2024 H2 Chem P1 Q23 — Acid base equilibrium Solution

The answer is C.
Acid strength depends on how stable the anion formed on losing \(\text{H}^+\) is. In phenoxide, a lone pair on the O atom is delocalised into the benzene ring, spreading out the negative charge, so phenoxide is more stable than hydroxide. Phenol therefore ionises more than water and has a larger \(K_\text{a}\). C is correct.
• A is wrong: ethanol does have a larger \(\text{p}K_\text{a}\) than phenol, but the reason is false. The ethoxide ion is less stable than phenoxide, because the electron-donating ethyl group intensifies the negative charge on O, while phenoxide's charge is delocalised into the ring.
• B is wrong: the electron-donating ethyl group intensifies the negative charge on O in ethoxide, making it less stable than hydroxide. Ethanol is a weaker acid than water, so it has a larger \(\text{p}K_\text{a}\), not a smaller one.
• D is wrong: phenol is a stronger acid than ethanol, so it has a larger \(K_\text{a}\), not a smaller one.