Edexcel A-Level Chemistry AS Paper 1, June 2018: Question 1

9 marks · Medium difficulty · Short Open Response

Describe covalent and dative covalent bonding, deduce the shape and bond angles of ammonia and boron trifluoride, and explain how their reaction changes these bond angles.

Practise this question

Question

Question text

1 This question is about covalent bonds.

(a) State what is meant by the term covalent bond.

(2)

(b) Draw a diagram of the ammonia molecule, clearly showing its shape.

Include any lone pairs of electrons and the value of the bond angle.

(2)

F B F

What is the bond angle in BF3?

(1)

A 90°

B 107°

C 109.5°

D 120°

(d)2 (i) Ammonia and boron trifluoride react to form a compound NH3BF3 which

contains a dative covalent bond. Each of the molecules, NH*P51459A0224*3and BF3, has a

different feature of its electronic structure that allows this to happen.

Use these two different features to explain how a dative covalent bond is formed.

(2)

(ii) During this reaction, the bond angles about the nitrogen atom and the

boron atom change.

State the new H N H and F B F bond angles.

(2)

(Total for Question 1 = 9 marks)

Mark scheme

Show the mark scheme

Question

Acceptable Answer Additional Guidance Mark

Number

1(a) An answer that makes reference to the following points: (2)

(strong electrostatic) attraction (1)

between two nuclei and the shared /bonding pair of

electrons (1)

Question

Acceptable Answer Additional Guidance Mark

Number

1(b) Example of diagram: (2)

diagram showing 3-D shape of ammonia,

including two bonds with one ‘wedge’ and

one ‘hatch’ and one N-H bond ‘in plane’ (1)

Allow any direction of the wedge and/or hatch

lone pair of electrons on nitrogen atom This mark can be scored on a dot and cross

diagram

and

bond angle of 107° labelled (1) Allow any angle between 106 and 108° inclusive.

Do not award M2 if the 107o bond angle is shown

as that between the lone pair and a bonding pair

Ignore name of shape even if incorrect

Question

Answer Mark

Number

1(c) The only correct answer is D (1)

A is not correct because this is approximately the angle given in the diagram

B is not correct because this is the angle for three bonds when there is also a lone pair on the central

atom

C is not correct because this is the angle when there are four pairs of bonding electrons

around the central atom

Question

Acceptable Answer Additional Guidance Mark

Number

1(d)(i) An answer that makes reference to the following points: (2)

donation of lone pair (of electrons) from nitrogen / Allow ‘non-bonding pair’ for lone pair

lone pair from ammonia (1) Allow ‘sharing’ for donation

Do not penalise donation to F atoms,

but can only score M1 in this case

to the boron (atom) which is electron deficient / has Allow just ‘boron has an incomplete

only 6 electrons in outer shell / has 6 valence outer shell’

electrons / can accept two electrons to complete octet Allow boron has an empty (p-)orbital

/can accept two electrons to get a full (outer) shell

Do not award M2 for just ‘nitrogen

(1) shares lone pair with boron atom’ or

similar

M1 may be scored from a diagram

here OR a diagram in (d)(ii)

e.g.

scores only M1

Question

Acceptable Answer Additional Guidance Mark

Number

1(d)(ii) May be shown on a diagram, including (2)

HNH angle is (approximately) 109.5° (1) on a diagram in 3(d)(i)

e.g

FBF angle is (approximately) 109.5° (1)

Allow 1 for just 109.5° if it has not

been made clear that this angle

applies to BOTH bond angles

Both angles change to 109.5°

scores 2

Allow 109-110°

(Total for Question 1 = 9 marks)

How to answer it

Study Guide: Covalent Bonding & Molecular Shapes

What this question tests

This question assesses core foundational concepts in covalent bonding and molecular structure for Edexcel AS Level Chemistry. It covers the definition of a covalent bond, VSEPR theory (predicting 3D shapes and bond angles using electron pairs), identifying multiple-choice structural properties, explaining dative covalent (coordinate) bonding mechanisms using electron pairs and electron deficiency, and understanding how molecular geometry changes upon forming addition compounds.

Part (a) — Definition of a Covalent Bond (2 Marks)

State what is meant by the term covalent bond.

✅ Correct Answer

A covalent bond is a strong electrostatic attraction between two nuclei and the shared pair of electrons.

💡 Key Knowledge

Must mention both key marking points: electrostatic attraction AND shared pair of electrons between nuclei.
Do not confuse this with ionic bonding (transfer of electrons) or metallic bonding (delocalized sea of electrons).

❌ Common Errors

Saying "sharing of electrons" without stating that the shared electrons are electrostatically attracted to the positive nuclei.
Using vague terms like "sharing a shell" or "holding atoms together".

Mark breakdown: 1 mark for strong electrostatic attraction + 1 mark for between two nuclei and the shared pair of electrons.

Part (b) — Shape and Bond Angle of Ammonia (2 Marks)

Draw a diagram of the ammonia molecule, clearly showing its shape. Include any lone pairs of electrons and the value of the bond angle.

✅ Correct Answer

A 3D drawing of trigonal pyramidal geometry for NH₃:

Central N atom bonded to three H atoms.
Use standard 3D notation: two normal lines for in-plane bonds, one wedge for a bond pointing forwards, and one hashed/dotted line for a bond pointing backwards.
Include a clearly visible lone pair on the nitrogen atom.
State bond angle: 107° (accept 106° to 108°).

🧠 Exam Technique

Always practice drawing 3D bonds (wedges and hatches) cleanly so spatial orientation is unambiguous.
Ensure the bond angle label ( 107° ) is clearly associated with the H—N—H angle, not jammed between the lone pair and bonding pair (which loses M2).

Mark breakdown: 1 mark for 3D diagram showing correct shape (wedge/hatch) and lone pair on nitrogen + 1 mark for the correct bond angle ( 107° ).

Part (c) — Bond Angle in BF₃ (1 Mark)

What is the bond angle in BF₃? (Options: A 90°, B 107°, C 109.5°, D 120°)

✅ Correct Answer

D: 120°

💡 Key Knowledge & Distractors

BF₃ has 3 bonding pairs and 0 lone pairs around the central boron atom, giving a trigonal planar shape with a 120° bond angle.
A (90°) is incorrect (typical for octahedral/linear geometries).
B (107°) is the angle for trigonal pyramidal molecules with 1 lone pair (like NH₃).
C (109.5°) is the angle for tetrahedral molecules with 4 bonding pairs (like CH₄).

Mark breakdown: 1 mark for selecting D.

Part (d)(i) — Dative Covalent Bonding Mechanism (2 Marks)

Use these two different features of electronic structure to explain how a dative covalent bond is formed in NH₃BF₃.

✅ Correct Answer

Feature 1 (Ammonia): Ammonia has a lone pair of electrons on the nitrogen atom available for donation.
Feature 2 (Boron trifluoride): Boron is electron-deficient (has only 6 electrons in its outer shell / an incomplete octet / can accept an electron pair).
The Bond Formation: The lone pair is donated from nitrogen to the boron atom.

🧠 Exam Technique

Top-level responses explicitly reference both participating molecules' specific features: NH₃ provides a lone pair, while BF₃ provides an electron-deficient / incomplete octet center.
Examiners accept terms like "incomplete outer shell" or "empty orbital" for boron.

Mark breakdown: 1 mark for stating donation of a lone pair from ammonia + 1 mark for stating boron is electron-deficient / has 6 electrons / incomplete outer shell.

Part (d)(ii) — Change in Bond Angles (2 Marks)

During this reaction, the bond angles about the nitrogen atom and the boron atom change. State the new H—N—H and F—B—F bond angles.

✅ Correct Answer

New H—N—H angle: 109.5° (accept 109° to 110°)
New F—B—F angle: 109.5° (accept 109° to 110°)

💡 Key Knowledge

When NH₃ forms a dative bond, its nitrogen now coordinates 4 pairs of bonding electrons (no lone pair involved in repulsion affecting this specific coordination geometry in the same way, becoming tetrahedral). Thus, the bond angle shifts from 107° up to tetrahedral 109.5° .
When BF₃ accepts the dative bond, boron changes from trigonal planar (3 bond pairs, 120° ) to tetrahedral (4 bond pairs, 109.5° ).

Mark breakdown: 1 mark for H—N—H angle of 109.5° + 1 mark for F—B—F angle of 109.5° . (Note: Allow 1 mark total if 109.5° is written once clearly applying to both).

Topics

Physical Chemistry · Topic 2: Bonding and Structure

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.