OCR A-Level Chemistry Unified chemistry (03), June 2019: Question 3
10 marks · Medium difficulty · Structured Questions
Determine the empirical formula of a lead oxide from reduction experimental data, evaluate the experimental procedure, and compare the lattice structures and bonding of silicon dioxide and carbon dioxide.
Practise this questionQuestion
Question text
3 This question is about elements and compounds in Group 14 (Group 4) of the periodic table.
(a) There are four oxides of lead: PbO, PbO2, Pb2O3 and Pb3O4.
A student carries out an experiment to identify an unknown lead oxide, which is one of the
four oxides of lead shown above.
The student plans to reduce the unknown lead oxide to lead by heating the lead oxide in a
stream of methane gas, CH4. The apparatus is shown below.
dish containing lead oxide
tube
methane gas
heat
Student’s method
• Weigh an empty dish.
Add the lead oxide to the dish and reweigh.
• Set up the apparatus and pass methane gas through the tube as shown.
Heat the dish for 10 minutes.
• Pass cold air through the tube to cool the dish and contents.
• Weigh the dish and contents.
(i) Write the equation for the reduction of Pb2O3 with CH4.
… [1]
(ii) The student uses safety glasses and a lab coat.
State, with a reason, one other important safety precaution the student should take when
carrying out this experiment.
… [1]
(iii) The student was not sure that all the oxygen had been removed from the lead oxide.
Suggest two modifications that the student could make to their method to be confident
that all the oxygen had been removed. Explain your reasoning.
1 …
2 …
[2]
(iv) The student makes suitable modifications to the method and repeats the experiment to
obtain the accurate results shown below.
Mass of dish / g 8.364
Mass of dish + lead oxide / g 11.818
Mass of dish + lead at end of experiment / g 11.496
Calculate the empirical formula of the lead oxide.
empirical formula = … [2]
(b) SiO2 and CO2 are oxides of other Group 14 (Group 4) elements.
Solid SiO2 melts at 2156 °C. Solid CO2 melts at −56 °C.
Suggest the type of lattice structure in solid SiO2 and in solid CO2 and explain the difference
in melting points in terms of the types of force within each lattice structure.
Structure in SiO2(s) …
Structure in CO2(s) …
Explanation …
… [4]
Mark scheme
Show the mark scheme
AO
Question Answer Marks Guidance
element
3 (a) (i) 4Pb2O3 + 3CH4 → 8Pb + 3CO2 + 6H2O 1 AO2.6 ALLOW multiples
OR
Pb2O3 + CH4 → 2Pb + CO + 2H2O IGNORE state symbols
OR
2Pb2O3 + 3CH4 → 4Pb + 3C + 6H2O
(ii) ONE Safety issue AND precaution 1 AO3.3 IGNORE use safety glasses, lab coat (in question)
From: and tying hair back, safety screen
Safety issue:
Compounds may be toxic/poisonous/flammable
AND
Precaution: Definite safety issue needed.
Use a fume cupboard/good ventilation Not just ‘harmful’ OR dangerous (Too vague).
------------------------------
Safety issue: FOR OTHER SAFETY ISSUES AND
Lead (compounds) is/are toxic/poisonous PRECAUTIONS, CONTACT TEAM LEADER
AND
Precaution:
Wear gloves
-----------------------------
Safety issue:
Methane is flammable
AND
Precaution:
Keep away from flame
-----------------------------
AO
9 element
(iii) Any 2 modifications 2 AO3.4 ALLOW response that implies heating to constant
from ×2 mass, e.g.
1. Heat to constant mass Heat again until the mass does not change
(Ensures all lead oxide has reacted) IGNORE ‘heat for longer’
Needs link to constant mass
2. Spread/stir/break up lead oxide
OR increase surface area IGNORE ‘weigh straight after heating’
OR use powder rather than lumps
IGNORE idea of repeating the experiment/
(Ensures all lead oxide has reacted) taking an average/ getting concordant results
/ larger sample size, etc.
3. Pass methane/inert gas/N2 through tube as it cools
OR don’t pass cold air
(Prevents O2 reacting with Pb)
4. Use excess methane OR more methane
(Ensures all lead oxide has reacted)
5. Bubble (escaping) gas through lime water
(Ensures all lead oxide has reacted
OR ensures all CO2 has been produced)
(iv) Pb : O 2 AO2.8
Masses(/g): 3.132 AND 0.322 ×2
3.132 0.322
OR Mole ratios: :
207.2 16.0
OR Mole ratios: 0.0151: 0.020125
NO ECF from incorrect masses
Empirical formula Pb3O4
H432/03 (must come from masses)Mark Scheme June 2019
AO
element
(b) 10 4 Throughout, IGNORE ‘ionic’ for SiO2
Type of lattice 2 marks
• SiO2: Giant (covalent lattice) AO1.1 FOR SiO2, IGNORE macromolecular
• CO2: Simple molecular/covalent (lattice) ×2 DO NOT ALLOW giant metallic
--------------------------------------
Explanation 2 marks Mark explanation independently on type of lattice
i.e. no ECF from incorrect lattice
1. Forces in CO For CO2
IGNORE
• Induced dipole–dipole interactions / London forces
AO1.1 • covalent bonds
×1 • van der Waals’ forces
• idid
• LDF
DO NOT ALLOW hydrogen bonds
OR permanent dipole interactions
For SiO2,
2. Comparison of forces with strength / melting point comparison needs just ‘bonds’ OR ‘forces’
• (Covalent) bonds in SiO2 are stronger
THAN intermolecular forces in CO2 For intermolecular, ALLOW ‘between molecules’
OR
• More energy to break (covalent) bonds in SiO For comparison,
AO2.1 ALLOW strong in SiO2 AND weak in CO2
THAN intermolecular forces in CO2
×1
DO NOT ALLOW responses containing
ORA
intermolecular forces in SiO2
IGNORE ‘More bonds’
Total 10
How to answer it
Group 14 Oxides: Practical Reduction, Formula Deduction & Lattice Bonding
Key Knowledge & Exam Skills Assessed:
- Redox balancing: Constructing balanced non-standard stoichiometric equations for reduction reactions using hydrocarbon reducing agents.
- Practical evaluation: Identifying hazards vs. precautions in organic/inorganic syntheses and modifying experimental designs to ensure reaction completion (heating to constant mass).
- Quantitative analysis: Calculating empirical formulas from experimental reduction mass data.
- Structure and bonding: Contrasting the physical properties of giant covalent lattices (SiO₂) with simple molecular lattices (CO₂) using exact chemical force terminology.
Equation for the Reduction of Pb₂O₃
Writing a balanced redox reduction equation
✅ Acceptable Equations
Any of the following balanced equations are awarded [1 mark]:
- 4Pb₂O₃ + 3CH₄ → 8Pb + 3CO₂ + 6H₂O (complete oxidation to CO₂)
- Pb₂O₃ + CH₄ → 2Pb + CO + 2H₂O (incomplete oxidation to CO)
- 2Pb₂O₃ + 3CH₄ → 4Pb + 3C + 6H₂O (reduction yielding soot/carbon)
🧠 Exam Technique
Lead is reduced from oxidation state +3 to 0 ( Pb ). Meanwhile, the carbon in methane ( CH₄ ) is oxidized.
When you're asked to write a reduction equation without specified products for the reducing agent, choose the simplest standard oxides ( CO₂ and H₂O ) and balance methodically: Pb first, then C, H, and finally check O.
Experimental Safety Precautions
Identifying chemical hazards and linked precautionary controls
✅ Creditworthy Answers (Hazard + Linked Precaution)
- Toxic / poisonous lead compounds: Wear gloves / avoid skin contact / wash hands after handling.
- Toxic or harmful gases produced: Carry out reaction in a fume cupboard / well-ventilated area.
- Methane is flammable / explosive: Ensure no naked flames nearby / keep apparatus away from sparks / ensure system is gas-tight.
❌ Common Errors & Pitfalls
- Repeating stem details: Stating "wear eye protection" or "wear a lab coat" scores 0 because the question explicitly states these are already in use.
- Vague descriptions: Describing chemicals simply as "dangerous" or "harmful" without naming the specific hazard (e.g., toxic, flammable).
- Unlinked precautions: Stating "wear gloves because methane is flammable" will not receive credit.
Modifications to Ensure Complete Reduction
Achieving quantitative completeness in thermal decompositions and reductions
✅ Two Modifications & Explanations (Any 2 pairs, 1 mark each)
- Heat to constant mass: Reheat the dish and contents for a further 5 minutes, cool, and reweigh until successive mass readings are identical. (Ensures all oxygen/oxide has fully reacted).
- Grind or spread the sample: Break up lumps or increase surface area before heating. (Allows methane gas to penetrate and contact all oxide particles).
- Cool under methane or inert gas (N₂): Do not pass cold air over the hot lead product. (Prevents hot lead from re-oxidising with atmospheric oxygen).
- Use an excess stream of methane: Pass methane at a higher flow rate or for a significantly longer duration. (Drives equilibrium/reaction to completion).
- Test exit gases with limewater: Bubble the exiting gas into limewater until CO₂ stops evolving. (Confirms reaction has ceased).
🧠 Top-Grade Insight
"Heat to constant mass" is the gold standard phrase in A-Level gravimetric analysis questions. Simply stating "heat for longer" does not score unless tied directly to reweighing until the mass stops changing.
Calculating Empirical Formula from Mass Data
Quantitative determination using moles
📐 Step-by-Step Calculation
Mass of dish = 8.364 g
Mass of dish + lead oxide = 11.818 g
Mass of dish + lead (end) = 11.496 g
Mass of Pb = 11.496 − 8.364 = 3.132 g
Mass of O = 11.818 − 11.496 = 0.322 g
Moles of Pb = 3.132 / 207.2 = 0.015116 mol
Moles of O = 0.322 / 16.0 = 0.020125 mol
[Award 1 mark for correct masses or mole values]
Divide both by 0.015116:
Pb: 0.015116 / 0.015116 = 1.00
O: 0.020125 / 0.015116 = 1.331 ≈ 4/3
Multiply by 3 to achieve whole numbers → Pb : O = 3 : 4
Final Answer: Pb₃O₄ [1 mark]
❌ Common Calculation Traps
- Dividing by O₂ (32.0): In empirical formula calculations, elements must be treated as individual atoms ( Aᵣ(O) = 16.0 ), not diatomic molecules!
- Premature rounding: Rounding 1.33 down to 1.0 gives a false formula of PbO . Remember: .33 = 1/3, .50 = 1/2, .67 = 2/3.
- No ECF from miscalculated masses: The mark scheme strictly specifies no error-carried-forward from incorrect masses. Always double-check your initial subtractions!
Structure & Bonding Comparison: SiO₂ vs. CO₂
Explaining drastic melting point differences (2156 °C vs. −56 °C)
✅ Mark Scheme Breakdown (4 Marks Total)
Lattice Structures (2 marks):
- SiO₂: Giant covalent (lattice) [1 mark]
- CO₂: Simple molecular (lattice) / simple covalent [1 mark]
Explanation & Forces (2 marks):
- Forces in CO₂: Induced dipole–dipole interactions / London dispersion forces between molecules [1 mark].
- Energy comparison: Covalent bonds in SiO₂ are much stronger than the intermolecular forces in CO₂ (OR significantly more energy is required to break strong covalent bonds in SiO₂ than the weak London forces between CO₂ molecules) [1 mark].
❌ Examiner Terminology Warnings
- Forbidden for SiO₂: Never call SiO₂ "ionic" or "giant metallic". Do not write that SiO₂ has "intermolecular forces".
- Disallowed terminology for CO₂: OCR guidance specifically states to IGNORE vague terms like "van der Waals", "idid", or "LDF". Use the full approved specification term: induced dipole–dipole interactions or London forces.
- Never say bonds break in CO₂ during melting: Melting solid dry ice does not break C=O covalent bonds; it only overcomes weak intermolecular forces between molecules.
💡 Model Comparison Answer Structure
"Solid SiO₂ has a giant covalent lattice with strong covalent bonds between silicon and oxygen atoms throughout the entire structure.
Solid CO₂ has a simple molecular lattice held together by weak induced dipole–dipole interactions (London forces) between CO₂ molecules.
Breaking the strong covalent bonds in SiO₂ requires far more thermal energy than overcoming the weak intermolecular forces in CO₂, which is why SiO₂ has a vastly higher melting point."
Topics
Module 1: Development of practical skills in chemistry · Module 2: Foundations in chemistry · Practical Activity Groups · 1.1 Practical skills assessed in a written examination · 2.1 Atoms and reactions · 2.2 Electrons, bonding and structure · PAG 1: Moles determination
Question and mark scheme from the OCR A-Level Chemistry examination, Unified chemistry (03), June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.