Edexcel A-Level Chemistry Paper 2, June 2022: Question 4

15 marks · Medium difficulty · Open Response

Calculate the volume of nitrogen gas using the ideal gas equation, draw dot-and-cross diagrams for a nitrogen molecule and a nitride ion, explain the shapes and bond angles of ammonia and the ammonium ion, and explain how butylamine acts as a nucleophile and base when reacting with ethanoyl chloride.

Practise this question

Question

A four-part exam question about nitrogen and nitrogen compounds. Part (a)(i) asks to calculate the volume of nitrogen gas in cm3 given 0.420 g, 120 kPa, and 20 degrees Celsius. Part (a)(ii) asks why nitrogen is used in crisp packets instead of air. Part (b) requires dot-and-cross diagrams for a nitrogen molecule and the nitride ion in sodium nitride. Part (c) asks to explain the different shapes and bond angles of ammonia and the ammonium ion. Part (d) provides a chemical equation for the reaction of butylamine with ethanoyl chloride and asks to explain how butylamine acts as a nucleophile and as a base.
Question text

4 This question is about nitrogen and some nitrogen compounds.

(a) A study of one brand of crisps found that each packet contained

0.420g of nitrogen gas at a pressure of 120kPa and a temperature of 20°C.

(i) Calculate the volume of nitrogen gas, in cm3, in one packet of crisps.

[R = 8.31 J mol–1 K–1]

(4)

(ii) Give a possible reason why nitrogen gas and not air is used in packets

of crisps.

(1)

(b) Draw dot-and-cross diagrams for a molecule of nitrogen gas and for the

nitride ion, N3–, in sodium nitride, Na N.

Use dots ( ) for nitrogen electrons and crosses (X) for electrons from sodium.

(2)

Nitrogen molecule Nitride ion

(c) Ammonia accepts a proton to form an ammonium ion.

NH + H+ → NH+

Explain why the ammonia molecule and the ammonium ion have different shapes

and different bond angles.

(4)

*P67094RA0624*

(d) Butylamine, C4H9NH2 , reacts with ethanoyl chloride.

2C H NH + CH COCl → C H NHCOCH + C H NH+Cl–

49 2 3 4 9 3 4 9 3

Explain how this equation illustrates that butylamine acts as a nucleophile and as

a base.

(4)

(Total for Question 4 = 15 marks)

Mark scheme

Show the mark scheme The mark scheme provides detailed answers and guidance for all four parts of Question 4. Part (a)(i) awards 4 marks for moles calculation, unit conversion, rearrangement of pV=nRT, and conversion to cm3. Part (a)(ii) awards 1 mark for mentioning prevention of oxidation. Part (b) awards 2 marks for correct dot-and-cross diagrams. Part (c) awards 4 marks based on lone pair repulsions, number of bond pairs, and specific bond angles of 107 degrees and 109.5 degrees. Part (d) awards 4 marks for defining the nucleophile electron pair donation, attack at the carbon-8 position, production of hydrogen chloride, and proton-accepting basicity.

Question

Answer Additional Guidance Mark

Number

4(a)(i) Example of calculation: (4)

• evaluation of number of moles of nitrogen (1) n = 0.42 ÷ 28 = 0.015 (mol)

• conversion of pressure and temperature to correct units (1) 120 kPa = 120 000 Pa,

20°C = 293 K

• rearrangement of ideal gas equation so V = nRT ÷ P and V= 0.015 x 8.31 x 293

evaluation of volume (1) 120 000

= 3.0435 x 10−4 (m3)

• answer converted into cm3 (1) = 3.0435 x 10−4 x 106

= 304 (cm3)

Ignore SF except 1SF

TE throughout

Correct answer without working

scores (4)

Question

Answer Additional Guidance Mark

Number

4(a)(ii) An answer that makes reference to (1)

Allow answers such as

• prevents oxidation (of the crisps) ‘keep the crisps fresh’ or ‘prevents the

crisps from going off/stale’

Allow reference to ‘crisps not reacting

with nitrogen but will with air’

Ignore reference to gas prevents crisps

from getting squashed/broken

Ignore nitrogen is less reactive than

air/oxygen or nitrogen is inert

Ignore reference to effects of moisture

Question

Answer Additional Guidance Mark

Number

4(b) Example of dot-and-cross diagrams (2)

• dot-and-cross diagram of nitrogen gas (1)

• dot-and-cross diagram of nitride ion (1)

Allow electrons to be paired horizontally

Allow the crosses to be paired in any way

Allow representation of inner shell with two electrons

Allow any pairing of dots and crosses

Ignore lines representing covalent bonds

Ignore missing circles

Ignore absence of brackets and charge on nitride ion

Ignore any diagram of the sodium ion

Question

Answer Additional Guidance Mark

Number

4(c) An explanation that makes reference to the following points: (4)

• the lone pair (of electrons) in ammonia repels more than the Standalone mark

bonded pairs (of electrons) (1)

The further three marks are scored as follows:

Six of the following scores (3) four or five scores (2) and two or Accept points made on labelled

three scores (1) diagrams

• the ammonia molecule has three bond pairs and

one lone pair

• the ammonium ion has four bond pairs

• the ammonia molecule is (trigonal) pyramidal

• ammonium ion is tetrahedral

• the bond angle in ammonia is 107(°)

• the bond angle in the ammonium ion is 109.5(°)

Question

Answer Additional Guidance Mark

Number

4(d) An explanation that makes reference to the following points: (4)

• nucleophiles are electron pair donors / attack areas of low

electron density / the nitrogen donates its lone pair of

electrons (1)

• so the amine group attacks as a (nucleophile) by attacking Allow the N/ butylamine for ‘the amine

the Cδ+ of the acyl chloride (1) group’

Allow shown in a mechanism

Do not award attacks carbocation

• which produces hydrogen chloride (1) Allow hydrochloric acid

• it’s a base because amine group reacts with the acid / Allow the N/ butylamine for ‘the amine

protons (to produce the salt / C4H9NH3Cl) (1) group’

Allow base is a proton acceptor

Do not award just ‘hydrogen’ for

proton

Do not award reference to ethanoyl

chloride as an acid/donating a proton

(Total Question 4 = 15 marks)

How to answer it

Nitrogen and Nitrogen Compounds

What this question tests

This comprehensive question assesses core physical, inorganic, and organic chemistry principles: applying the Ideal Gas Equation with strict unit conversions, evaluating industrial/commercial applications of gases, drawing bonding representations (dot-and-cross diagrams), explaining shapes of molecules and ions using Valence Shell Electron Pair Repulsion (VSEPR) theory, and analysing amine reaction mechanisms as both nucleophiles and bases.

Part (a)(i) — Ideal Gas Calculation (4 marks)

Calculate the volume of nitrogen gas, in cm³, in one packet of crisps.

📐 Step-by-Step Calculation

  1. Moles of N₂: n = mass / Mr = 0.420 / 28.0 = 0.015 mol
  2. Convert Pressure: 120 kPa = 120 000 Pa (Multiply by 10³)
  3. Convert Temperature: 20 °C = 293 K (Add 273)
  4. Rearrange Ideal Gas Equation: V = nRT / P
  5. Evaluate Volume (m³): (0.015 × 8.31 × 293) / 120 000 = 3.0435 × 10⁻⁴ m³
  6. Convert to cm³: 3.0435 × 10⁻⁴ × 10⁶ = 304 cm³

❌ Common Errors & Traps

  • Unit conversion failures: Forgetting to convert kPa to Pa (multiplying by 10³) or failing to convert m³ to cm³ at the final step (multiplying by 10⁶ instead of dividing or using wrong powers).
  • Temperature trap: Using Celsius (20) directly instead of Kelvin (293 K).
  • Mr calculation: Using N instead of N₂ ( Mr = 28.0 ).
Mark Scheme Breakdown: (1) Evaluation of moles of nitrogen; (2) Conversion of pressure and temperature to correct SI units; (3) Rearrangement of Ideal Gas equation and calculation of volume in m³; (4) Final conversion into cm³.
Part (a)(ii) — Applied Chemistry (1 mark)

Give a possible reason why nitrogen gas and not air is used in packets of crisps.

✅ Correct Answer

It prevents oxidation of the crisps (or keeps them fresh / prevents them from going off/stale).

❌ What NOT to write

Examiners strict on this: Do not credit statements like "nitrogen is inert", "nitrogen is unreactive", "nitrogen prevents crisps getting crushed", or "prevents moisture/dampness". Focus strictly on chemical oxidation/spoilage.

Part (b) — Bonding & Structure (2 marks)

Draw dot-and-cross diagrams for a molecule of nitrogen gas and for the nitride ion.

💡 Key Knowledge

  • Nitrogen molecule (N₂): Requires a triple covalent bond. Show 3 pairs of electrons shared between the two nitrogen atoms, plus one lone pair on each outer shell.
  • Nitride ion (N³⁻): Nitrogen atom has gained 3 electrons (shown as crosses), giving a total of 8 electrons in its outer shell. Enclose in square brackets with a 3- charge outside.

🧠 Exam Technique

Follow the stem instructions carefully: use dots ( • ) for nitrogen electrons and crosses ( × ) for transferred electrons (from sodium). Examiners allow horizontal pairing and do not penalise missing inner shells or inner circle boundaries.

Mark Scheme Breakdown: (1) Correct dot-and-cross diagram for N₂ molecule; (2) Correct dot-and-cross diagram for the N³⁻ ion (including brackets and charge).
Part (c) — VSEPR Theory & Shapes (4 marks)

Explain why the ammonia molecule and the ammonium ion have different shapes and different bond angles.

✅ Correct Model Answer

  • Ammonia (NH₃): Trigonal pyramidal shape with a bond angle of 107°. It has 3 bond pairs and 1 lone pair.
  • Ammonium ion (NH₄⁺): Tetrahedral shape with a bond angle of 109.5°. It has 4 bond pairs and 0 lone pairs.
  • Repulsion rule: Lone pairs repel more than bonded pairs, which compresses the bond angles in ammonia down from the tetrahedral 109.5° to 107°.

❌ Common Errors

Students often lose marks by stating generic repulsion ("electrons repel each other") rather than specifically stating that lone pair-bond pair repulsion is greater than bond pair-bond pair repulsion.

Mark Scheme Breakdown: (1) Stating that lone pair-bond pair repulsion > bond pair-bond pair repulsion (standalone mark); plus 3 marks awarded for correctly identifying shapes, number of electron pairs, and exact bond angles for both species.
Part (d) — Organic Mechanisms & Properties (4 marks)

Explain how this equation illustrates that butylamine acts as a nucleophile and as a base.

💡 Key Knowledge: Nucleophile vs Base

  • As a Nucleophile: The nitrogen atom donates an electron pair to attack areas of low electron density (specifically the electron-deficient carbon, C⁺ , of the acyl chloride).
  • As a Base: Butylamine accepts a proton ( H⁺ ) released during the reaction, reacting with the resulting hydrogen chloride to form a salt ( C₄H₉NH₃⁺Cl⁻ ).

🧠 Examiner Guidance

Top-tier responses explicitly link the lone pair on nitrogen to nucleophilic attack on Cδ⁺ , and separately define the base role as accepting protons or reacting with the generated acid ( HCl ). Avoid vague phrases like "attacks the carbocation" or just saying "it accepts hydrogen".

Mark Scheme Breakdown: (1) Nucleophile definition (electron pair donor); (2) Application to amine attacking the delta-plus carbon of acyl chloride; (3) Production of hydrogen chloride; (4) Base definition / action (proton acceptor reacting with acid/protons to form the salt).

Topics

Physical Chemistry · Organic Chemistry · Topic 5: Formulae, Equations and Amounts of Substance · Topic 2: Bonding and Structure · Topic 12: Acid-base Equilibria · Topic 18: Organic Chemistry III

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 2, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.