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A2 · Practice questions · Relative acidity of carboxylic acids

Stabilise the anion.

Six original Cambridge-style questions on the strength of carboxylic acids and the oxidation of methanoic and ethanedioic acids.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Withdraw electrons, stronger acid.

01
Explain
[3 marks]

Explain why trichloroethanoic acid is a much stronger acid than ethanoic acid.

  • Cl is more electronegative than H, so each Cl withdraws electron density along the σ bonds. ✓
  • This spreads the negative charge of Cl3CCOO− and stabilises it. ✓
  • Three Cl atoms have a large combined effect, so the acid ionises far more (pKa 0.65 against 4.76). ✓
02
Calculation
[2 marks]

Using the pKa values 4.76 (ethanoic acid) and 0.65 (trichloroethanoic acid), calculate Ka for each and how many times stronger trichloroethanoic acid is.

  • Ka = 10−4.76 = 1.74 × 10−5 and 10−0.65 = 0.224 mol dm−3. ✓
  • Ratio = 0.224 ÷ (1.74 × 10−5) = 1.29 × 104. ✓
03
Analysis
[2 marks]

Put these in order of increasing acid strength and explain the position of the first one: propanoic acid, 2-chloropropanoic acid, 3-chloropropanoic acid.

  • Propanoic acid < 3-chloropropanoic acid < 2-chloropropanoic acid. ✓
  • Propanoic acid has no electron-withdrawing group, only an electron-donating ethyl group, so its anion is least stable. ✓
04
Recall
[3 marks]

Describe what is seen when methanoic acid is warmed with (a) Tollens' reagent, (b) Fehling's solution, (c) acidified KMnO4, and write an equation using [O].

  • (a) a silver mirror; (b) a brick-red precipitate of Cu2O from a blue solution; (c) purple to colourless. ✓
  • HCOOH + [O] → CO2 + H2O. ✓
05
Calculation
[3 marks]

25.0 cm3 of ethanedioic acid solution needs 20.0 cm3 of 0.0200 mol dm−3 KMnO4 in warm acid. Calculate the concentration of the ethanedioic acid.

  • n(MnO4−) = 0.0200 × 0.0200 = 4.00 × 10−4 mol. ✓
  • n(H2C2O4) = 4.00 × 10−4 × 5/2 = 1.00 × 10−3 mol. ✓
  • Concentration = 1.00 × 10−3 ÷ 0.0250 = 0.0400 mol dm−3. ✓
06
Analysis
[2 marks]

Describe a test to distinguish methanoic acid from ethanoic acid, and explain why only one reacts.

  • Warm each with Tollens' reagent: methanoic acid gives a silver mirror, ethanoic acid gives no change. ✓
  • Methanoic acid, HCOOH, has an H attached to the C=O carbon, which can be oxidised (to CO2); in ethanoic acid that carbon is bonded to CH3, OH and =O, with no H. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.