OCR A-Level Chemistry Synthesis and analytical techniques (02), June 2025: Question 4
1 mark · Easy difficulty · Multiple Choice
Identify the correct apparatus setup for heating under reflux to convert ethanol to ethanoic acid.
Practise this questionQuestion
Question text
4 Which diagram shows the correct way to set up apparatus for converting ethanol to ethanoic
acid?
water outwater outwater outwater out water inwater inwater inwater in
A B
water inwater inwater inwater in water outwater outwater outwater out
water outwater outwater outwater out water inwater inwater inwater in
water inwater inwater inwater in water outwater outwater outwater out
C D
Your answer
[1]
Mark scheme
Show the mark scheme
4 A 1
How to answer it
Apparatus Setup for the Oxidation of Primary Alcohols
- Distinguishing between reflux and distillation setups for organic preparations.
- Reaction conditions for the complete oxidation of a primary alcohol (ethanol) to a carboxylic acid (ethanoic acid).
- Correct laboratory technique for water flow through a Liebig condenser (water in at bottom, out at top).
Choosing the Correct Oxidation Apparatus
Multiple Choice [1 Mark]
✅ Correct Answer
A
To prepare ethanoic acid from ethanol, the mixture must be heated under reflux (vertical condenser) with acidified potassium dichromate(VI). In a Liebig condenser, water must always enter at the lowest point ("water in" at the bottom) and leave at the highest point ("water out" at the top).
💡 Key Knowledge
- Ethanol to Ethanal (Partial Oxidation): Requires gentle heating with acidified potassium dichromate(VI) under distillation so the volatile aldehyde distils off immediately before it can react further.
- Ethanol to Ethanoic Acid (Complete Oxidation): Requires excess acidified potassium dichromate(VI) heated under reflux to ensure any evaporated vapours condense and drip back into the flask for full oxidation:
CH₃CH₂OH + 2[O] → CH₃COOH + H₂O - Condenser Direction: Water flows against gravity (in at the bottom, out at the top) to completely fill the jacket and prevent air bubbles or air pockets from forming, which would cause inefficient cooling.
🧠 Exam Technique: The Two-Step Filter
- Step 1: Identify the process required.
Converting ethanol to ethanoic acid is complete oxidation. This requires heating under reflux.
👉 Instantly eliminate C and D (which show distillation). - Step 2: Check water flow through the jacket.
Water must go in at the bottom and out at the top.
👉 A has water in at the bottom, water out at the top. Correct!
👉 B has water reversed (in at top, out at bottom). Incorrect!
❌ Common Errors & Misconceptions
- Selecting Distillation (C or D): Confusing the production of an aldehyde (ethanal, requires distillation) with the production of a carboxylic acid (ethanoic acid, requires reflux).
- Ignoring Water Flow (Choosing B): Recognising reflux but forgetting the golden rule of condensers: water flows uphill to ensure the condenser jacket is completely full.
- Adding a Stopper (Safety Trap): Although not an option here, students frequently draw reflux apparatus with a stopper at the top in written papers. A sealed reflux system creates dangerous pressure buildup! Diagram A correctly shows an open top.
Topics
Module 4: Core organic chemistry · Module 1: Development of practical skills in chemistry · Practical Activity Groups · 4.2 Alcohols, haloalkanes and analysis · 1.1 Practical skills assessed in a written examination · 1.2 Practical skills assessed in the practical endorsement · PAG 5: Synthesis of an organic liquid
Question and mark scheme from the OCR A-Level Chemistry examination, Synthesis and analytical techniques (02), June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.