Advanced Placement (AP) Chemistry Practice Exam

In the context of buffers, what do weak acids or bases do?

1. They completely dissociate in water to form strong ions.

2. They react with strong acids or bases to stabilize pH changes.

3. They create a low-pH environment for chemical reactions.

4. They prevent any change in the system's temperature.

The correct answer is: They react with strong acids or bases to stabilize pH changes.

Weak acids and weak bases play a critical role in buffer solutions by reacting with strong acids or bases to minimize changes in pH when small amounts of acids or bases are added to the solution. A buffer typically consists of a weak acid and its conjugate base or a weak base and its conjugate acid. When a strong acid is added to a buffer solution, the weak base component of the buffer will react with the strong acid, effectively neutralizing it and limiting the increase in hydrogen ion concentration. Conversely, if a strong base is introduced, the weak acid in the buffer will react with the strong base, reducing the increase in hydroxide ion concentration. This ability to moderate pH changes is essential in many biological and chemical systems where maintaining a stable pH is crucial for proper functioning. The other options discuss misconceptions about the behavior of weak acids and bases. For instance, they do not completely dissociate in water (the first choice), nor do they necessarily create an acid or basic environment exclusively for chemical reactions (the third choice). Additionally, while buffers can influence temperature in some contexts, they do not outright prevent temperature changes (the fourth choice).