Edexcel A-Level Chemistry Paper 2, June 2018: Question 4

12 marks · Medium difficulty · Calculations

Calculate the minimum mass of sodium azide needed to inflate an airbag, balance a redox equation with oxidation numbers, explain the reaction of metal oxides with silicon dioxide, and determine the effect of cooling on a Maxwell-Boltzmann distribution.

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Question

Question text

4 Many vehicles are fitted with airbags which provide a gas-filled safety cushion to

protect the occupant of the vehicle if there is a crash.

(a) The first reaction in airbags is the thermal decomposition of sodium azide, NaN3,

to form sodium and nitrogen gas.

(i) Write the equation for this decomposition of sodium azide.

State symbols are not required.

(1)

(ii) In the reaction in (a)(i), a typical airbag is inflated by about 67 dm3 of gas.

Calculate the minimum mass of sodium azide, in grams, needed to produce

this volume of gas. Use the Ideal Gas Equation and give your answer to an

appropriate number of significant figures.

For the purpose of this calculation, assume that the temperature is 300°C and

the pressure is 140000Pa.

(4)

(b) The second reaction in the airbag is between the sodium produced in the

reaction (a)(i) and potassium nitrate.

… Na + … KNO3 → … K2O + … Na2O + … N2

Balance the above equation, justifying your answer in terms of the changes in

oxidation numbers.

(3)

State the type of reaction taking place and justify why this reaction is necessary.

(3)

(d) The Maxwell-Boltzmann distribution diagram shows the molecular energies for

the gaseous system immediately after the airbag has been deployed.

Number of molecules *P52293A0824*

with a given energy, E

Energy, E

What is the change in shape of the curve when the airbag cools?

(1)

A the peak would shift to the left and be higher

B the peak would shift to the left and be lower

C the peak would shift to the right and be higher

D the peak would shift to the right and be lower

(Total for Question 4 = 12 marks)

Mark scheme

Show the mark scheme

Question

Acceptable Answer Additional guidance Mark

Number

4(a)(i) correct equation Example of equation: (1)

2NaN3 → 2Na + 3N2

Allow multiples

Ignore state symbols even if incorrect

Question

Acceptable Answer Additional guidance Mark

Number

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

conversion of volume and temperature to correct 67 dm3 = 0.067 m3,

units (1) 300°C = 573 K

rearrangement of ideal gas equation so n=pV ÷ RT n(N2) = 140 000 x 0.067 =

and calculation of n(N2) in moles (1) 8.31 x 573

= 1.9699…..(mol)

evaluation of n(NaN3) (1) n(NaN3) =

(2/3 x 1.9699…..=) 1.313…. (mol)

answer converted into mass to 2/3 SF (1) m= (1.313 …. x 65 = 85.3629..=)

= 85.4 / 85 (g)

Allow TE at each stage

Correct answer without working scores

(4)

Question

Acceptable Answer Additional guidance Mark

Number

4(b) An answer that makes reference to the following points: Look for oxidation numbers annotated (3)

on the equation

Nitrogen (is reduced) from +5 to 0 (1) Do not award potassium oxidised

Sodium (is oxidised) from 0 to +1 (1) Penalise omission of “+” sign, once only

Balanced equation (1) Example of balanced equation:

10Na +2KNO3 → K2O+ 5Na2O + N2

Allow multiples

Question

Acceptable Answer Additional guidance Mark

Number

4(c) An answer that makes reference to the following points: (3)

Neutralisation reaction / acid base reaction (1) Allow salt formation

Sodium and/or potassium oxides are caustic / Allow “metal oxides”

corrosive (1) Ignore “harmful” / “alkaline”

Salts (silicates) formed are inert / unreactive (1) Allow “not harmful”/ “not caustic”

Ignore “neutral”

Question

Acceptable Answer Mark

Number

4(d) The only correct answer is A (1)

B is incorrect because the peak would shift to the left and be higher

C is incorrect because the peak would shift to the left not to the right

D is incorrect because the peak would be shift to the left not to the right

(Total for Question 4 = 12 marks)

How to answer it

Chemistry Study Guide: Car Airbag Chemistry

What this question tests

This comprehensive multi-part question assesses core physical and inorganic chemistry concepts: writing and balancing decomposition and redox equations, applying the Ideal Gas Equation in complex stoichiometry calculations, understanding acid-base neutralisation involving metal oxides, and interpreting Maxwell-Boltzmann distribution curves under varying temperature conditions.

Question 4(a)(i) — Thermal Decomposition

Decomposition of Sodium Azide

✅ Correct Answer

2NaN₃ → 2Na + 3N₂

(Allow multiples. State symbols are not required as per question stem).

💡 Key Knowledge

Sodium azide ( NaN₃ ) thermally decomposes into solid sodium metal and nitrogen gas.
Nitrogen gas is generated rapidly to inflate the safety cushion.

Mark breakdown: (1) Unit conversions • (2) Ideal gas rearrangement & moles • (3) Reacting ratio evaluation • (4) Final mass to 2/3 SF

Question 4(a)(ii) — Ideal Gas Calculation

Calculating Minimum Mass of Sodium Azide

📐 Step-by-Step Calculation

Convert Units:
Volume: 67 dm³ = 0.067 m³ (divide by 1000)
Temperature: 300 °C + 273 = 573 K
Rearrange Ideal Gas Equation ( pV = nRT ):
n(N₂) = pV / RT
n(N₂) = (140,000 × 0.067) / (8.31 × 573) = 1.9699... mol
Use Reacting Ratios:
From part (a)(i), 2 moles of NaN₃ produce 3 moles of N₂ .
n(NaN₃) = (2 / 3) × 1.9699... = 1.313... mol
Calculate Mass & Apply Significant Figures:
Molar mass of NaN₃ = 23.0 + (3 × 14.0) = 65.0 g mol⁻¹
Mass = 1.313... × 65.0 = 85.36 g
Final Answer: 85 g or 85.4 g (to 2 or 3 significant figures).

❌ Common Calculation Traps

Unit Failure: Forgetting to convert dm³ to m³ when using R = 8.31 J mol⁻¹ K⁻¹ .
Temperature Mistake: Using 300 directly instead of adding 273 to convert to Kelvin.
Ratio Inversion: Multiplying by 3/2 instead of 2/3 for the mole ratio.

Mark breakdown: (1) Nitrogen oxidation number change • (2) Sodium oxidation number change • (3) Fully balanced equation

Question 4(b) — Redox Balancing

Balancing Redox Equations via Oxidation Numbers

✅ Correct Balanced Equation

10Na + 2KNO₃ → K₂O + 5Na₂O + N₂

🧠 Justification & Exam Technique

Nitrogen: Changes from +5 in KNO₃ to 0 in N₂ (Reduced).
Sodium: Changes from 0 in Na to +1 in Na₂O (Oxidised).
Examiner Tip: Always explicitly write the + sign for positive oxidation states (e.g., +1 , +5 ) to secure marks. Do not state that potassium is oxidised.

Mark breakdown: (1) Identify neutralisation/acid-base • (2) Toxicity/caustic nature of metal oxides • (3) Inert/safe nature of silicate salts formed

Question 4(c) — Reaction Type & Justification

Neutralisation of Hazardous Metal Oxides

✅ Acceptable Answers

Type: Neutralisation reaction / acid-base reaction (Silicon dioxide acts as a weak acid, metal oxides act as bases).
Necessity 1: Sodium and potassium oxides are strongly caustic and corrosive.
Necessity 2: The resulting silicate salts are stable, inert, and completely harmless to vehicle occupants.

❌ Common Errors

Vague descriptions like "making it safe" without specifying that the metal oxides are caustic/corrosive.
Failing to explicitly name the reaction type as neutralisation or acid-base.

Mark breakdown: (1) Correct multiple-choice selection (A)

Question 4(d) — Maxwell-Boltzmann Distribution

Effect of Cooling on Molecular Energies

✅ Correct Option: A

The peak would shift to the left and be higher.

💡 Explanation & Rationale

When an airbag cools, the average kinetic energy of the gas molecules decreases.
This shifts the most probable energy (the peak) to the left (lower energy).
Because the total number of molecules (area under the curve) remains constant, a lower, narrower spread means the peak must become higher.

Topics

Physical Chemistry · Inorganic Chemistry · Topic 5: Formulae, Equations and Amounts of Substance · Topic 3: Redox I · Topic 9: Kinetics I · Topic 4: Inorganic Chemistry and the Periodic Table

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