2022 H2 Chem P1 Q3 — Acid base equilibrium Solution

The answer is D (3 only).

A smaller \(\text{p}K_\text{a}\) means a larger \(K_\text{a}\) and a stronger acid. Thioacetic acid (\(\text{p}K_\text{a}\) = 3.33, \(K_\text{a} = 4.7 \times 10^{-4}\)) is a stronger acid than ethanoic acid (\(\text{p}K_\text{a}\) = 4.76, \(K_\text{a} = 1.7 \times 10^{-5}\)).

• Statement 1 is false: \(\text{H}^+\) is more easily removed from thioacetic acid, the stronger acid, not from ethanoic acid. The data show that the weaker S-H bond outweighs the greater polarity of the O-H bond.
• Statement 2 is false: ethanoic acid has the larger \(\text{p}K_\text{a}\), so its \(K_\text{a}\) is smaller. A stronger O-H bond makes ethanoic acid ionise less, which lowers \(K_\text{a}\), not raises it.
• Statement 3 is true: the S-H bond is weaker than the O-H bond, so thioacetic acid ionises to a greater extent. At the same concentration it gives a higher \([\text{H}^+]\) and so a lower pH.

• A is wrong: statements 1 and 2 are false.
• B is wrong: statements 1 and 2 are both false, and it leaves out statement 3, which is true.
• C is wrong: statement 2 is false.