Crude Oil & Alkanes

IGCSE Edexcel Chemistry
4.1–4.18 Organic chemistry basics, crude oil, combustion, and cracking
Key Concepts: Crude oil is a mixture of hydrocarbons separated by fractional distillation. Alkanes are saturated hydrocarbons with the general formula CnH2n+2. Combustion of alkanes produces CO₂ and H₂O (complete) or CO and C (incomplete). Cracking converts long-chain alkanes into shorter, more useful products.

Section A — Organic Chemistry Basics

1. Define each term. [4]

a) Hydrocarbon

b) Homologous series

c) Functional group

d) Isomerism

2. Complete the table for the alkane homologous series. [8]
NameMolecular formulaStructural formulaState at room temp.
Methane
Ethane
Propane
Butane
3. State the general formula for alkanes. Explain why alkanes are described as saturated. [2]
4. Draw the displayed formula for propane (C₃H₈). [2]
5. Draw and name two isomers of butane (C₄H₁₀). [3]

Section B — Crude Oil and Fractional Distillation

6. Describe the process of fractional distillation of crude oil. [4]
7. Complete the table for the main fractions of crude oil. [8]
FractionApproximate boiling point rangeCarbon chain lengthMain use
Refinery gas
Gasoline (petrol)
Kerosene
Diesel oil
Fuel oil
Bitumen
8. State the trend in each property as you move from the top to the bottom of a fractionating column. [3]

a) Boiling point

b) Viscosity

c) Flammability

Section C — Combustion

9. Write the balanced symbol equation for the complete combustion of: [4]

a) Methane (CH₄)

b) Propane (C₃H₈)

10. Compare complete and incomplete combustion. State when each occurs and the products formed. [4]
11. Explain why carbon monoxide is dangerous to health. [2]
12. Explain how each pollutant is produced and the environmental problem it causes. [4]

a) Sulfur dioxide (SO₂)

b) Nitrogen oxides (NOx)

Section D — Cracking

13. Explain why cracking is necessary. [2]
14. State the conditions required for catalytic cracking. [2]
15. Complete the cracking equation: C₁₀H₂₂ → C₆H₁₄ + ______ [2]
16. Explain why alkenes (not alkanes) are produced in cracking. [2]

Total marks: 56

Mark Scheme

1. a) Compound containing only carbon and hydrogen; b) A series of compounds with the same functional group and general formula, with properties that change gradually; c) An atom or group of atoms responsible for the characteristic reactions of a compound; d) Compounds with the same molecular formula but different structural formulae [4]
2. Methane CH₄, CH₄, gas; Ethane C₂H₆, CH₃CH₃, gas; Propane C₃H₈, CH₃CH₂CH₃, gas; Butane C₄H₁₀, CH₃(CH₂)₂CH₃, gas [8 — 2 per row]
3. General formula: CnH2n+2 [1]; saturated means all C–C bonds are single bonds (no double bonds) [1] [2]
4. Correct displayed formula: three C in a chain, each C bonded to correct number of H [2]
5. n-butane (straight chain CH₃CH₂CH₂CH₃) [1]; methylpropane/isobutane (branched: CH₃CH(CH₃)CH₃) [1]; both correctly named/drawn [1] [3]
6. Crude oil is heated until it vaporises [1]; the vapour rises up a fractionating column that is cooler at the top [1]; hydrocarbons condense at different heights according to their boiling points [1]; fractions are collected as liquids at different levels [1] [4]
7. Refinery gas: <40°C, C1–C4, LPG/fuel; Gasoline: 40–75°C, C5–C10, car fuel; Kerosene: 150–250°C, C10–C16, jet fuel; Diesel: 250–350°C, C14–C20, lorries/cars; Fuel oil: 300–400°C, C20+, ships/power stations; Bitumen: >400°C, C40+, road surfacing [8 — accept approximate ranges]
8. a) Increases; b) Increases; c) Decreases [3]
9. a) CH₄ + 2O₂ → CO₂ + 2H₂O [2]; b) C₃H₈ + 5O₂ → 3CO₂ + 4H₂O [2] [4]
10. Complete combustion: occurs with excess oxygen; products CO₂ + H₂O [2]; Incomplete combustion: occurs when oxygen is limited; products CO + C (soot) + H₂O [2] [4]
11. CO binds irreversibly to haemoglobin in red blood cells [1]; reducing the blood's ability to carry oxygen → can cause death [1] [2]
12. a) SO₂: formed when sulfur impurities in fossil fuels burn; S + O₂ → SO₂; dissolves in rain to form sulfurous/sulfuric acid (acid rain) which damages forests and buildings [2]; b) NOx: formed in car engines when N₂ and O₂ from air react at high temperatures; contribute to acid rain and photochemical smog [2] [4]
13. Crude oil contains more long-chain hydrocarbons than are needed [1]; demand for short-chain fuels and alkenes (for plastics) is greater, so long chains must be broken down [1] [2]
14. High temperature (~600–700°C) [1]; catalyst (silica/alumina or zeolite) [1] [2]
15. C₄H₈ (butene) [2]
16. When long chains break, there are insufficient hydrogen atoms to form all single bonds [1]; some carbon atoms form double bonds with each other, resulting in alkenes [1] [2]