Question

Determine whether or not each mixture is a buffer. (a) \(\mathrm{HCl}\) and \(\mathrm{HF}\) (b) \(\mathrm{NaOH}\) and \(\mathrm{NH}_{3}\) (c) HF and NaF (d) \(\mathrm{HC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\) and \(\mathrm{KC}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\)

Short answer

Only mixtures (c) HF and NaF and (d) HC2H3O2 and KC2H3O2 are buffers, as they consist of a weak acid and its conjugate base.

Step-by-step solution

Understanding what a buffer is

A buffer solution is one that can resist changes in pH when small amounts of an acid or a base are added. A common way to make a buffer is by mixing a weak acid with its conjugate base, or a weak base with its conjugate acid.

Analyzing mixture (a): HCl and HF

HCl is a strong acid and HF is a weak acid. Since HCl dissociates completely, it won't pair with the fluoride ion (F-) to form a buffer. A strong acid cannot be part of a buffering pair.

Analyzing mixture (b): NaOH and NH3

NaOH is a strong base and NH3 (ammonia) is a weak base. When NaOH dissociates, it creates OH- ions that do not form a buffer with NH3. Therefore, this mixture also does not constitute a buffer.

Analyzing mixture (c): HF and NaF

HF is a weak acid and NaF is a salt that dissociates into Na+ and F- ions in solution. The fluoride ion is the conjugate base of HF. Therefore, this mixture of a weak acid and its conjugate base can act as a buffer solution.

Analyzing mixture (d): HC2H3O2 and KC2H3O2

HC2H3O2 (acetic acid) is a weak acid, and KC2H3O2 (potassium acetate) is a salt that will dissociate into K+ and C2H3O2- ions. Here, C2H3O2- is the conjugate base of acetic acid, making this mixture a buffer solution.

Key concepts

Conjugate Acid-Base Pair

In acid-base chemistry, a conjugate acid-base pair consists of two species that transform into each other by the gain or loss of a proton (H⁺). This concept is pivotal in understanding why certain mixtures can act as buffers. For example, in our buffer exercise, HF (hydrofluoric acid) and its corresponding conjugate base F^- (fluoride ion), which comes from NaF (sodium fluoride), form such a pair. When HF loses a proton, it turns into F^-, and the F^- can gain a proton to become HF again. This back-and-forth helps maintain pH levels when acids or bases are added to a solution.

Conjugate pairs are necessary for a solution to be a buffer, as they neutralize additional acid or base. If a strong acid like HCl is part of a mixture, it disrupts balance by completely dissociating, leaving no partnership for buffering action. As we can see with HF and NaF, the weak acid and its conjugate base are more suited to the task.

pH Stability

pH stability refers to the capacity of a solution to maintain its pH level, even when small quantities of acid or base are introduced. This property is essential for many biological and chemical processes that require a consistent pH environment. Buffer solutions, such as the HF and NaF mixture, exemplify this stability. In contrast, mixtures without a conjugate acid-base pair, like HCl and HF or NaOH and NH₃, lack this resistance and thus their pH can change significantly with the addition of acids or bases.

Buffer solutions owe their pH stability to the presence of both a weak acid or base and its conjugate form. When an acid is added, the conjugate base neutralizes it, and when a base is added, the weak acid neutralizes it, helping the solution to maintain a relatively constant pH.

Chemistry of Buffers

Buffer solutions play a crucial role in biochemical systems and industrial applications because of their ability to resist pH changes. The chemistry of buffers is rooted in their components: typically a weak acid and its conjugate base or a weak base and its conjugate acid. In the exercises, we see that mixtures (c) HF and NaF, and (d) HC₂H₃O₂ and KC₂H₃O₂ function as buffer solutions.

These buffers work by having the weak acid react with any added bases and the conjugate base react with any added acids. For instance, if we add hydroxide ions (OH^-) to a buffer made from acetic acid and sodium acetate, the acetate ion (C₂H₃O₂^-) reacts with the OH^- to form water and acetic acid, therefore minimizing the change in pH. This dynamic equilibrium is what gives buffer solutions their important characteristics.

Acid-Base Chemistry

Acid-base chemistry is a subset of chemistry that deals with acids, bases, their reactions, and their equilibrium in solutions. It tells us that acids release protons (H⁺) and bases accept them. The strength of an acid or base is determined by how completely it dissociates in water. Strong acids, like HCl, fully dissociate and provide no buffer qualities. In contrast, weak acids, such as HF or acetic acid (HC₂H₃O₂), do not completely dissociate, allowing them to form conjugate acid-base pairs with their salts.

Understanding the dissociation and the constants associated with acids and bases (Ka and Kb) is critical when designing buffer solutions. In summary, a suitable buffer solution typically contains a weak acid or base along with its conjugate counterpart, enabling it to resist changes in pH—a fundamental aspect of acid-base chemistry crucial for maintaining the conditions necessary for various chemical reactions and biological functions.