Question

Could a buffered solution be made by mixing aqueous solutions of HCl and NaOH? Explain. Why isn't a mixture of a strong acid and its conjugate base considered a buffered solution?

Short answer

A buffered solution cannot be made by mixing aqueous solutions of HCl (a strong acid) and NaOH (a strong base) because buffered solutions require the presence of a weak acid and its conjugate base, or a weak base and its conjugate acid. HCl and NaOH are both strong and, when mixed, they neutralize each other to form water and a salt (NaCl). Since there is neither a weak acid/base and its conjugate pair present, a buffered solution cannot be made. A strong acid and its conjugate base cannot form a buffered solution because the strong acid, such as HCl, dissociates completely in water, while its conjugate base (Cl-) is a very weak base and does not readily accept protons. Consequently, the addition of more acid or base to this solution would cause a significant change in pH, as the solution does not exhibit the pH resistance characteristic of buffered solutions.

Step-by-step solution

Understanding Buffered Solutions

A buffered solution is a solution that resists significant changes in pH when small amounts of an acid or a base are added to it. This resistance is due to the presence of a weak acid and its conjugate base, or a weak base and its conjugate acid. The most commonly used example of a buffer system is a mixture of a weak acid (like acetic acid) and its conjugate base (like the acetate ion).

Analyzing the given solutions

In this exercise, we are given two solutions: one containing HCl (hydrochloric acid) and the other containing NaOH (sodium hydroxide). Both HCl and NaOH are considered strong acids and strong bases, respectively. This means that they dissociate completely in water. \(HCl \rightarrow H^+ + Cl^-\) \(NaOH \rightarrow Na^+ + OH^-\)

Determining if a buffered solution can be made

Buffered solutions require a weak acid and its conjugate base, or a weak base and its conjugate acid. In our case, HCl and NaOH are both strong, not weak. When they are mixed, they neutralize each other, forming water and a salt. \(HCl + NaOH \rightarrow H_2O + NaCl\) Since we have neither a weak acid and its conjugate base nor a weak base and its conjugate acid, a buffered solution cannot be made. It would just be a neutral salt solution.

Explaining why a strong acid and its conjugate base cannot form a buffered solution

A strong acid like HCl dissociates completely in water, forming a relatively high concentration of hydrogen ions. However, its conjugate base (Cl-) is a very weak base, meaning it does not readily accept protons (H+) to become HCl again. Therefore, the addition of more acid or base would cause a significant change in pH, and the solution would not resist the change, which is the main characteristic of a buffered solution.

Key concepts

Weak Acid and Conjugate Base

Buffered solutions are pivotal in many chemical and biological processes because they maintain a stable pH despite the addition of acids or bases. A key feature of such solutions is the presence of a weak acid and its conjugate base. Weak acids, unlike strong acids, do not dissociate completely in water. Instead, they establish an equilibrium between the acid and its conjugate base.

For example, acetic acid (\(CH_3COOH\)) is a weak acid that partially dissociates to form acetate ions (\(CH_3COO^-\)) and hydrogen ions (\(H^+\)). This equilibrium allows the buffered solution to negate fluctuations in pH. When an acid is added, the conjugate base can react with the additional hydrogen ions, minimizing the pH change. Similarly, if a base is added, the weak acid can donate hydrogen ions to counter the base, again resisting significant pH shifts. This harmonious interplay is why a mixture of a weak acid and its conjugate base serves as an effective buffer.

Resistance to pH Change

The resistance to pH change in buffered solutions is a characteristic that essentially defines them. It’s important to understand that pH is a measure of the hydrogen ion (\(H^+\)) concentration in a solution. In a buffer system, when these ions are added or removed, the weak acid-conjugate base pair or the weak base-conjugate acid pair mitigates the change by absorbing or releasing hydrogen ions or hydroxide ions (\(OH^-\)), respectively.

Consider the acetic acid/acetate buffer again. If we add \(H^+\) ions to this buffered solution, the acetate ions (\(CH_3COO^-\)) available will 'capture' these extra ions to form acetic acid, keeping the pH relatively stable. Conversely, if \(OH^-\) ions are introduced, the acetic acid can donate \(H^+\) to neutralize \(OH^-\) and form water, therefore, preventing the pH from rising significantly. This dynamic equilibrium is the buffer's main defense mechanism against pH alterations.

Strong Acid and Strong Base Neutralization

In contrast to buffered solutions, a mixture of a strong acid and a strong base leads to complete neutralization, which is vastly different from the buffering action we’ve discussed. Strong acids, such as hydrochloric acid (\(HCl\)), ionize fully in water to yield a high concentration of \(H^+\) ions. When they come into contact with a strong base like sodium hydroxide (\(NaOH\)), which also disassociates completely to give \(OH^-\) ions, the two react to form water (\(H_2O\)) and a neutral salt (in this case, sodium chloride, or NaCl).

During this neutralization reaction, none of the reactants retain their acid or base characteristics, as they form completely new substances. This reaction doesn't have the give-and-take relationship necessary for a buffered solution because once neutralized, there's no weak acid or weak base present to counteract further additions of \(H^+\) or \(OH^-\) ions, meaning the solution's pH will drastically shift with the addition of more acid or base. Therefore, the neutralized solution of a strong acid and strong base lacks the key properties to qualify as a buffer.