The questions are on the esterification test. Take it under real conditions before reading the mark scheme.
Quick answer: Carboxylic acids react with alcohols, in the presence of concentrated sulfuric acid, to form esters and water in a reversible equilibrium. This mark scheme gives the model answers and the marks for the 8 questions of the Chemistry 11-12 test on esterification, from naming an ester to reactivity of acyl chlorides, with the equivalent answers to accept.
The answers include a mark scheme for each question. Award marks for correct chemistry even if the wording differs, but do not accept a single arrow for esterification or an ester name with the parts reversed.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: A Level 3.3.9
Question 1 Naming an ester (2 marks)
- Ethyl propanoate.
- Propanoic acid and ethanol.
Marking: 1 mark each.
Question 2 Esterification (2 marks)
- CH₃COOH + CH₃CH₂CH₂OH ⇌ CH₃COOCH₂CH₂CH₃ + H₂O.
- Concentrated sulfuric acid.
Marking: 1 mark each.
Question 3 Alkaline hydrolysis (2 marks)
- CH₃COOCH₃ + NaOH → CH₃COONa + CH₃OH.
- The acid is converted into the ethanoate ion, which does not react with methanol, so the reverse reaction cannot occur.
Marking: 1 mark each.
Question 4 Making an acyl chloride (2 marks)
- Thionyl chloride, SOCl₂.
- Sulfur dioxide and hydrogen chloride.
Marking: 1 mark each.
Question 5 Addition-elimination (3 marks)
- Nucleophilic addition-elimination.
- A tetrahedral intermediate where the carbonyl carbon carries O⁻, Cl, CH₃ and the ethanol oxygen.
- CH₃COOC₂H₅ and HCl.
Marking: 1 mark each.
Question 6 Equilibrium amounts (3 marks)
- Kc = [ester][water] / ([acid][alcohol]).
- x² / (0.800 − x)² = 4.0, so x / (0.800 − x) = 2.0 and x = 1.60 − 2x, giving x = 0.533 mol.
Marking: 1 for the expression; 1 for setting up the equation; 1 for the answer.
Question 7 Identify an ester (3 marks)
- A is ethanoic acid (M = 60.0). B is propan-2-ol: its oxidation product is a ketone (propanone).
- Propan-2-yl ethanoate, CH₃COOCH(CH₃)₂.
Marking: 1 for A; 1 for B; 1 for the ester name.
Question 8 Reactivity of acyl chlorides (3 marks)
- The carbonyl carbon is more δ+ because it is bonded to O and Cl, and Cl⁻ is a good leaving group, so the nucleophile attacks faster.
- Advantage: irreversible and fast, so a high yield. Disadvantage: corrosive and gives off HCl fumes.
Marking: 1 for the carbon being more positive; 1 for the leaving group; 1 for advantage and disadvantage together.
