Edexcel A-Level Chemistry Paper 2, June 2018: Question 5

7 marks · Medium difficulty · Short Open Response

Describe and explain aspects of the oxidation of alcohols including half-equations, reflux apparatus setup, anti-bumping granules, and the use of reflux versus distillation.

Practise this question

Question

Question text

5 Some alcohols can be oxidised by acidified sodium dichromate(VI), Na2Cr2O7.

(a) Balance the ionic half-equation for the reduction of the dichromate(VI) ion.

Give the colours of all of the species involved, or state colourless if appropriate.

(2)

Cr O2− + H+ + e− → Cr3+ + H O

27 … 2

Colour …

(b) Reflux apparatus can be used to carry out the oxidation of alcohols.

(i) This Liebig condenser has been set up incorrectly. Add shading to the

diagram to show the water in the condenser, illustrating the effect of the

incorrect water flow.

(1)

open top

water in

Liebig

condenser

water out

pear-shaped

flask

anti-bumping

granules

heat

(ii) State how the granules prevent bumping.

(1)

Which alcohol would be oxidised under reflux to produce this carboxylic acid?

(1)

A 1,1-dimethylethanol

B 2-methylpropan-1-ol

C 2-methylbutan-1-ol

D propan-2-ol

(d) Using the apparatus for distillation instead of reflux is not an efficient way to

produce ethanoic acid from ethanol. Explain why.

(2)

(Total for Question 5 = 7 marks)

Mark scheme

Show the mark scheme

Question

Acceptable Answer Additional guidance Mark

Number

5(a) Example of equation: (2)

Balanced equation (1) Cr O 2−+14H++6e−→2Cr3++7H O

27 2

Allow multiples

Appropriate colours (1) Orange colourless green colourless

Allow ‘No colour’ for colourless

Do not award ‘blue’ for ‘green’

Do not award ‘clear’ for colourless

Do not award if any spaces left blank

Ignore any colour given for electrons

Ignore any shades of colour

Question

Acceptable Answer Additional guidance Mark

Number

5(b)(i) A diagram with any shading that is not 100% An example of a suitable diagram: (1)

Allow shaded area to show ‘air pockets’

Question

Acceptable Answer Additional guidance Mark

Number

5(b)(ii) An answer that makes reference to the following (1)

prevention of uncontrolled boiling by:

distributing the heat more evenly Do not award provide surface area for

reaction

providing a surface for bubbles to form/allow Ignore reference to mixing the

smaller bubbles to form/provides nucleation sites reagents/provide smooth boiling

for bubbles

Question

Acceptable Answer Mark

Number

5(c) The only correct answer is B (1)

A is incorrect because not a systematic name

C is incorrect because it has five carbons

D is incorrect because it has only three carbons

Question

Acceptable Answer Additional guidance Mark

Number

5(d) An explanation that makes reference to the following Allow aldehyde for ethanal (2)

ethanol would be oxidised to ethanal (1) Allow ethanal will be formed

because ethanal has a low boiling temperature Allow ethanal is (more) volatile

ethanal will distil before ethanoic acid can be Accept reverse argument in terms of

formed (1) reflux condensing ethanal for oxidation

to ethanoic acid

(Total for Question 5 = 7 marks)

How to answer it

Oxidation of Alcohols & Practical Techniques

What this question tests

This question assesses your knowledge of redox half-equations involving acidified dichromate(VI), practical laboratory setups for reflux (including condenser water flow and anti-bumping granules), alcohol identification from structural formulas, and the chemical rationale behind why distillation fails for producing carboxylic acids.

Part (a) - Redox Half-Equation

Balancing Dichromate Reduction & Species Colours

✅ Correct Answer

Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O

Colours: Orange, colourless, green, colourless

💡 Key Knowledge

Dichromate(VI) ions act as strong oxidising agents in acidic conditions.
The chromium changes oxidation state from +6 (orange) to +3 (green).
Hydrogen ions ( H⁺ ) supply the acidity required for the reduction to proceed.

❌ Common Errors

Writing "blue" instead of "green" for the Cr³⁺ ion.
Writing "clear" instead of "colourless" (remember: clear simply means transparent; solutions can be clear and coloured!).
Leaving any answer lines blank in the table.

Marks: 2 marks (1 mark for the balanced equation, 1 mark for all correct colours).

Parts (b)(i) and (b)(ii) - Reflux Practical Setup

Condenser Water Flow & Anti-Bumping Granules

✅ Correct Answers

(i) Shading: Partial shading (not 100%) showing water jackets correctly filled from the bottom up to ensure the condenser stays completely full.

(ii) Granules: Distributes heat evenly OR provides a surface/nucleation sites for smaller bubbles to form, preventing violent boiling.

🧠 Exam Technique

When shading diagrams for condensers, ensure the water enters at the bottom and leaves at the top so the cooling jacket never drains dry.
For anti-bumping granule explanations, always use terminology like "nucleation sites" or "small bubbles" rather than vague phrases like "so it mixes better".

❌ Common Errors

Stating that anti-bumping granules "provide a larger surface area for reaction" (they are inert; they provide a surface for bubble formation, not chemical reaction).

Marks: 1 mark for (b)(i) + 1 mark for (b)(ii). Total = 2 marks.

Part (c) - Identifying the Alcohol

Matching Carboxylic Acids to Precursor Alcohols

✅ Correct Answer

B (2-methylpropan-1-ol)

💡 Key Knowledge

Oxidation under reflux converts primary alcohols directly into carboxylic acids while keeping the carbon skeleton intact. By counting the carbon atoms in the given skeletal formula of the carboxylic acid (4 carbons with a methyl branch on carbon 2), we trace it back to 2-methylpropan-1-ol.

❌ Common Errors

A: 1,1-dimethylethanol is not a systematic IUPAC name.
C: 2-methylbutan-1-ol contains 5 carbons, which would yield a 5-carbon chain acid instead of the 4-carbon acid shown.
D: propan-2-ol is a secondary alcohol, which oxidises to a ketone, not a carboxylic acid.

Marks: 1 mark.

Part (d) - Distillation vs Reflux

Why Distillation Fails to Produce Carboxylic Acids Efficiently

✅ Correct Answer

Ethanol would first be oxidised to ethanal. Ethanal has a low boiling temperature (is volatile) and would distil out of the mixture before it has a chance to be further oxidised into ethanoic acid.

🧠 Exam Technique

To score full marks, you must link the physical property (low boiling point / volatility of the intermediate aldehyde) to the practical flaw (it escapes the reaction vessel before complete oxidation can occur).
Accept the reverse argument: reflux is required to trap and condense volatile intermediates back down into the flask for complete oxidation.

Marks: 2 marks (1 mark for identifying ethanal formation, 1 mark for explaining its volatility/distilling off before conversion). Total = 2 marks.

Topics

Organic Chemistry · Core Practicals · Physical Chemistry · Core Practical 5: Investigate the oxidation of ethanol · Topic 6: Organic Chemistry I · Topic 3: Redox I

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 2, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.