Ethanol can be oxidised to form ethanoic acid. Describe the properties of ethanoic acid that show it is a weak acid, and explain what is meant by the term 'weak acid'. Include in your answer what happens when ethanoic acid reacts with magnesium.

WJEC A-Level Chemistry (Wales) — 2.7 Alcohols and carboxylic acids · Describe · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

Ethanoic acid is a weak acid because it only partially dissociates in water:

CH₃COOH ⇌ CH₃COO⁻ + H⁺

Only a small fraction of the molecules give up a proton, so the concentration of H⁺ in solution is lower than that of a strong acid of the same molarity. Consequently, a solution of ethanoic acid has a higher pH than a strong acid of the same concentration.

When ethanoic acid reacts with magnesium metal, the acid protonates the metal and a salt is formed:

2 CH₃COOH + Mg → Mg(CH₃COO)₂ + H₂↑

Thus magnesium ethanoate is produced together with hydrogen gas.

Examiner tips

  • Use the dissociation equation to show partial ionisation; mention the equilibrium symbol. State the effect on H⁺ concentration and pH. Give the balanced reaction with Mg and note H₂ gas. Use the term ‘weak acid’ exactly as defined.
  • Remember to include the salt name (magnesium ethanoate) and the gas produced.

Mark scheme (5 marks)

  1. Ethanoic acid partially dissociates / ionises in water (does not fully dissociate)
  2. A weak acid produces a lower concentration of hydrogen ions (H⁺) than a strong acid of the same concentration
  3. Ethanoic acid has a higher pH than a strong acid of the same concentration
  4. Ethanoic acid reacts with magnesium to produce a magnesium salt (magnesium ethanoate)
  5. Hydrogen gas is also produced in the reaction between ethanoic acid and magnesium

Key terms in this question

weak acid · ethanoic acid

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