Edexcel A-Level Chemistry Paper 3, November 2021: Question 3

10 marks · Hard difficulty · Open Response

Complete the electronic configuration of an aluminium atom, draw a dot-and-cross diagram, predict the shape and bond angle of AlCl3, draw the mechanism for the alkylation of benzene using AlCl3 as a catalyst, and illustrate the dimerisation of AlCl3 including the type of bonding involved.

Practise this question

Question

Question text

3 This question is about aluminium chloride.

(a) Complete the electronic configuration of an aluminium atom.

(1)

1s2

(b) At high temperatures, aluminium chloride exists as AlCl3 molecules.

(i) Draw a dot-and-cross diagram of an aluminium chloride molecule, AlCl3.

Show the outer shell electrons only.

(1)

(ii) Predict the shape of an AlCl3 molecule and the Cl−Al−Cl bond angle.

(2)

Shape of AlCl3

Cl−Al−Cl bond angle

(iii) Aluminium chloride is used as a catalyst in the alkylation of benzene.

Draw the mechanism for the reaction between benzene and chloromethane

using aluminium chloride as the catalyst.

Include an equation for the formation of the electrophile, and any relevant

curly arrows.

(4)

*P67806A0636*

(i) Draw a diagram to show how two AlCl3 molecules are joined together in

the dimer.

(1)

(ii) State the type of bond that joins the two AlCl3 molecules together.

(1)

(Total for Question 3 = 10 marks)

Mark scheme

Show the mark scheme

Question Answer Additional Guidance Mark

Number

3(a) Allow numbers of electrons as subscripts or large (1)

• (1s2)2s22p63s23p1 numbers

Allow p orbitals designated as x, y and z

Ignore 1s2 repeated

Number

3(b)(i) Example of diagram (1)

• dot-and-cross diagram

Allow electrons in overlapping circles

Allow all dots / all crosses

Ignore inner shell electrons, even if incorrect

Ignore lines as bonds e.g.

•

Do not award diagram with lone pair on Al

Number

3(b)(ii) An answer that makes reference to the following points: Mark independently (2)

• shape – trigonal planar (1) Both words needed

Allow triangular for trigonal – but not just tri

• bond angle - 120° (1) Allow marks for labelled diagram

Note

If shape is pyramidal, no mark for M1 but allow (1)

for 107°

No TE for any other shape

Number

3(b)(iii) Example of mechanism (4)

CH Cl + AlCl → CH + + [AlCl ]−

33 3 4

Allow AlCl − / δ+CH -AlCl δ−

• equation for the formation of the electrophile (1) 4 3 4

+ (1) Allow curly arrow from anywhere within the

• curly arrow from on or within the circle to CH3

hexagon

Allow curly arrow to any part of CH +, including

the + charge

Do not award curly arrow from outside the hexagon

• structure of intermediate including charge with some part Allow dotted / dashed lines for horseshoe

of the charge within the horseshoe Do not award dotted bonds to H and CH unless

and horseshoe covering at least 3 carbon atoms clearly part of a 3D structure

and facing the tetrahedral carbon

• curly arrow from C-H bond to anywhere in the hexagon (1) −

Ignore any involvement of AlCl4 in the final step

reforming the delocalised structure /HCl

(1) Note

Correct Kekulé structures score full marks

Number

3(c)(i) Example of diagram (1)

• diagram showing two AlCl3 molecules joined through two

chlorine atoms

Allow dot-and-cross diagram

Ignore missing arrows / direction of arrows

Ignore missing lone pairs

Number

3(c)(ii) (1)

Allow this labelled on diagram in (i)

• dative (covalent) bonds

or Do not award this mark if dative bonds shown as

coordinate bonds arrows starting from aluminium in (c)(i)

(Total for Question 3 = 10 marks)

How to answer it

Aluminium Chloride Chemistry Study Guide

What this question tests

This comprehensive question assesses core inorganic and physical chemistry principles applied to Group 13 elements, including electronic configurations, VSEPR theory for predicting molecular shapes and bond angles, electrophilic substitution reaction mechanisms (Friedel-Crafts alkylation), and advanced bonding concepts such as electron-deficient molecules and dative covalent (coordinate) bonding in dimers.

Question Part (a)

Electronic Configuration of Aluminium

✅ Correct Answer

2s² 2p⁶ 3s² 3p¹ (or 1s² 2s² 2p⁶ 3s² 3p¹ )

Awarded 1 mark

💡 Key Knowledge

Aluminium has an atomic number (Z) of 13.
Fill sublebs in order of increasing energy levels according to the Aufbau principle.

Question Part (b)(i)

Dot-and-Cross Diagram for AlCl₃

✅ Correct Answer

A central Al atom bonded to 3 chlorine atoms using single covalent bonds, showing only outer shell electrons (Al contributes 3 electrons, each Cl contributes 1, plus 6 non-bonding electrons around each Cl).

Awarded 1 mark

❌ Common Errors

Students frequently lose marks by mistakenly drawing a lone pair on the central aluminium atom. Remember that aluminium in AlCl₃ only has 6 electrons in its outer shell (electron deficient).

Question Part (b)(ii)

Shape and Bond Angle of AlCl₃

✅ Correct Answer

Shape of AlCl₃: Trigonal planar
Cl-Al-Cl bond angle: 120°

Awarded 2 marks (1 mark per point, marked independently)

🧠 Exam Technique

Both words ( trigonal planar ) are strictly required for the first mark. 'Triangular' is accepted, but 'tri' alone is not. Make sure you memorise standard VSEPR geometries for 3-coordinate species with no lone pairs.

Question Part (b)(iii)

Mechanism of Friedel-Crafts Alkylation

✅ Correct Answer

Equation for electrophile formation: CH₃Cl + AlCl₃ ➔ CH₃⁺ + [AlCl₄]⁻
Curly arrow 1: From inside or on the benzene ring circle to the electrophile (CH₃⁺).
Intermediate structure: Horseshoe shape covering at least 3 carbon atoms, with a positive charge inside and the incoming CH₃ group explicitly attached to the ring.
Curly arrow 2: From the C-H bond back into the delocalised ring to restore aromaticity and release H⁺.

Awarded 4 marks total

❌ Common Errors

Starting the first curly arrow from outside the delocalised ring or away from the electrons.
Failing to extend the horseshoe symbol over at least 3 carbon atoms.
Omitting the positive charge entirely from the intermediate structure.

Question Part (c)(i)

Dimer Structure of Aluminium Chloride (Al₂Cl₆)

✅ Correct Answer

A diagram showing two AlCl₃ units linked together through two bridging chlorine atoms, forming a central four-membered Al-Cl-Al ring.

Awarded 1 mark

💡 Key Knowledge

Aluminium chloride vaporises to form a dimer to complete an octet of electrons around each aluminium atom, overcoming its electron deficiency.

Question Part (c)(ii)

Bonding Type in Al₂Cl₆ Dimer

✅ Correct Answer

Dative (covalent) bonds or coordinate bonds.

Awarded 1 mark

❌ Common Errors

Do not draw conventional reaction arrows starting from aluminium in part (c)(i) to indicate these bonds, as the chlorine lone pair is actually being donated to the aluminium atom.

Topics

Physical Chemistry · Organic Chemistry · Topic 1: Atomic Structure and the Periodic Table · Topic 2: Bonding and Structure · Topic 18: Organic Chemistry III

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.