Which of the following statements correctly explains the dissociation of ethanoic acid in water?

Question

BlackTom AI · Accepted Answer

The carboxylate anion is resonance stabilised, so...

1. Relevant knowledge points and concepts
- Brønsted–Lowry acid: CH3COOH donates a proton to water to give CH3COO^- (conjugate base) and H3O+.
- Resonance: the negative charge on CH3COO^- is delocalised over the two oxygen atoms via two equivalent resonance structures, lowering its energy and stabilising the anion.
- pKa/Ka relationship: pKa of acetic acid ≈ 4.76, so Ka = 10^(-pKa) quantifies the equilibrium extent.

2. Step-by-step reasoning
- Proton transfer: CH3COOH + H2O ⇌ CH3COO^- + H3O+. This is the acid donating H+ and water accepting H+.
- Conjugate-base stability controls acid strength: a more stable (lower-energy) conjugate base makes the forward reaction more favorable.
- For CH3COO^-, resonance spreads the negative charge across two oxygens and into the C=O bond, so the anion is significantly stabilised compared with a localized alkoxide (RO^-).

3. Calculations / intermediate steps
- Using pKa 4.76: Ka = 10^(-4.76) = 10^(-5) × 10^(0.24) ≈ 1.0×10^-5 × 1.74 ≈ 1.74×10^-5. This small Ka shows partial dissociation, consistent with a weak acid whose conjugate base is stabilised by resonance.

4. Why the correct answer is correct
- Resonance stabilisation of CH3COO^- lowers its energy and therefore makes proton loss from CH3COOH more favorable than it would be if the negative charge were localized. This is the fundamental molecular explanation for ethanoic acid’s ability to dissociate in water.

5. Why the other options are incorrect
- b: Generation of H3O+ makes the solution acidic, not basic — so statement is false.
- c: Water here acts as a base (proton acceptor), not an electrophile; calling it an electrophile is incorrect.
- d: While the equilibrium for acetic acid does lie largely to the left (weak acid), saying the equilibrium lies left does not explain why dissociation occurs; it also would contradict the idea that dissociation is explained (it merely states extent).
- e: Oxygen is electronegative, not electropositive; the statement is wrong.

6. Conclusion
Resonance stabilisation of the carboxylate anion correctly explains ethanoic acid dissociation; this confirms choice a.

Which of the following statements correctly explains the dissociation of ethanoic acid in water?单项选择题

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