Carboxylic Acids Exam Questions

(a) The carboxyl group, -COOH [1]

(b) CnH2n+1COOH [1]

(c) Methanoic acid, HCOOH [1]

Examiner note: In the -COOH group, one oxygen is double-bonded to carbon (C=O) and the other is bonded as -O-H. The acidic hydrogen is the one in the -O-H part of -COOH. Do not confuse with the -OH group of alcohols. Ethanoic acid (CH3COOH) is the most commonly examined member.

(a) Effervescence / fizzing / bubbles of gas produced [1]

(b) ethanoic acid + sodium carbonate → sodium ethanoate + water + carbon dioxide [1]

(c) 2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2 [2] (1 mark for correct formulae, 1 mark for balancing)

Examiner note: This reaction follows the same pattern as any acid with a carbonate: acid + carbonate → salt + water + carbon dioxide. The gas produced can be tested with limewater (turns milky). The salt is named by combining the metal name with the acid name modified: ethanoic acid gives ethanoate.

(a) ethanoic acid + magnesium → magnesium ethanoate + hydrogen [1]

(b) Hydrogen [1]

(c) Hold a burning splint near the gas — it burns with a squeaky pop [1]

Examiner note: Carboxylic acids react with metals just like mineral acids do: acid + metal → salt + hydrogen. However, the reaction is slower and less vigorous than with a strong acid like hydrochloric acid because ethanoic acid is a weak acid. This slower reaction rate is a common exam comparison point.

• A strong acid completely dissociates/ionises in water — all molecules release H+ ions [1]
• Example: HCl → H+ + Cl- (complete ionisation) [1]
• A weak acid only partially dissociates/ionises in water — only a small proportion of molecules release H+ ions at any given time [1]
• Example: CH3COOH ⇌ CH3COO- + H+ (reversible/equilibrium, most molecules remain un-ionised) [1]

Examiner note: Strong/weak refers to the degree of ionisation, NOT concentration. A concentrated weak acid can have more H+ ions per dm3 than a dilute strong acid. The reversible arrow (⇌) in the ethanoic acid equation is essential — using a forward arrow (→) implies complete dissociation and loses the mark.

(a) Esterification [1]

(b) Ethyl ethanoate [1]

(c) ethanoic acid + ethanol → ethyl ethanoate + water [1]

(d) Any one from: as a solvent, in flavourings, in perfumes, in nail varnish remover [1]

Examiner note: Ester naming: the first part comes from the alcohol (ethanol → ethyl), the second part from the acid (ethanoic acid → ethanoate). Concentrated sulfuric acid acts as a catalyst and a dehydrating agent. The reaction is reversible and reaches equilibrium.

(a) CH3COOH + NaOH → CH3COONa + H2O [1]

(b) Sodium ethanoate [1]

(c) Neutralisation [1]

Examiner note: This is the same pattern as any acid-base reaction: acid + alkali → salt + water. Carboxylic acids react with bases, metal oxides, and metal hydroxides. The pH of the resulting solution depends on whether excess acid or excess alkali is used. A titration with indicator can determine the exact volumes needed.

(a) Hydrochloric acid has the lower pH [1] because it is a strong acid and completely dissociates, producing more H+ ions per dm3 at the same concentration [1]

(b) Rate: the reaction with hydrochloric acid is initially faster because there are more H+ ions available to react with the magnesium [1] Total gas: the same total volume of hydrogen is produced from both acids because they have the same concentration and volume, so the same number of moles of acid [1]

Examiner note: This is a high-level question that tests understanding of strong vs weak acids. Although the weak acid reacts more slowly at first, it continues to dissociate as H+ ions are used up, so eventually the same total amount of product forms. The final volume of gas depends on the total moles of acid, not the degree of ionisation.