Exam Practice

Topic 3.3.9: Carboxylic Acids & Derivatives

Test your understanding of carboxylic acids, esters, and acyl derivatives with exam-style questions.

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📋 Structured Questions

Question 1: Acid Properties & Carbonates

4 marks

(a) State why carboxylic acids behave as weak acids in aqueous solution. [1]

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They only partially dissociate / ionise in aqueous solution to release hydrogen ions (H+) [1]

Examiner tip: Ensure you include the word "partially" or "incomplete" dissociation. Stating that they simply do not fully ionise is also acceptable.

(b) Write a balanced chemical equation for the reaction of propanoic acid with sodium hydrogencarbonate. State one observation. [3]

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Equation: CH3CH2COOH + NaHCO3 -> CH3CH2COONa + H2O + CO2 [2]

(1 mark for correct reactants and products; 1 mark for correct balancing)

Observation: Effervescence / bubbling / solid NaHCO3 dissolves [1]

Examiner tip: Carboxylic acids react with carbonates or hydrogencarbonates to produce carbon dioxide gas. Standard observations include effervescence or fizzing. Alcohols do not undergo this reaction, making this a useful test.

Question 2: Ester Hydrolysis

6 marks

An ester has the molecular formula C4H8O2. When heated with dilute sulfuric acid, it hydrolyses to form compound A and compound B. Compound A is oxidized by acidified potassium dichromate(VI) to form compound B.

(a) Identify the ester and compounds A and B. Write a balanced chemical equation for the hydrolysis reaction. [4]

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  • Ester: ethyl ethanoate [1]
  • Compound A: ethanol [1]
  • Compound B: ethanoic acid [1]
  • Equation: CH3COOCH2CH3 + H2O <=> CH3COOH + CH3CH2OH [1]
Examiner tip: Since A oxidizes to B, both must have the same carbon skeleton (2 carbons each). This rules out methyl propanoate or propyl methanoate. Dilute acid hydrolysis is reversible, so a reversible arrow is preferred in the equation.

(b) Contrast the hydrolysis of ethyl ethanoate using hot aqueous sodium hydroxide with the acid-catalysed hydrolysis in part (a). State two differences. [2]

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Any two differences from:

  • Base hydrolysis goes to completion / is not reversible (acid hydrolysis is a reversible equilibrium) [1]
  • Base hydrolysis produces the carboxylate salt (sodium ethanoate) (acid hydrolysis produces the carboxylic acid / ethanoic acid) [1]
  • Base hydrolysis requires one mole of base per mole of ester as a reactant (acid hydrolysis uses the acid as a catalyst) [1]
Examiner tip: Under alkaline conditions, the carboxylic acid produced reacts immediately with the alkali (NaOH) to form a carboxylate salt. This shifts the equilibrium position fully to the right, making the reaction irreversible.

Question 3: Nucleophilic Addition-Elimination

6 marks

(a) Ethanoyl chloride reacts with ammonia to form ethanamide. Describe the role of ammonia in this mechanism and explain why this reaction is faster than the reaction of ethanoyl chloride with water. [2]

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  • Ammonia acts as a nucleophile / electron pair donor [1]
  • Ammonia is a stronger nucleophile than water because the nitrogen atom is less electronegative than the oxygen atom (so its lone pair is more available for donation) [1]
Examiner tip: Ensure you relate nucleophile strength to electronegativity. A less electronegative atom has a lone pair that is more easily donated to form a covalent bond.

(b) Ethanoic anhydride can also be reacted with ammonia to produce ethanamide. Explain one advantage and one disadvantage of using ethanoic anhydride rather than ethanoyl chloride in industrial synthesis. [2]

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Advantage (any one):

  • It is cheaper to manufacture / purchase [1]
  • It is less corrosive / less dangerous to handle [1]
  • It does not produce toxic hydrogen chloride (HCl) gas as a byproduct (produces non-toxic ethanoic acid instead) [1]
  • It reacts less violently, making the temperature easier to control [1]

Disadvantage (any one):

  • It has a lower atom economy [1]
  • It reacts more slowly [1]
Examiner tip: This is a common question in the context of aspirin manufacture. The absence of toxic HCl emissions and lower corrosivity are major industrial safety benefits.

(c) Draw the structure of the organic product formed when excess ethanoyl chloride reacts with butane-1,4-diol. [2]

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Structure showing a diester link at both ends of the butane chain: CH3COOCH2CH2CH2CH2OOCCH3 [2]

(1 mark for correct ester linkage at one end, 2 marks if both ends are esterified correctly)

Product name: butane-1,4-diyl diethanoate.

Examiner tip: Diols contain two hydroxyl (-OH) groups. When reacted with an excess of an acyl chloride, both groups undergo nucleophilic addition-elimination to yield a diester.

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