Question

One way to make an acetic acid buffer is to mix substantial amounts of acetic acid and its salt, sodium acetate, in water. Another way to make the same buffer is to add a substantial amount of acetic acid to water and then add half as much \(\mathrm{NaOH}\) to the water. Explain how and why this latter procedure makes a buffer.

Short answer

Adding half as much \(\mathrm{NaOH}\) to a solution containing acetic acid results in a reaction that produces both sodium acetate and water, while some acetic acid remains unreacted. The solution now contains both acetic acid and sodium acetate, like in the initial buffer system, which enables it to resist changes in pH when small amounts of acid or base are added. Hence, the latter procedure creates a buffer solution for the same reason as directly mixing acetic acid and sodium acetate.

Step-by-step solution

Understanding buffer solutions

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

Examining the reaction between acetic acid and sodium acetate

When acetic acid (\(\mathrm{CH_3COOH}\)) and sodium acetate (\(\mathrm{CH_3COONa}\)) are mixed in water, sodium acetate dissociates into sodium ions (\(\mathrm{Na^+}\)) and acetate ions (\(\mathrm{CH_3COO^-}\)). Acetate ions are the conjugate base of acetic acid and can react with hydrogen ions (\(\mathrm{H^+}\)) to form acetic acid. By providing both the weak acid and its conjugate base, this system can resist changes in pH.

Analyzing the reaction between acetic acid and \(\mathrm{NaOH}\)

When acetic acid is mixed with a strong base like sodium hydroxide (\(\mathrm{NaOH}\)), the base reacts with the acid to form water and a salt. In this case, the reaction is as follows: \(\mathrm{CH_3COOH + NaOH \rightarrow H_2O + CH_3COONa}\) The salt produced is sodium acetate, the same salt used to create the buffer in the first method.

Understanding why adding half as much \(\mathrm{NaOH}\) can create a buffer

Adding half as much \(\mathrm{NaOH}\) means that only half of the acetic acid reacts with the \(\mathrm{NaOH}\), producing sodium acetate while some acetic acid remains unreacted. This mixture now contains substantial amounts of both acetic acid and sodium acetate, just like the initial buffer system. Since we now have both the weak acid (acetic acid) and its conjugate base (acetate ion from sodium acetate) present, this solution can also act as a buffer system, resisting changes in pH when small amounts of acid or base are added. Thus, the latter procedure creates a buffer solution for the same reason as mixing acetic acid and sodium acetate directly.

Key concepts

Acetic Acid Buffer

An acetic acid buffer is a specific type of buffer solution that includes acetic acid, a weak acid, and its conjugate base, the acetate ion. Buffer solutions are designed to maintain a stable pH even when small quantities of an acid or base are introduced. When acetic acid and sodium acetate are combined, they form a buffer system that minimizes pH fluctuations due to the interactions between the weak acid component and its conjugate base.
The presence of both acetic acid and acetate ions allows the solution to neutralize added acids and bases. This makes acetic acid buffers particularly useful in laboratory settings and various chemical applications where pH stability is crucial.

pH Resistance

Buffer solutions exhibit impressive pH resistance, meaning they can fight off significant changes in pH levels when exposed to acids or bases. This resistance is due to the balanced presence of a weak acid and its conjugate base. For example, in an acetic acid buffer, both acetic acid and acetate ions are present. When an acid is added, acetate ions can absorb the excess hydrogen ions, preventing a significant pH shift.
On the other hand, if a base is added, acetic acid can donate hydrogen ions to neutralize the base's effect. This dynamic system allows buffers to stabilize pH levels, making them invaluable in maintaining the proper conditions for chemical reactions and biological processes.

Weak Acid and Conjugate Base

A buffer system primarily comprises two components: a weak acid and its conjugate base. Acetic acid (\(\mathrm{CH_3COOH} \)) acts as the weak acid in acetic acid buffers. Its conjugate base, the acetate ion (\(\mathrm{CH_3COO^-} \)), is a result of the weak dissociation of acetic acid in water.
The balance between these two ensures that the buffer can effectively neutralize any additional acid or base. The weak acid part donates hydrogen ions when needed, and the conjugate base accepts them when excess hydrogen ions are present. This relationship is crucial to the buffer's function, allowing it to maintain consistent pH levels.

Sodium Acetate

Sodium acetate is an essential component in forming a buffer with acetic acid. It acts as a source of acetate ions, which are the conjugate base of acetic acid. When sodium acetate is dissolved in water, it separates into sodium ions (\(\mathrm{Na^+} \)) and acetate ions (\(\mathrm{CH_3COO^-} \)).
These acetate ions are vital to the buffer's functionality as they can combine with hydrogen ions when additional acid is introduced. This neutralizes the acid's effect, preventing a pH change. Sodium acetate not only contributes to the buffer system's ability to maintain pH but also enriches the solution with ions necessary for many chemical interactions.

Acid-Base Reactions

In the context of buffer solutions, understanding acid-base reactions is crucial. When acetic acid and a strong base like sodium hydroxide (\(\mathrm{NaOH} \)) interact, a neutralization reaction occurs. During this process, acetic acid reacts with \(\mathrm{NaOH} \) to produce water and sodium acetate. This newly-formed sodium acetate contributes to the buffer system.\[\mathrm{CH_3COOH + NaOH \rightarrow H_2O + CH_3COONa}\]
Additionally, when only half of the acetic acid is neutralized by \(\mathrm{NaOH} \), both acetic acid and sodium acetate coexist in the solution. This co-presence is what establishes the buffer system. Thus, an understanding of how acids and bases interact underpins the mechanism by which buffers resist pH changes and maintain stability.