Consider an acetate buffer system made up of acetic acid and sodium acetate, initially at the same pH as its pka (4.76). When small amount of base is added to this buffer which of the following reactions is most likely to occur which will prevent increase in pH of the buffer? O a. CH3COOH + N2OH – CH3C00+ H20 Ob. None of the above OC CH3CO0″ + HCI CH3COOH + Cr Od CHạCO0 + NAOH – CH3COOH + Na* Oe. CH3COOH + HCI – CH3CO0 + H20
The Correct Answer and Explanation is :
The correct answer is d. CH₃COO⁻ + NaOH → CH₃COOH + Na⁺.
Explanation:
In an acetate buffer system, acetic acid (CH₃COOH) and sodium acetate (CH₃COONa) are in equilibrium. The pKa of acetic acid is 4.76, so the buffer will maintain a pH near this value when the concentrations of acetic acid and acetate ions are roughly equal.
When a small amount of base (such as NaOH) is added to this buffer system, it will try to increase the pH. However, a buffer resists changes in pH by neutralizing the added base with its acidic component. In this case, the base (OH⁻ from NaOH) will react with the acidic component (CH₃COOH).
The acetate ion (CH₃COO⁻), which is the conjugate base of acetic acid, will accept the OH⁻ ion (from NaOH). This results in the formation of more acetic acid (CH₃COOH), and the pH increase is neutralized. This reaction can be represented as:
CH₃COO⁻ + OH⁻ → CH₃COOH + H₂O
Thus, when NaOH is added, the acetate ions (CH₃COO⁻) act to neutralize the hydroxide ions (OH⁻), converting them into water and forming more acetic acid, which prevents a significant rise in pH. This is why the reaction that occurs in response to the addition of a base is CH₃COO⁻ + NaOH → CH₃COOH + Na⁺.
Let’s briefly analyze the other options:
- a. CH₃COOH + N₂OH → CH₃COO⁻ + H₂O: This is an unlikely reaction in this context because N₂OH is not a relevant species in a typical buffer solution.
- b. None of the above: This is incorrect because option d describes the correct reaction.
- c. CH₃COO⁻ + HCl → CH₃COOH + Cl⁻: This represents a neutralization between the acetate ion and HCl, which would be relevant when an acid is added, not a base.
- e. CH₃COOH + HCl → CH₃COO⁻ + H₂O: This is incorrect because HCl would donate H⁺ ions, not accept them.
In summary, the acetate buffer works by using the conjugate base (acetate) to neutralize any added acid or base, and the reaction that occurs when a base is added is CH₃COO⁻ + NaOH → CH₃COOH + Na⁺, which prevents a significant pH increase.