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

13 marks · Medium difficulty · Open Response

Analyze the reactions, isomerism, mechanisms, polymerisation, incineration, and manufacturing conditions related to alkenes, halogenoalkanes, and chloroethene.

Practise this question

Question

Question text

5 This question concerns alkenes and some halogen compounds.

(a) The alkene, propene, reacts with hydrogen chloride.

(i) This reaction is best described as

(1)

A electrophilic substitution

B electrophilic addition

C nucleophilic substitution

D nucleophilic addition

(ii) The reaction of propene with hydrogen chloride can produce two isomeric products:

CH3 CH2 CH2 Cl

Cl CH3 CH CH3

1-chloropropane 2-chloropropane

1-chloropropane and 2-chloropropane are

(1)

A cis-trans isomers

B E/Z isomers

C structural isomers

D stereoisomers

(iii) Draw the mechanism for the reaction of propene with hydrogen chloride to

produce 2-chloropropane. Include curly arrows, and any relevant dipoles and

lone pairs.

(4)

(b) The halogenoalkane chloroethene is used to make the important polymer

poly(chloroethene), PVC.

(i) Draw a displayed formula of two repeat units of poly(chloroethene).

(1)

(ii) Some polymers are disposed of by incineration. Ignoring any economic*P51460RA01224*

considerations, explain why incineration is not a suitable method for the

disposal of poly(chloroethene).

(2)

(iii) Chloroethene has a boiling temperature of 260K and is known to be

carcinogenic. Use these facts to state one precaution that chemists should

take when using this compound.

(1)

(i) In stage 1, chlorine is reacted with ethene at a temperature between 50°C and 80°C

Cl + CH CH CH Cl CH Cl ΔH = −178 kJ mol−1

22 2 2 2

Give one reason why a temperature below 50°C and another reason, apart

from costs, why a temperature above 80°C would not be used for this process.

(2) 13

*P51460RA01324*

(ii) In stage 2, the product from the first reaction is converted to chloroethene:

CH Cl CH Cl → CH CHCl + HCl ΔH = +71 kJ mol−1

22 2

Both products are required for use in other processes.

Which method would be most suitable for the separation of these two products?

(1)

A fractional distillation

B solvent extraction using a separating funnel

C heating under reflux

D bubble through dilute alkali

(Total for Question 5 = 13 marks)

Mark scheme

Show the mark scheme

Question Acceptable Answer Mark

Number

5(a)(i) The only correct answer is B

A is not correct because reaction is not substitution

C is not correct because reaction is not substitution, nor nucleophilic

D is not correct because reaction is not nucleophilic (1)

Question

Acceptable Answer Mark

Number

5(a)(ii) The only correct answer is C

A is not correct because no C=C present

B is not correct because no C=C present

D is not correct because these are not stereoisomers (1)

Question Acceptable Answer Additional Guidance Mark

Number

An answer which shows the following: Example of mechanism

5(a)(iii)

curly arrow from double bond to H

atom of HCl/space between double

bond and H atom of HCl (1)

δ+

correct dipole on HCl molecule and :

curly arrow from H-Cl bond to Cl _

atom (1) δ

intermediate with + charge shown

on correct carbon (1)

curly arrow from lone pair on

chloride ion to correct carbon (1)

incorrectly drawn starting molecule loses M1, e.g. missing H or

pentavalent carbon.

incorrect starting molecule, e.g. butene will lose M3.

if product is 1-chloropropane M3 only is lost. Other errors in

end product lose M4

use of HBr in place of HCl loses M2 only

use of H+ and Cl- loses M1 and M2 (4)

Question Acceptable Answer Additional Guidance Mark

Number

5(b)(i)

must show two repeat units fully displayed

allow head to head, head to tail, tail to tail,

syndiotactic and isotactic stuctures

do not award any other type of formula

ignore brackets and n

(1)

Question Acceptable Answer Additional Guidance Mark

Number

5(b)(ii) An explanation that makes reference to the M2 is dependent on M1

following:

allow chlorine

(incineration produces) HCl/chlorinated molecules ignore carbon dioxide and its consequences

(1)

allow adverse effect on ozone layer

which are corrosive/toxic /cause acid rain (1) (2)

Question Acceptable Answer Additional Guidance Mark

Number

5(b)(iii) An answer that makes reference to the following:

any appropriate precautions to deal with toxic allow good ventilation required

vapours/use fume cupboard etc. allow gas mask/respirator

do not award just mask

ignore gloves, lab coat (1)

Question Acceptable Answer Additional Guidance Mark

Number

5(c)(i) An answer that makes reference to the following: allow reverse argument

at lower temperatures (below 50oC) the

reaction will be slow (1)

at higher temperatures (above 80oC) yield will

allow other products produced at higher

be lower because (forward) reaction is temperatures

exothermic

(1) (2)

Question Acceptable Answer Mark

Number

5(c)(ii) The only correct answer is A

B is not correct because separating funnel is inappropriate for an industrial process

C is not correct because not a separation process

D is not correct because both will react with alkaline solution (1)

(Total for Question 5 = 13 mark)

How to answer it

Alkenes and Halogen Compounds

What this question tests

This AS Level Chemistry question assesses your knowledge of alkene reactions (electrophilic addition), reaction mechanisms with curly arrows, structural isomerism, polymerisation of alkenes (PVC), the environmental impacts of waste disposal, practical safety precautions, and the application of equilibria and rates to industrial chemical processes.

Part (a): Reactions of Propene with Hydrogen Chloride

(i) Type of Reaction

✅ Correct Answer

B: electrophilic addition

💡 Key Knowledge

Alkenes are electron-rich due to the C=C double bond (pi bond), making them targets for electrophiles (electron pair acceptors).

(ii) Type of Isomerism

✅ Correct Answer

C: structural isomers

❌ Common Errors

Students often incorrectly choose stereoisomers or E/Z isomers. Remember that 1-chloropropane and 2-chloropropane have different connectivity of atoms, defining them as structural position isomers.

(iii) Reaction Mechanism

✅ Correct Answer & Mark Scheme Breakdown

Mark 1: Curly arrow starting from the C=C double bond pointing to the H atom of HCl.
Mark 2: Correct dipole (δ+ on H, δ- on Cl) on HCl and a curly arrow breaking the H-Cl bond.
Mark 3: Intermediate carbocation structure with a positive charge located correctly on the middle carbon.
Mark 4: Curly arrow originating from the lone pair on the chloride ion (Cl⁻) pointing directly to the positively charged carbon.

🧠 Exam Technique

Curly arrows must start precisely from bond lines or lone pairs. Ensure your carbocation intermediate clearly shows the positive charge on the central carbon atom to form 2-chloropropane as the major product.

Part (b): Poly(chloroethene) (PVC) and Disposal

(i) Repeat Units of Poly(chloroethene)

✅ Correct Answer

Display a polymer chain showing two fully displayed repeat units: -CH(Cl)-CH₂-CH(Cl)-CH₂- with all single bonds drawn out clearly.

❌ Common Errors

Drawing only one repeat unit or leaving bonds dangling incorrectly without showing two full repeating units will lose the mark.

(ii) Incineration of PVC

✅ Correct Answer

Incineration produces toxic or corrosive gases, specifically hydrogen chloride ( HCl ), which can cause acid rain.

💡 Examiner Insight

M2 is dependent on M1. You must link the formation of chlorinated molecules/HCl (M1) to their hazardous environmental or toxic consequences (M2).

(iii) Laboratory Safety Precaution

✅ Correct Answer

Use a fume cupboard (or ensure good ventilation/wear a respirator) due to the toxicity and carcinogenicity of chloroethene vapor.

🧠 Exam Technique

Avoid generic answers like "wear safety glasses" or "wear a lab coat". The question specifies that chloroethene is a carcinogenic gas, so vapor containment (fume cupboard) is essential.

Part (c): Industrial Manufacture of Chloroethene

(i) Temperature Control in Stage 1

✅ Correct Answer

Below 50°C: The reaction rate becomes too slow due to insufficient kinetic energy.
Above 80°C: The equilibrium yield decreases because the forward reaction is exothermic ( ΔH = -178 kJ mol⁻¹ ).

❌ Common Errors

Students often forget Le Chatelier's principle when explaining high temperatures. Always explicitly mention that high temperatures shift the exothermic equilibrium position in the backward direction.

(ii) Separation of Products

✅ Correct Answer

A: fractional distillation

💡 Key Knowledge

Fractional distillation exploits differences in boiling points to separate volatile liquid products efficiently on an industrial scale, unlike separating funnels which are suited for immiscible liquids in smaller laboratory settings.

Topics

Organic Chemistry · Physical Chemistry · Topic 6: Organic Chemistry I · Topic 10: Equilibrium I

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