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 deprotonation of ethanoic acid is favored relative to deprotonation of an alcohol.

1) Relevant knowledge points and concepts
- Brønsted–Lowry acid/base: acid donates H+; conjugate base stability increases acidity.
- Resonance (delocalization) stabilizes charge by spreading it over multiple atoms.
- Acetic acid (CH3COOH) ⇌ CH3COO– + H3O+; acetate ion has two equivalent resonance forms that delocalize the negative charge over both oxygens.
- pKa comparison: acetic acid pKa ≈ 4.76 vs. ethanol pKa ≈ 16; greater conjugate-base stabilization → stronger acid.

2) Step-by-step reasoning
- Remove H+ from COOH → get CH3COO–. The negative charge can be drawn on either oxygen in two resonance forms:
  Resonance form A: negative on the single-bonded O; form B: negative on the carbonyl O with double bond shifted.
- These resonance structures mean the negative charge is not localized on one O but shared (delocalized) across both O atoms and the C–O framework.
- Delocalization lowers the energy (stabilizes) of the acetate ion relative to a hypothetical localized anion (e.g., ethoxide), so the conjugate base is more stable.
- A more stable conjugate base makes the parent acid more willing to donate a proton (higher acidity), so dissociation occurs to a measurable extent.

3) Intermediate/calculation notes
- Acid dissociation constant: Ka = [CH3COO–][H3O+]/[CH3COOH]. Ka ≈ 1.8 × 10^−5 for acetic acid; this nonzero Ka results from acetate stabilization by resonance.
- Compare pKa values to quantify stronger acidity relative to non-resonance-stabilized alcohols.

4) Why the correct answer is correct
- Option e directly identifies the core reason: resonance stabilization of the carboxylate anion lowers its energy and thus promotes proton loss from acetic acid.

5) Why the other options are incorrect
- a: Hydronium (H3O+) makes the solution acidic, not basic — statement is wrong.
- b: It’s true that equilibrium lies mostly to the left (weak acid), but that is an observation of position, not the underlying explanation for why acetic acid can dissociate at all; resonance stabilization explains why dissociation is possible to the observed extent.
- c: “Oxygen is an electropositive element” is false — oxygen is electronegative.
- d: Water acts as a base (proton acceptor) here, not an electrophile.

6) Conclusion
The explanation based on resonance stabilization of the carboxylate anion is correct; therefore the correct answer is (e), matching the provided answer.

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

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