugate base, which lacks buffering capacity.

Key concepts

Acid-Base Equilibrium in Buffer Solutions

Understanding acid-base equilibrium is essential for grasping how buffer solutions work. A buffer solution is a special chemical system that resists changes in pH when small amounts of an acid or a base are added. At the heart of this phenomenon is an equilibrium between a weak acid or base and its conjugate pair.

For instance, consider a solution containing acetic acid, a weak acid that partially dissociates in water. The reaction can be represented as:

(CH₃COOH ↔ CH₃COO⁻ + H⁺)

When a strong acid (like HCl) is added, it increases the concentration of H⁺ ions, which would tend to lower the pH. However, in the buffer solution, these extra H⁺ ions will mostly combine with the acetate ions (CH₃COO⁻) to form more undissociated acetic acid, hence the pH does not drop significantly. This is a prime example of acid-base equilibrium at work, demonstrating the buffer's ability to 'soak up' added acidity without drastically altering the pH.

The Role of Weak Acids and Bases in Buffer Solutions

Let's delve into the characteristics of weak acids and bases. Unlike strong acids and bases, which completely dissociate in water, weak acids and bases only partially dissociate. This incomplete dissociation is what makes them such key components in buffer solutions.

For example, acetic acid (CH₃COOH) in vinegar is weak because it doesn't fully ionize in the solution. Its ions, CH₃COO⁻ and H⁺, reach a state of dynamic equilibrium with the undissociated molecules of acetic acid. Addition of sodium acetate (CH₃COONa), which provides more CH₃COO⁻ ions, strengthens the buffer as the system can now counter both acids and bases. When a base is added, these acetate ions react to neutralize the base and maintain the pH. Similarly, if an acid is introduced, the acetic acid can neutralize it. Thus, a buffer relies on the presence of these weak interactors to maintain its equilibrium and keep the pH steady.

Conjugate Acid-Base Pairs and Buffer Capacity

Conjugate acid-base pairs are at the core of buffering action. In a buffer solution, the solution contains a weak acid and its conjugate base, or a weak base and its conjugate acid. A conjugate base is formed when an acid loses a proton (H⁺), and a conjugate acid is formed when a base gains a proton.

Take the buffer consisting of acetic acid and its conjugate base, acetate. When acetic acid dissociates, it forms H⁺ and CH₃COO⁻. If a strong acid is added to the system, the H⁺ from the strong acid will combine with CH₃COO⁻, thereby reducing the impact on pH. Conversely, if a base is added, the acetic acid will donate a proton to the OH⁻ from the base, neutralizing it and again resisting a pH change.

Importantly, chloride ion (Cl⁻), the conjugate base of strong hydrochloric acid (HCl), is not effective in a buffer as it is a much weaker base than acetate and does not recombine readily with hydrogen ions. That's why HCl and NaCl cannot form a buffer system. The ability of conjugate pairs to buffer a solution is intrinsic to their strength and behavior in aqueous solutions.