Edexcel A-Level Chemistry AS Paper 1, November 2021: Question 3

8 marks · Medium difficulty · Open Response

Write a balanced equation for the reaction of ammonia with sodium, draw 3D shapes and lone pairs for an ammonia molecule and an amide ion, state the H-N-H bond angle in ammonia, explain the difference in bond angle between ammonia and the amide ion, and give a reason why sodium amide is stored in oil.

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Question

Question text

3 Ammonia reacts with sodium to form sodium amide, NaNH2, and hydrogen.

(a) (i) Write the equation for this reaction.

State symbols are not required.

(1)

(ii) Draw diagrams showing the 3-dimensional shape of an ammonia molecule

and of an amide ion, NH–.

Include any lone pairs of electrons in each species.

(3)

ammonia molecule amide ion

(iii) What is the H–N–H bond angle in an ammonia molecule?

(1)

A 104.5°

B 107°

C 109.5°

D 120°

(iv) Explain the difference between the H–N–H bond angle in ammonia and in

8 the amide ion.

*P67083A0824* (2)

(b) Give a possible reason why samples of sodium amide are stored in oil.

(1)

(Total for Question 3 = 8 marks)

Mark scheme

Show the mark scheme

Question Answer Additional Guidance Mark

Number

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

• balanced equation 2Na + 2NH3 → 2NaNH2 + H2

Allow multiples

Ignore state symbols, even if incorrect

Number

3(a)(ii) Examples of diagrams (3)

• dot and wedge diagram of ammonia molecule with

one or two wedges or one or two series of dots / (1)

dashes

• diagram of amide ion with two bonds which are

clearly not at 180o to each other (1)

Allow any combination of dots and wedges for the

• one lone pair of electrons on the nitrogen of amide ion

ammonia and two on the amide ion Ignore lack of charge and brackets

(1)

Question Answer

Mark

Number

3(a)(iii) The only correct answer is B (107o) (1)

A is not correct because this is the expected angle for 2 bonding pairs and 2 lone pairs of electrons

C is not correct because this is the expected angle for 4 bonding pairs and no lone pairs of electrons

D is not correct because this is the expected angle for 3 bonding pairs and no lone pairs of electrons

Number

3(a)(iv) An explanation that makes reference to the following (2)

points:

• H‒N‒H bond angle is smaller in the amide ion /

104.5o / 105o (1)

• (because the amide ion has two / more) lone pairs of Allow lone pair-lone pair repulsion is greater than

electrons which repel the bonding pairs more than the lone-pair bonding pair repulsion

(1)

one lone pair of electrons in ammonia

Number

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

• sodium amide reacts (vigorously / violently) with Allow reacts with air

water / oxygen Allow will oxidise if not stored in oil / if stored in air

Do not award sodium reacts with water / oxygen / air

(Total for Question 3 = 8 marks)

How to answer it

Ammonia, Amide Ions, and Sodium Amide Chemistry

What this question tests

This question tests your understanding of chemical equations, 3D molecular shapes using Electron Pair Repulsion theory (VSEPR), bond angles and the factors that influence them (lone pair vs. bonding pair repulsion), and the physical properties/reactivity of reactive alkali metal compounds like sodium amide.

Question 3 (a)(i)

Writing a Balanced Chemical Equation

Reaction of ammonia with sodium

✅ Correct Answer

2Na + 2NH₃ → 2NaNH₂ + H₂

(Multiples and omitting state symbols are accepted)

💡 Key Knowledge

Sodium reacts with ammonia to produce sodium amide and hydrogen gas.
Always check atom balances for hydrogen and nitrogen across the arrow.

🎯 Mark: 1 mark for a fully balanced equation.

Question 3 (a)(ii)

3D Molecular Shapes and Lone Pairs

Ammonia molecule vs. Amide ion

✅ Correct Answer Requirements

Ammonia (NH₃): Trigonal pyramidal 3D shape using normal bonds, wedges, and dashes, with 1 lone pair on nitrogen.
Amide ion (NH₂⁻): Non-linear/bent shape (bonds clearly not at 180° to each other), with 2 lone pairs on nitrogen. Brackets and negative charge are good practice.

🧠 Exam Technique

When drawing 3D shapes, standard convention requires at least one wedge (coming out of the page) and one dash (going into the page) alongside normal lines to show tetrahedral electron pair arrangements clearly.

❌ Common Errors

Students frequently forget to draw the correct number of lone pairs or incorrectly draw the amide ion as linear (180° bond angle) because they ignore lone pair-lone pair repulsion effects.

🎯 Marks: 3 marks total (1 for ammonia 3D wedges/dots, 1 for non-linear amide ion structure, 1 for correct lone pair counts on both species).

Question 3 (a)(iii)

Bond Angle Identification

H—N—H bond angle in ammonia

✅ Correct Answer

B: 107°

💡 Key Knowledge & Distractor Analysis

A (104.5°): Expected for 2 bonding pairs and 2 lone pairs (e.g., H₂O).
B (107°): Correct for 3 bonding pairs and 1 lone pair (NH₃).
C (109.5°): Expected for 4 bonding pairs and 0 lone pairs (e.g., CH₄).
D (120°): Expected for 3 bonding pairs and 0 lone pairs in a trigonal planar arrangement (e.g., BF₃).

🎯 Mark: 1 mark for selecting B.

Question 3 (a)(iv)

Explaining Bond Angle Differences

Ammonia vs. Amide Ion

✅ Correct Answer Points

The H—N—H bond angle is smaller in the amide ion (around 104.5° / 105°).
This is because the amide ion has two lone pairs of electrons, whereas ammonia only has one.
Lone pair–lone pair repulsion is greater than lone pair–bonding pair repulsion, pushing the bonding pairs closer together.

🧠 Exam Technique

Always structure VSEPR explanations systematically: (1) State which angle is smaller/larger, (2) Compare the number of lone pairs on the central atom, and (3) Explicitly state the relative repulsion strengths (lone pair–lone pair > lone pair–bond pair > bond pair–bond pair).

🎯 Marks: 2 marks total (1 for stating the angle is smaller in the amide ion, 1 for linking it to having two lone pairs causing greater repulsion).

Question 3 (b)

Storage of Reactive Compounds

Why sodium amide is stored in oil

✅ Correct Answer

Sodium amide reacts vigorously/violently with water or moisture in the air (or would oxidise if left exposed).

❌ Common Errors

Do not simply state "sodium reacts with water". The question specifies sodium amide (NaNH₂), so reference must be made to sodium amide reacting with moisture, water, or air.

🎯 Mark: 1 mark for stating it reacts with water/moisture/air.

Topics

Physical Chemistry · Inorganic Chemistry · Topic 2: Bonding and Structure · Topic 5: Formulae, Equations and Amounts of Substance · Topic 4: Inorganic Chemistry and the Periodic Table

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, November 2021. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.