Problem 98 - General Chemistry: Principles and Modern Applications

Question

Write equations to show how each of the following buffer solutions reacts with a small added amount of a strong acid or a strong base: (a) HCOOH-KHCOO; (b) \(\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{2}-\mathrm{C}_{6} \mathrm{H}_{5} \mathrm{NH}_{3}^{+} \mathrm{Cl}^{-}\) (c) \(\mathrm{KH}_{2} \mathrm{PO}_{4}-\mathrm{Na}_{2} \mathrm{HPO}_{4}\)

Step-by-Step Solution

Answer

In the presence of a strong acid, the base component of the buffer reacts to form its conjugate. When a strong base is present, the acid component reacts to form water and its conjugate.

Step 1: Reaction with strong acid

When an acid (represented here as H+) is added to a buffer solution, the base component of the buffer reacts to form its conjugate. Therefore, the reactions are: (a) \( KHCOO^- + H^{+} \rightarrow HCOOH \)(b) \( C_6H_5NH_2 + H^{+} \rightarrow C_6H_5NH_3^+ \)(c) \( Na_2HPO_4^- + H^{+} \rightarrow KH_2PO_4 \)

Step 2: Reaction with strong base

When a base (represented here as OH-) is added to the solution, the acid component of the buffer reacts to form water and its conjugate. Therefore, the reactions are: (a) \( HCOOH + OH^- \rightarrow H_2O + HCOO^- \)(b) \( C_6H_5NH_3^+Cl^- + OH^- \rightarrow H_2O + C_6H_5NH_2 \)(c) \( KH_2PO_4 + OH^- \rightarrow H_2O + Na_2HPO_4^- \)

Key Concepts

Acid-Base Reaction

An acid-base reaction involves the interaction of hydrogen ions ( H^+ ) with hydroxide ions ( OH^- ). This type of chemical reaction occurs when an acid donates a proton to a base, resulting in the formation of water and/or a conjugate acid-base pair. In the context of buffer solutions, these reactions are highly significant. When an acid is introduced to a buffer solution, the base component of the buffer reacts with the added hydrogen ions to neutralize the acid. Conversely, when a base is added, the acid part of the buffer will react with the hydroxide ions to neutralize the base. Thus, the pH of the solution remains relatively stable, which is the key role of buffers. The stability of pH is important in many biological and chemical processes that require a consistent environment.

Conjugate Acid-Base Pairs

In chemistry, a conjugate acid-base pair refers to two species that transform into each other by the gain or loss of a proton ( H^+ ). By definition, an acid becomes its conjugate base once it donates a proton, and a base becomes its conjugate acid once it gains a proton. Here’s how it works in a buffer solution:

• When a strong acid is added, the base component of the buffer accepts protons to form its conjugate acid.
• When a strong base is added, the acid component of the buffer donates protons to form its conjugate base.

For example, in the formic acid ( HCOOH ) and formate ( HCOO^- ) buffer, adding H^+ to formate forms more formic acid. Similarly, adding OH^- reacts with formic acid to regenerate formate. Thus, conjugate pairs play a vital role in the buffering action by continuously interconverting to balance the pH of the solution.

Buffer Capacity

Buffer capacity refers to the ability of a buffer solution to resist changes in pH when an acid or base is added. It's a measure of how well a buffer can neutralize added acids or bases. A strong buffer can handle significant additions of acids or bases with only a minimal change in pH. Several factors influence buffer capacity:

• The concentration of the acid and base components: Higher concentrations tend to result in a higher buffer capacity.
• The ratio of acid to base: Buffers are most effective when the acid and base are present in nearly equal amounts.

Buffer solutions with optimal capacity are crucial in environments where maintaining a consistent pH is necessary. Biological systems often rely on regions of high buffer capacity to ensure stable conditions for enzyme activity and cellular function.