Edexcel A-Level Chemistry Paper 3, June 2023: Question 3

11 marks · Medium difficulty · Short Open Response

Write the dehydration equation for ethanol, draw the dot-and-cross diagram for sulfuric acid, and analyse the experimental setup for dehydrating ethanol over aluminium oxide including gas calculations.

Practise this question

Question

$A multi-part chemistry question about the dehydration of ethanol. Part (a) asks for the equation of ethanol dehydration using structural formulae (1 mark). Part (b) asks for the formula of phosphoric(V) acid (1 mark). Part (c) requires a dot-and-cross diagram of sulfuric acid with outer shell electrons only (2 marks). Part (d) presents a labeled diagram of an apparatus for dehydrating ethanol using aluminium oxide, followed by sub-questions (i) to (vi) covering boiling tube angle, heating errors, safety issues at position X, the purpose of a Bunsen valve, an alkene test, and a gas volume calculation from $2.759 \times 10^{20}$ molecules at r.t.p. (total 11 marks).$

Question text

3 Ethanol can be dehydrated using concentrated phosphoric(V) acid,

concentrated sulfuric acid or aluminium oxide.

(a) Write the equation for the dehydration of ethanol using structural formulae.

State symbols are not required.

(1)

(b) Give the formula of phosphoric(V) acid.

(1)

Clearly differentiate between sulfur, oxygen and hydrogen electrons.

Show outer shell electrons only.

(2)

(d) Ethanol may be dehydrated using the catalyst aluminium oxide, Al2O3 .

The apparatus is shown.

rubber coated clamp

aluminium oxide

alkene gas

X bung

mineral wool

soaked in

ethanol heat

Bunsen valve –

expanded top view

Bunsen valve

delivery

tube glass

slit

plug

(i) Give a possible reason for the boiling tube to be clamped at the angle shown.

(1)

… *P71914A0532*

(ii) Describe the problem if the ethanol is heated instead of the catalyst.

(1)

(iii) Identify a safety issue if the heat source was moved to the position labelled X

on the diagram.

(1)

(iv)Give a possible reason for the use of the Bunsen valve in the apparatus.*P71914A0632*

(1)

(v) Describe a test, with the positive result, which would confirm the presence of

an alkene in the test tube.

(1)

(vi) Calculate the volume of 2.759 × 1020 molecules of alkene gas

at room temperature and pressure (r.t.p.).

(2)

(Total for Question 3 = 11 marks)

Mark scheme

Show the mark scheme

Question

Answer Additional Guidance Mark

Number

3(a) Example of equation (1)

• equation CH3CH2OH → CH2CH2 + H2O

Allow displayed, semi-displayed, skeletal

formulae and C2H5OH and C2H4

Ignore any catalyst, even if incorrect, written

above the arrow

Ignore state symbols even if incorrect

Do not award use of molecular formula for

ethanol

Do not award inclusion of catalyst on both

sides of the equation

Do not award reversible arrow

Question

Answer Additional Guidance Mark

Number

3(b) (1)

• H3PO4 Ignore any state symbols even if incorrect

Ignore (V) written after the correct formula

Question

Answer Additional Guidance Mark

Number

Example of diagram (2)

3(c)

• 2 double bonds and 2 single bonds for sulfur (1)

• rest of diagram correct (1)

Allow

2 dative covalent bonds and 2 single bonds for sulfur scores (1)

rest of diagram correct scores (1)

Allow bond pairs to be show horizontally

Ignore lines drawn between atoms to show covalent bonds

Ignore arrows drawn between atoms to show coordinate bonds

Allow (1) for diagram with all dots/crosses

Question

Answer Additional Guidance Mark

Number

An answer that makes reference to the following point: (1)

3(d)(i)

• (excess liquid) ethanol could ‘run’/leak down the tube (to the catalyst) Allow if test tube is horizontal then liquid

ethanol would run down the tube

Allow ‘to keep the ethanol where it is’ which

implies movement otherwise

Allow to prevent ethanol/ (mineral ) wool

and catalyst mixing

Allow reactant for ethanol

Allow the catalyst could slide (towards the

ethanol)

Ignore any reference to heating

Question

Answer Additional Guidance Mark

Number

An answer that makes reference to the following point: (1)

3(d)(ii)

• ethanol (would evaporate) Ignore references to combustion and

and flammability of ethanol

pass over the catalyst without reaction Ignore references to the mineral wool

Question

Answer Additional Guidance Mark

Number

An answer that makes reference to the following point: (1)

3(d)(iii)

• the clamp may burn Allow the bung may burn / be set on fire /

rubber may melt

Question

Answer Additional Guidance Mark

Number

An answer that makes reference to the following point: (1)

3(d)(iv)

• (the valve) prevents the flow of water / suck-back (up the delivery tube) Allow prevents water entering the delivery

tube

Question

Answer Additional Guidance Mark

Number

An answer that makes reference to the following point: (1)

3(d)(v)

• bromine (water/solution)

and

decolourisation Ignore clear

(orange / brown / yellow / red) to colourless Allow any combination of stated colours

Allow

Acidified potassium manganate((VII))

and

decolourisation / purple to colourless

Question

Answer Additional Guidance Mark

Number

Example of calculation (2)

3(d)(vi)

• calculation of number of moles of ethene (1) n(Ethene)= (2.759 × 1020 ÷ 6.02 × 1023)

= 4.5831 × 10−4 (mol)

• calculation of the volume of ethene (1) V(Ethene) = (4.5831 × 10−4 × 24 000)

= 10.999 / 11.0 / 11 (cm3)

V(Ethene) = (4.5831 × 10−4 × 24 =)

= 0.010999/ 0.011 (dm3)

Ignore SF except 1SF

Correct answer without working scores (2)

TE for the volume from number of moles

Allow (1) for no. of molecules x 24(000)

Allow use of pV=nRT for both marks

298 K gives 11.349 (cm3)

293 K gives 11.159 (cm3)

(Total Question 3 = 11 marks)

How to answer it

Ethanol Dehydration & Practical Analysis Study Guide

📌 What this question tests

This question assesses organic chemistry fundamentals, specifically the dehydration of alcohols to form alkenes. It tests your ability to write balanced structural equations, recall inorganic formulae, draw complex dot-and-cross diagrams (including dative covalent bonds), understand practical laboratory setup and safety precautions for gas collection, perform chemical tests for unsaturation, and execute mole-to-volume gas calculations.

Part (a): Equation for the Dehydration of Ethanol

Write the equation for the dehydration of ethanol using structural formulae. State symbols are not required.

✅ Correct Answer

CH₃CH₂OH → CH₂=CH₂ + H₂O

(Allowed: Displayed, semi-displayed, or skeletal formulae, and molecular formula C₂H₄ for ethene).

❌ Common Errors

Using the molecular formula for ethanol ( C₂H₅OH ) when structural or semi-structural formulae are required.
Including the catalyst ( Al₂O₃ or H₃PO₄ ) on both sides of the equation.
Using a reversible arrow ( ⇌ ) instead of a single reaction arrow ( → ).

🎯 Mark: 1 mark

Part (b): Formula of Phosphoric(V) Acid

Give the formula of phosphoric(V) acid.

✅ Correct Answer

H₃PO₄

🧠 Exam Technique

Roman numerals in IUPAC nomenclature indicate oxidation states (here, phosphorus is +5). Combine this with known common ions to construct the formula swiftly.

🎯 Mark: 1 mark

Part (c): Dot-and-Cross Diagram of Sulfuric Acid

Draw the dot-and-cross diagram of sulfuric acid, H₂SO₄. Clearly differentiate between sulfur, oxygen, and hydrogen electrons. Show outer shell electrons only.

✅ Correct Answer

Sulfur (central atom) forms 2 double bonds and 2 single bonds to oxygen atoms (or 2 dative covalent bonds and 2 single bonds). Hydrogen atoms are bonded to the single-bonded oxygens.

Outer shells must clearly show distinct symbols (e.g., dots for O, crosses for S, triangles or distinct dots/crosses for H).

💡 Key Knowledge

Sulfuric acid expands its octet. In the Lewis structure, sulfur shares a total of 12 outer-shell electrons (2 double bonds with two oxygens, and 2 single bonds linked to -OH groups).

❌ Common Errors

Failing to use different symbols for sulfur, oxygen, and hydrogen electrons, or drawing an invalid octet around the sulfur atom.

🎯 Mark: 2 marks (1 mark for sulfur bonding arrangement, 1 mark for the rest of the diagram correct).

Part (d)(i) to (iii): Practical Setup & Safety

Analyse the apparatus setup for dehydrating ethanol using aluminium oxide.

✅ (i) Tube Angle

Answer: To prevent excess liquid ethanol from running/leaking down the tube and touching the hot catalyst prematurely, or to keep the liquid ethanol pooled at the mineral wool.

✅ (ii) Heating Ethanol Instead of Catalyst

Answer: The ethanol would simply evaporate and pass over the cold catalyst without reacting (no catalytic dehydration would occur).

✅ (iii) Heat Source at Position X

Answer: The rubber-coated clamp, bung, or delivery tube near position X may burn, melt, or catch fire.

🎯 Marks: 1 mark per section (Total: 3 marks)

Part (d)(iv) and (v): Bunsen Valve & Alkene Test

Explain the purpose of the Bunsen valve and describe the confirmation test for the alkene product.

✅ (iv) Bunsen Valve Purpose

Answer: The valve prevents water from flowing or sucking back up the delivery tube into the hot apparatus (preventing thermal shock and glass breakage).

✅ (v) Alkene Confirmation Test

Answer: Add bromine water (or bromine solution).
Positive Result: Decolourises from orange/brown/yellow/red to colourless.
Alternative: Acidified potassium manganate(VII) turning from purple to colourless.

🎯 Marks: 1 mark per section (Total: 2 marks)

Part (d)(vi): Gas Calculation

Calculate the volume of 2.759 × 10²⁰ molecules of alkene gas at room temperature and pressure (r.t.p.).

📐 Step-by-Step Calculation

Calculate moles of ethene:
Number of molecules ÷ Avogadro constant ( 6.02 × 10²³ )
n = (2.759 × 10²⁰) ÷ (6.02 × 10²³) = 4.5831 × 10⁻⁴ mol
Calculate volume at r.t.p. (molar volume = 24.0 dm³ mol⁻¹ or 24000 cm³ mol⁻¹):
In cm³: 4.5831 × 10⁻⁴ × 24000 = 11.0 cm³ (or 10.999 cm³)
In dm³: 4.5831 × 10⁻⁴ × 24 = 0.011 dm³ (or 0.010999 dm³).

❌ Common Calculation Traps

Forgetting to convert units between cm³ and dm³ if using standard molar volumes.
Using incorrect Avogadro constants or inverse division ( N ÷ NA vs NA ÷ N ).
Rounding intermediate values too early, leading to rounding errors in the final significant figures.

🎯 Marks: 2 marks (1 mark for calculating moles, 1 mark for calculating volume with correct units).

Topics

Organic Chemistry · Physical Chemistry · Core Practicals · Topic 2: Bonding and Structure · Topic 5: Formulae, Equations and Amounts of Substance · Topic 6: Organic Chemistry I

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.