Using the acid – base conjugate pair below as an example, explain how a buffer solution is able to resist change in pH when ( ) – 4 . 2 . 1 a small amount of 1 0 ml of 0 . 5 M NaOH ( aq ) is added. 4 . 2 . 2 a small amount of 1 0 ml of 0 . 5 M H C l ( a q ) is added.
Answer
🔬 Buffer Action of the CH3COOH / CH3COO− System
A buffer solution resists changes in pH upon the addition of small amounts of acid or base. The system given here involves acetic acid (CH3COOH), a weak acid, and its conjugate base acetate ion (CH3COO−).
📌 Case 1: Addition of NaOH (Aqueous Base)
- The OH⁻ ions would normally increase the pH by reducing H⁺ concentration.
- However, in a buffer solution, the added OH⁻ reacts with the available H⁺ ions to form water:
- As H⁺ is consumed, the equilibrium shifts to the right (according to Le Chatelier’s Principle) to produce more H⁺:
- This minimizes the increase in pH and keeps the solution buffered.
📌 Case 2: Addition of HCl (Aqueous Acid)
- These H⁺ ions would normally lower the pH.
- But the buffer system neutralizes this change as the H⁺ reacts with the conjugate base CH₃COO⁻ to form CH₃COOH:
- This reaction removes the added H⁺ ions and shifts the equilibrium to the left, preventing a significant drop in pH.
✅ Conclusion
The buffer system containing acetic acid and acetate ion effectively maintains a relatively constant pH by neutralizing added acids (H⁺) and bases (OH⁻) through reversible reactions. This showcases the core principle of buffer action in acid-base chemistry.