Question

The formation of a buffer solution is an example of the common-ion effect. Explain how a buffer works with reference to a solution containing acetic acid and sodium acetate.

Short answer

A buffer solution of acetic acid and sodium acetate stabilizes pH through the common-ion effect by resisting changes caused by added acid or base through a balance of the acid-base equilibrium.

Step-by-step solution

Define a Buffer Solution

A buffer solution is a solution that resists changes in pH when small amounts of acid or base are added. It is typically made from a weak acid and its conjugate base, or a weak base and its conjugate acid. In this case, our buffer solution is made of acetic acid (a weak acid) and sodium acetate (its conjugate base).

Understand Acetic Acid and Sodium Acetate Equilibrium

Acetic acid ( ext{CH}_3 ext{COOH}) partially ionizes in water to form acetate ions ( ext{CH}_3 ext{COO}^-) and hydronium ions ( ext{H}_3 ext{O}^+). The equilibrium can be represented by the equation: \[ ext{CH}_3 ext{COOH} ightleftharpoons ext{CH}_3 ext{COO}^- + ext{H}^+ \]Adding sodium acetate introduces more ext{CH}_3 ext{COO}^- ions into the solution.

Explain Common-Ion Effect

The common-ion effect occurs when a second compound providing a common ion is added to the solution, altering the equilibrium position. Here, adding sodium acetate increases the concentration of acetate ions ( ext{CH}_3 ext{COO}^-), thus shifting the equilibrium toward the left (the direction that forms more acetic acid), decreasing dissociation of acetic acid and maintaining a stable pH.

Describe Buffer Action Against Added Acid

When a small amount of an acid is added to the buffer solution, ext{H}^+ ions from the acid react with the acetate ions ( ext{CH}_3 ext{COO}^-) to form more ext{CH}_3 ext{COOH}, reducing the change in pH. Due to the large concentration of the conjugate base, the few added ext{H}^+ ions are effectively neutralized, minimizing any pH change.

Describe Buffer Action Against Added Base

Conversely, if a small amount of a base is added, the ext{OH}^- ions react with the hydronium ions to form water, causing more ext{CH}_3 ext{COOH} to dissociate and restore the hydronium ion concentration. Again, this process minimizes changes to the pH level, owing to the vast presence of the weak acid.

Key concepts

Common-Ion Effect

The common-ion effect plays a crucial role in understanding how buffer solutions work. It occurs when a solution containing a weak acid or base is combined with another substance providing the same ion as the weak component. In the case of a buffer solution containing acetic acid (\(\text{CH}_3\text{COOH}\)) and sodium acetate, the shared ion is the acetate ion (\(\text{CH}_3\text{COO}^-\)).

This phenomenon results in a shift in the equilibrium position. More specifically, when sodium acetate is added, it increases the concentration of acetate ions. According to Le Chatelier's Principle, this causes the equilibrium to shift towards the formation of more acetic acid and fewer ions, thus reducing the dissociation of acetic acid. Consequently, the pH remains relatively unchanged. This makes the solution more resistant to pH alterations when small amounts of acid or base are introduced.

Acetic Acid

Acetic acid is a weak acid, which means it does not completely dissociate in water. Its chemical formula is \(\text{CH}_3\text{COOH}\), and in aqueous solution, it partly ionizes to produce acetate ions (\(\text{CH}_3\text{COO}^-\)) and hydrogen ions (\(\text{H}^+\)).

The equilibrium reaction can be represented as:\[\text{CH}_3\text{COOH} \rightleftharpoons \text{CH}_3\text{COO}^- + \text{H}^+\]

In the buffer solution, this equilibrium is delicate. Acetic acid provides the capacity to neutralize added bases by supplying hydrogen ions that combine with hydroxide ions (\(\text{OH}^-\)) to form water. This clever balance is essential for maintaining a stable pH in the buffer.

Sodium Acetate

Sodium acetate is the salt of acetic acid. Its chemical formula is \(\text{CH}_3\text{COONa}\), and when dissolved in water, it breaks down into sodium ions (\(\text{Na}^+\)) and acetate ions (\(\text{CH}_3\text{COO}^-\)).

The role of sodium acetate in the buffer is to supply a significant amount of acetate ions, which are crucial for the common-ion effect. This addition reinforces the buffer's ability to resist pH changes. When additional hydrogen ions (\(\text{H}^+\)) are introduced from an external acid or when \(\text{OH}^-\) ions are added from a base, the acetate ions in the solution help neutralize these added ions. This mitigates any significant impact on the pH level.

pH Stability

The primary function of a buffer solution is to maintain pH stability, meaning it holds the pH at a near-constant level, even when small acids or bases are added. This stability is essential in many chemical processes and biological systems.

In a buffer solution made of acetic acid and sodium acetate, the copious amount of the conjugate base (acetate ions) allows the solution to effectively "soak up" added \(\text{H}^+\) ions from acids or \(\text{OH}^-\) ions from bases. This capacity to neutralize additional ions is what keeps the pH stable, as the shifts in pH that would typically occur are neatly countered by the equilibrium reactions.

In practical terms, buffer solutions like these are integral in fields ranging from biochemistry, where pH control is necessary for enzymes to function properly, to industrial processes that require strict pH ranges for optimal reactions efficiency.