2017 H2 Chem P1 Q20 — Carboxylic Acid Solution

The answer is C (1 only).
A lower \(\text{p}K_\text{a}\) means a stronger acid. Chloroethanoic acid (2.87) and fluoroethanoic acid (2.57) are stronger than ethanoic acid (4.76). Acid strength depends on how easily the O-H bond breaks and on how stable the carboxylate anion is.
• Statement 1 is true: the electronegative F and Cl atoms draw electron density away from the O-H bond in the carboxyl group, weakening it so that \(\text{H}^+\) is lost more readily. F is more electronegative than Cl, so fluoroethanoic acid has the lower \(\text{p}K_\text{a}\).
• Statement 2 is false: by drawing electron density towards themselves, F and Cl disperse the negative charge on the carboxylate anion, which stabilises it, not destabilises it.
• Statement 3 is false: the electron-donating methyl group in \(\text{CH}_3\text{CO}_2^-\) intensifies the negative charge on the carboxylate anion, which destabilises it. That is why ethanoic acid is the weakest of the three.
• A is wrong: it includes the false statements 2 and 3.
• B is wrong: it includes the false statement 2.
• D is wrong: statements 2 and 3 are both false, and the true statement 1 is left out.