Which of the following occurs when HCl is added to a buffer containing (CH3)3N and (CH3)3NH+?
The correct answer and explanation is:
Correct Answer:
(CH₃)₃N reacts with HCl to form (CH₃)₃NH⁺.
Explanation (300 words):
A buffer solution is made from a weak base and its conjugate acid or from a weak acid and its conjugate base. In this case, the buffer contains trimethylamine ((CH₃)₃N), which is a weak base, and its conjugate acid trimethylammonium ion ((CH₃)₃NH⁺).
When a strong acid like hydrochloric acid (HCl) is added to this buffer, it introduces hydrogen ions (H⁺) into the solution. These hydrogen ions would normally lower the pH sharply in a solution that lacks a buffering component. However, the role of a buffer is to resist changes in pH by neutralizing added acids or bases.
In this buffer system, (CH₃)₃N acts as the base. It reacts with the added H⁺ ions from HCl according to this chemical reaction:
(CH₃)₃N + H⁺ → (CH₃)₃NH⁺
This reaction shows that the base (CH₃)₃N accepts the proton (H⁺) and gets converted into its conjugate acid (CH₃)₃NH⁺. Because the base component of the buffer consumes the added acid, the pH does not change significantly. Only a small change occurs due to the altered ratio of base to conjugate acid.
This is the key function of a buffer system: it uses the components present to neutralize added acids or bases. The result is a solution that maintains a relatively constant pH. Since the added HCl reacts specifically with (CH₃)₃N and not with (CH₃)₃NH⁺, the correct answer is that (CH₃)₃N reacts with HCl to form (CH₃)₃NH⁺, preserving the buffering action.
This behavior is typical of acid-base equilibrium reactions in buffer solutions, and it demonstrates the importance of having both components of the conjugate pair present to handle incoming acids or bases effectively.