OxfordAQA A-Level Chemistry: Carboxylic acids and derivatives (9620) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

This set covers section 3.3.9 (Carboxylic acids and derivatives, subsections 3.3.9.1 and 3.3.9.2, with required practical 10) of OxfordAQA’s International AS and A-level Chemistry specification (9620), Version 5.3, for International AS and A-level exams from May/June 2020. The whole section is International A2 content, so it is International A-level only. Questions on it appear in Unit 4 (Organic 2 and Physical 2), and any section can also appear in Unit 5 (Practical and synoptic). Question 11 also uses the oxidation of alcohols from section 3.3.5.1.

Revise from the study guide and the revision notes before attempting these. For planning, use the course hub, the printable checklist and the free diagnostics.

Data. Use Ar values Na = 23.0, O = 16.0, C = 12.0 and H = 1.0.

Questions

1. (a) Name the esters HCOOCH₂CH₂CH₂CH₃ and CH₃CH₂COOCH₂CH₂CH₂CH₃. [2] (b) Which carboxylic acid and which alcohol would you use to make CH₃CH₂COOCH₂CH₂CH₂CH₃? [1] (c) State two common uses of esters. [1]

2. Pentanoic acid, CH₃CH₂CH₂CH₂COOH, is added to potassium carbonate.

(a) Write an equation for the reaction. [2] (b) Pentanoic acid reacts with carbonates, yet it is described as a weak acid. Explain why. [1]

3. State the family to which each compound belongs: (CH₃CH₂CO)₂O; CH₃CH₂CH₂COCl; CH₃CH₂CONHCH₃; (CH₃)₂CHCOOCH₃. [4]

4. Ethyl pentanoate, CH₃CH₂CH₂CH₂COOCH₂CH₃, is hydrolysed.

(a) Give the products of heating it under reflux with (i) dilute sulfuric acid; (ii) aqueous sodium hydroxide. [2] (b) Explain why the reaction with aqueous sodium hydroxide is not reversible. [1] (c) State how pentanoic acid can be obtained from the mixture in (a)(ii). [1]

5. Glyceryl trioctanoate, (C₇H₁₅COO)₃C₃H₅ (Mr = 470.0), is a triester found in some vegetable oils.

(a) Give the structural formula of the alcohol from which all oils and fats are made. [1] (b) Write an equation for the hydrolysis of glyceryl trioctanoate by sodium hydroxide. [2] (c) Calculate the maximum mass of soap, sodium octanoate (Mr = 166.0), that 18.8 g of the triester can form. [2] (d) State what biodiesel is, and give the reagent and condition used to make it from this oil. [2]

6. Write equations for the reactions of hexanoyl chloride, CH₃(CH₂)₄COCl, with:

(a) water [1] (b) ethanol [1] (c) excess ammonia [1] (d) excess propylamine, CH₃CH₂CH₂NH₂. [1]

7. Octanoyl chloride, CH₃(CH₂)₆COCl, reacts with methanol.

(a) State the name of this type of mechanism. [1] (b) Outline it, describing each curly arrow and the intermediate. [3] (c) Name the organic product. [1]

8. Write equations for the reactions of pentanoic anhydride, (CH₃CH₂CH₂CH₂CO)₂O, with:

(a) water [1] (b) propan-2-ol, (CH₃)₂CHOH [1] (c) excess methylamine, CH₃NH₂. [1]

9. Aspirin is made industrially from 2-hydroxybenzoic acid (Mr = 138.0) and ethanoic anhydride (Mr = 102.0).

(a) Give an equation for this reaction. [1] (b) Calculate the minimum mass of ethanoic anhydride needed to react with 8.28 g of 2-hydroxybenzoic acid. [2] (c) Give three industrial advantages of ethanoic anhydride over ethanoyl chloride for this process. [3]

10. For required practical 10, a student makes a solid amide, Z, whose data-book melting point is 130 °C. The crude solid (3.10 g) melts over the range 119–125 °C. After one recrystallisation, 2.35 g of dry crystals melt over 129–130 °C.

(a) Explain why the student uses only the minimum volume of hot solvent to dissolve the crude solid. [1] (b) State why the hot solution is filtered before it is cooled. [1] (c) Explain why the crystals are washed with cold, not hot, solvent. [1] (d) Use the melting point data to compare the purity of the two samples. [2] (e) Suggest one reason, other than the removal of impurities, why the mass falls. [1]

11. Ester X, C₆H₁₂O₂, is heated under reflux with excess sodium hydroxide solution. The products are sodium propanoate and an alcohol, B. When B is warmed with acidified potassium dichromate(VI) and the product is distilled off as it forms, the solution turns from orange to green. The distillate gives no silver mirror with Tollens’ reagent.

(a) Identify B, explaining your reasoning. [2] (b) Give the structural formula of X. [1] (c) Calculate the mass of sodium propanoate formed from 4.64 g of X. [2] (d) Write an equation for the reaction of B with propanoic anhydride to form X, and explain why this route gives a higher yield of X than heating B with propanoic acid and an acid catalyst. [2]

Answers

1. (a) Butyl methanoate [1]; butyl propanoate [1] (b) Propanoic acid and butan-1-ol [1] (c) Any two of: solvents, plasticisers, perfumes, food flavourings [1] [4] Examiner insight: In (a), the alcohol word comes first; “propyl butanoate” for the second ester is a different compound and earns nothing.

2. (a) 2CH₃CH₂CH₂CH₂COOH + K₂CO₃ → 2CH₃CH₂CH₂CH₂COOK + CO₂ + H₂O: correct species [1]; balanced [1] (b) It only partially dissociates into ions in water [1] [3] Examiner insight: A weak acid is defined by the extent of dissociation, so answers about pH, fizzing rate or concentration do not explain “weak”.

3. Acid anhydride [1]; acyl chloride [1]; amide (N-substituted) [1]; ester [1] [4] Examiner insight: Read the atom after C=O: a second C=O through an O is an anhydride, Cl an acyl chloride, N an amide, an O then a carbon chain an ester.

4. (a) (i) Pentanoic acid and ethanol [1]; (ii) sodium pentanoate and ethanol [1] (b) The acid forms as the pentanoate ion, which does not react with ethanol, so the reverse reaction cannot occur [1] (c) Add a strong acid, such as dilute hydrochloric acid [1] [4] Examiner insight: In (a)(ii), “pentanoic acid” is the product of the wrong conditions; name the salt.

5. (a) HOCH₂CH(OH)CH₂OH (propane-1,2,3-triol) [1] (b) (C₇H₁₅COO)₃C₃H₅ + 3NaOH → 3C₇H₁₅COONa + C₃H₅(OH)₃: correct products [1]; 3 : 3 balancing [1] (c) Amount of triester = 18.8 ÷ 470.0 = 0.0400 mol, so soap = 3 × 0.0400 = 0.120 mol [1]; mass = 0.120 × 166.0 = 19.9 g [1] (d) Methyl esters of long-chain carboxylic acids, as a mixture [1]; made with methanol in the presence of a catalyst [1] [7] Examiner insight: A 1 : 1 ratio in (c) gives 6.64 g; show the factor of 3 explicitly so an examiner can see your method.

6. (a) CH₃(CH₂)₄COCl + H₂O → CH₃(CH₂)₄COOH + HCl [1] (b) CH₃(CH₂)₄COCl + CH₃CH₂OH → CH₃(CH₂)₄COOCH₂CH₃ + HCl [1] (c) CH₃(CH₂)₄COCl + 2NH₃ → CH₃(CH₂)₄CONH₂ + NH₄Cl [1] (d) CH₃(CH₂)₄COCl + 2CH₃CH₂CH₂NH₂ → CH₃(CH₂)₄CONHCH₂CH₂CH₃ + CH₃CH₂CH₂NH₃Cl [1] [4] Examiner insight: With “excess” ammonia or amine, a balanced equation needs two molecules; HCl left beside a base is chemically inconsistent.

7. (a) Nucleophilic addition–elimination [1] (b) Curly arrow from an O lone pair in CH₃OH to the δ+ C of C=O, plus a curly arrow from the C=O π bond onto O [1]; intermediate showing O⁻, Cl still attached, and O⁺ (bonded to H and CH₃) from the methanol [1]; C=O re-formed from the O⁻ lone pair, C–Cl electrons moving onto Cl as Cl⁻ leaves, and H⁺ lost from O⁺ [1] (c) Methyl octanoate [1] [5] Examiner insight: Begin every curly arrow at a bond or a lone pair; arrows that start on a charge or an atom symbol do not show the source of the electrons.

8. (a) (C₄H₉CO)₂O + H₂O → 2C₄H₉COOH [1] (b) (C₄H₉CO)₂O + (CH₃)₂CHOH → C₄H₉COOCH(CH₃)₂ + C₄H₉COOH [1] (c) (C₄H₉CO)₂O + 2CH₃NH₂ → C₄H₉CONHCH₃ + C₄H₉COO⁻ CH₃NH₃⁺ [1] [3] Examiner insight: The anhydride by-product is the carboxylic acid (or its salt with excess amine), never HCl.

9. (a) HOC₆H₄COOH + (CH₃CO)₂O → CH₃COOC₆H₄COOH + CH₃COOH [1] (b) Amount of 2-hydroxybenzoic acid = 8.28 ÷ 138.0 = 0.0600 mol [1]; 1 : 1 ratio, so mass = 0.0600 × 102.0 = 6.12 g [1] (c) Any three: cheaper [1]; less corrosive [1]; less readily hydrolysed by water or moist air [1]; reacts less vigorously, so easier to control; no corrosive, toxic hydrogen chloride gas formed [6] Examiner insight: “Safer” alone is too vague to earn a mark; name the property, such as no HCl gas or slower hydrolysis.

10. (a) So the solution is saturated when hot and as much product as possible crystallises on cooling [1] (b) To remove insoluble impurities [1] (c) Hot solvent would dissolve some of the crystals; cold solvent rinses away impurities with little product loss [1] (d) The crude solid melts below 130 °C and over a 6 °C range, so it is impure [1]; the recrystallised sample melts sharply at the data-book value, so it is much purer [1] (e) Some Z stays dissolved in the cold solvent and is lost in the filtrate [1] [6] Examiner insight: For (d), use both features of the data, the temperature and the width of the range, for each sample.

11. (a) Sodium propanoate has three carbons, so B has 6 − 3 = 3 carbons [1]; B is oxidised, but the distillate is not an aldehyde (a primary alcohol would give propanal), so B is a secondary alcohol: propan-2-ol [1] (b) CH₃CH₂COOCH(CH₃)₂ [1] (c) Mr of X = 116.0, so amount = 4.64 ÷ 116.0 = 0.0400 mol [1]; Mr of CH₃CH₂COONa = 96.0, so mass = 0.0400 × 96.0 = 3.84 g [1] (d) (CH₃CH₂CO)₂O + (CH₃)₂CHOH → CH₃CH₂COOCH(CH₃)₂ + CH₃CH₂COOH [1]; the anhydride reaction is not reversible, whereas esterification with the acid reaches an equilibrium [1] [7] Examiner insight: Link each observation to a conclusion in (a); a bare name for B cannot earn the reasoning mark.

Where marks are usually lost

• Naming an ester with the acid word first.
• Writing pH or reaction speed, instead of partial dissociation, to explain “weak acid”.
• Giving the free acid instead of the salt for alkaline hydrolysis.
• Missing the factor of 3 between a triglyceride and its soap or methyl ester.
• Writing HCl as a product of an acid anhydride.
• Using only one ammonia or amine molecule per acyl chloride.
• Leaving the charges off the addition–elimination intermediate.
• Writing “safer” or “better” without naming the property.
• Comparing melting points using only the temperature, without the range.

Next steps

• Carboxylic acids and derivatives revision notes
• Carboxylic acids and derivatives study guide
• Alcohols
• OxfordAQA A-Level Chemistry hub
• Printable checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

OxfordAQA, International AS and A-level Chemistry (9620) specification, Version 5.3, for International AS and A-level exams May/June 2020 onwards: section 3.3.9 Carboxylic acids and derivatives (3.3.9.1 and 3.3.9.2) and required practical 10, International A2 content.