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 solution because it contains a weak acid and its conjugate base, allowing it to resist pH changes when small amounts of an acid or base are added. However, a mixture of HCl (hydrochloric acid) and NaCl (sodium chloride) cannot act as a buffer solution since it contains a strong acid and its conjugate base, which do not have the ability to resist pH changes.

Step-by-step solution

Understanding buffer solutions

A buffer solution is a mixture of a weak acid and its conjugate base, or a weak base and its conjugate acid. Buffer solutions resist a change in pH when small amounts of an acid or base are added to them. The capacity of a buffer solution to resist pH changes depends on the presence of both the weak acid/base and its corresponding conjugate base/acid in equilibrium. Buffer solutions can be made from two components: 1. A weak acid (HA) and its conjugate base (A-) in a solution 2. A weak base (B) and its conjugate acid (HB+) in a solution The ability of a buffer solution to resist pH changes depends on two factors: 1. The concentration ratio of weak acid/base to its conjugate base/acid 2. The acid dissociation constant (Ka) of the weak acid/base

Analyzing the mixture of acetic acid and sodium acetate

The mixture of CH3COOH (acetic acid) and CH3COONa (sodium acetate) contains a weak acid and its conjugate base. Acetic acid is a weak acid that partially ionizes in water, forming the acetate anion (CH3COO-) and the hydronium ion (H3O+): \[CH_3COOH \rightleftharpoons CH_3COO^{-} + H_3O^+\] Sodium acetate is a salt that dissociates completely in water, releasing the acetate anion (CH3COO-) and the sodium cation (Na+): \[CH_3COONa \rightarrow CH_3COO^{-} + Na^{+}\] In the mixture, we have both the weak acid (CH3COOH) and its conjugate base (CH3COO-). When an acid or base is added to this mixture, the weak acid and its conjugate base can react to minimize pH changes. Therefore, this mixture can act as a buffer solution.

Analyzing the mixture of hydrochloric acid and sodium chloride

The mixture of HCl (hydrochloric acid) and NaCl (sodium chloride) contains a strong acid instead of a weak acid or base. Hydrochloric acid is a strong acid that completely ionizes in water, forming chloride anion (Cl-) and the hydronium ion (H3O+): \[HCl \rightarrow Cl^{-} + H_3O^+\] Sodium chloride is a salt that dissociates completely in water, releasing the chloride anion (Cl-) and the sodium cation (Na+): \[NaCl \rightarrow Cl^{-} + Na^{+}\] In the mixture, we have a strong acid (HCl) and its conjugate base (Cl-), which is a very weak base. However, the strong acid does not have the ability to resist pH changes when small amounts of an acid or base are added. Consequently, this mixture cannot act as a buffer solution.

Conclusion

A mixture of CH3COOH (acetic acid) and CH3COONa (sodium acetate) can act as a buffer solution because it contains a weak acid and its conjugate base. This mixture can resist pH changes when small amounts of an acid or base are added. On the other hand, a mixture of HCl (hydrochloric acid) and NaCl (sodium chloride) cannot act as a buffer solution because it contains a strong acid and its conjugate base, which do not have the ability to resist pH changes.

Key concepts

Weak Acid and Conjugate Base

In the world of chemistry, a buffer solution is a superhero of sorts. It steps in to keep the pH levels from changing dramatically. One of the vital aspects of a buffer is the presence of a weak acid and its conjugate base. This dynamic duo prevents large shifts in pH when acidic or basic substances are introduced.
For example, in a mixture of acetic acid (\( ext{CH}_3 ext{COOH} \)) and sodium acetate (\( ext{CH}_3 ext{COONa} \)), acetic acid acts as the weak acid. In water, acetic acid only partially dissociates, producing acetate ions (\( ext{CH}_3 ext{COO}^- \)) and hydrogen ions. Sodium acetate, on the other hand, provides additional acetate ions. This ensures that the solution has both the weak acid and its conjugate base.

• The weak acid component can react with added bases, neutralizing them by donating a proton.
• Similarly, the conjugate base can react with added acids, accepting protons and minimizing pH change.

Together, they maintain a balance, blocking abrupt pH changes whether acids or bases try to disrupt the equilibrium.

Acid Dissociation Constant

A fundamental piece in the buffer puzzle is the acid dissociation constant, denoted as \( K_a \). It’s a measure of how easily an acid donates protons in an aqueous solution. A small \( K_a \) value indicates a weak acid, implying that only a fraction of the acid molecules dissociate, maintaining a robust reserve to support buffering action.
In acetic acid's case, the \( K_a \)
is considered small, favoring the existence of both the undissociated acid and its conjugate base (\( ext{CH}_3 ext{COO}^- \)). This equilibrium is crucial because:

• The pH of a solution is directly linked to \( K_a \). A moderate change in this value (when molecules are added or subtracted) doesn't drastically alter pH.
• It dictates the strength and capability of a buffer to respond to external pH changes.

Consequently, knowing the \( K_a \) helps in predicting the behavior and efficiency of a buffer solution in maintaining pH stability.

pH Resistance

The unique ability of a buffer to resist changes in pH is what makes it vitally important in many biological and chemical processes. The mechanism of buffering involves the weak acid and its conjugate base acting in tandem to neutralize added acids or bases. This resisting action is aptly termed 'pH resistance'.
When a small amount of strong acid or base is introduced:

• The weak acid component neutralizes the added base as it donates a hydrogen ion.
• Conversely, the conjugate base neutralizes the added acid by accepting a hydrogen ion.

This back and forth interplay between the acid and its conjugate base makes the change in pH minimal.
However, for a mixture involving a strong acid and its conjugate base, as in the case of HCl and NaCl, pH resistance is absent. Strong acids completely dissociate, leaving no room for buffer action. Thus, understanding how weak acid-conjugate base buffers work ensures the effective use of buffers in both scientific and everyday applications where pH stability is paramount.