Question

. A(n) \(\quad\) solution contains a conjugate acid-base pair and through this is able to resist changes in its pH.

Short answer

A solution containing a conjugate acid-base pair is called a buffer solution, which is able to resist changes in pH when small amounts of acids or bases are added to them. This is due to the presence of both a weak acid and its conjugate base in the solution. The conjugate acid-base pair can react with added acids or bases to neutralize them while maintaining the overall pH of the solution. For example, if HCl (a strong acid) is added to a buffer solution containing acetic acid and its conjugate base (acetate ion), the acetate ions will react with the added H+ ions to form more acetic acid, thus maintaining the pH. Similarly, if NaOH (a strong base) is added, the acetic acid will react with the OH- ions to form water and acetate ions, again maintaining the overall pH of the solution.

Step-by-step solution

Definition of Conjugate Acid-Base Pair

A conjugate acid-base pair consists of two species that are related by the transfer of a proton. When an acid donates a proton (H+) to a base, it forms a conjugate base and conjugate acid, respectively. For example, when an acid HA (where A represents the anion) donates a proton to water, it forms its conjugate base A- and hydronium ion H3O+ (the conjugate acid in this case). HA + H2O → A- + H3O+ The reverse reaction can also occur, where A- (acting as a base) takes up a proton from the hydronium ion, forming HA (acid) and water. A- + H3O+ → HA + H2O

Buffer Solutions

A solution that contains a conjugate acid-base pair is called a buffer solution. Buffer solutions are able to resist changes in pH when small amounts of acids or bases are added to them. This is due to the presence of both a weak acid and its conjugate base in the solution. The conjugate acid-base pair can react with added acids or bases to neutralize them while maintaining the overall pH of the solution. For example, consider a buffer solution that contains equal concentrations of acetic acid (CH3COOH) and its conjugate base, the acetate ion (CH3COO-).

Addition of Acid to a Buffer Solution

If a small amount of strong acid, such as HCl, is added to this buffer solution, the acetate ions will react with the added H+ ions to form more acetic acid, while consuming the added strong acid: CH3COO- + H+ → CH3COOH By consuming the added acid and converting it into its weak counterpart, the buffer maintains the overall pH of the solution.

Addition of Base to a Buffer Solution

Similarly, if a small amount of strong base, such as NaOH, is added to the buffer solution, the acetic acid will react with the OH- ions to form water and acetate ions: CH3COOH + OH- → CH3COO- + H2O Again, the buffer neutralizes the added strong base by converting it into its weak counterpart, maintaining the overall pH of the solution.

Key concepts

Conjugate Acid-Base Pair

A conjugate acid-base pair is an important concept in chemistry for understanding the behavior of acids and bases. When an acid donates a proton (H⁺) to a base, it results in the formation of a conjugate base. Conversely, the base gains a proton to become a conjugate acid. This relationship is crucial because it allows reversible reactions where protons can be transferred back and forth between species. For example, if you have an acid, like hydrochloric acid (HCl), and it donates a proton to water, it forms its conjugate base called chloride ion (Cl^-), and hydronium ion (H₃O⁺) as the conjugate acid.

• When acid HA donates a proton it becomes A^- (conjugate base)
• Water, acting as a base, becomes H₃O⁺ (conjugate acid)

This interchange highlights the delicate balance in acid-base chemistry, enabling buffer solutions to function effectively.

pH Resistance

Buffer solutions are remarkable for their ability to resist pH changes, a property known as pH resistance. This resistance is achieved by the presence of a weak acid and its conjugate base within the solution. When acids or bases are added to a buffer solution, the conjugate acid-base pair reacts to neutralize these changes, thereby maintaining the overall pH at a relatively constant level. For example, if an acid is added, the base component of the buffer will react and convert it into a less harmful form (such as a corresponding weak acid). Conversely, if a base is added, the acid component of the buffer will react and neutralize it by forming water or a related weak base. This dynamic reaction helps in stabilizing the pH, which is crucial in many biological and chemical applications, keeping environments stable where large pH fluctuations could be harmful.

• Buffer solutions consist of a weak acid and its conjugate base
• They neutralize added acids or bases, maintaining pH balance

Acetic Acid and Acetate Ion

Acetic acid and its conjugate base, the acetate ion, form a classic example of a buffer system. Acetic acid has the chemical formula CH₃COOH, and when it donates a proton, it becomes the acetate ion, CH₃COO^-. This pair is effective in creating a buffer due to its ability to resist adds or removals of protons. In practical terms, if you add a strong acid like HCl to an acetic acid/acetate buffer, the acetate ion will readily accept the protons from HCl to form more acetic acid, which minimizes the change in pH:

• CH₃COO^- + H⁺ → CH₃COOH

Similarly, if a strong base like NaOH is added, acetic acid will donate protons to neutralize the base, thus forming more acetate ion and water:

• CH₃COOH + OH^- → CH₃COO^- + H₂O

This chemical dynamic provides a buffering action, making acetic acid and acetate solutions valuable in various applications where pH stability is required.