3.3Unit 1 · Physical & Organic Chemistry
Crude oil — Edexcel IGCSE Chemistry Modular - UNIT 1 Revision Notes & Topical Questions
10 min read 7 parts Lesson 3 of 5 1 quiz
Lesson summary
Describe how crude oil is separated by fractional distillation and state uses of each fraction.Crude oil is a finite, non-renewable resource.Complete combustionhydrocarbon + oxygen → carbon dioxide + water.Incomplete combustion produces CO, which is colourless, odourless and toxi…
- Cracking conditions
- Fractions in order from the top
- Combustion of fuels
- Nitrogen oxides and sulfur dioxide
- Colour of fractions
- Products from crude oil
Section 1 of 7Crude oil
Essential knowledge
Crude oil
Crude oil is a mixture of many hydrocarbons, mostly alkanes. It is not useful as it is, so it is separated into fractions that each contain molecules of similar chain length and similar boiling points.
How crude oil is separated
How crude oil is separated in fractional distillation the oil is vaporised and passed into a tall column with a temperature gradient (hot at the bottom, cool at the top). Hydrocarbons condense at the height where the temperature falls below their boiling point and are drawn off.
Fractions in order from the top
Fractions in order from the top refinery gases, gasoline, kerosene, diesel oil, fuel oil and bitumen. Moving down, molecules get bigger, boiling point and viscosity increase, and they become harder to ignite and less volatile.
Combustion of fuels
Combustion of fuels complete combustion forms CO2 and H2O. With insufficient oxygen, incomplete combustion produces carbon monoxide, which combines with haemoglobin and reduces the blood’s capacity to carry oxygen, and soot.
Nitrogen oxides and sulfur dioxide
Nitrogen oxides and sulfur dioxide many fossil fuels contain sulfur, which burns to form SO2. In car engines the high temperature makes nitrogen and oxygen from the air react to form nitrogen oxides. Both gases dissolve in rain to form acid rain, which corrodes limestone and metal and harms aquatic life and trees.
Cracking
Cracking there is more long-chain fuel oil than is needed and not enough gasoline. Cracking breaks long alkanes into shorter, more useful alkanes (for petrol) and alkenes (for making polymers), using a silica or alumina catalyst at 600–700 °C.
Cracking equations must balance and always produce at least one alkene, because there are not enough hydrogen atoms for all products to be alkanes.
Colour of fractions
As chain length increases down the column, the fractions become darker in colour as well as thicker, less volatile and higher boiling.
Products from crude oil
Besides fuels, crude oil fractions are the raw material for polymers, lubricants, solvents, detergents and adhesives. This variety exists because carbon can form strong covalent bonds to other carbon atoms in chains of any length.
Predicting combustion products
With limited oxygen a fuel can give carbon monoxide, e.g. 2CH4 + 3O2 → 2CO + 4H2O, or soot, e.g. CH4 + O2 → C + 2H2O. This happens in faulty boilers, stoves and car engines.
Sulfur dioxide and acid rain
Sulfur impurities burn to sulfur dioxide
- S + O2 → SO2.
- In the air it reacts with oxygen and rainwater to form sulfuric acid: 2SO2 + O2 + 2H2O → 2H2SO4.
Nitrogen dioxide and acid rain
Nitrogen dioxide from engines (and lightning) reacts with oxygen and rainwater to form nitric acid: 4NO2 + O2 + 2H2O → 4HNO3. Nitrogen oxides also cause photochemical smog and trigger asthma.
Cracking is thermal decomposition
In catalytic cracking the long-chain hydrocarbon is vaporised and passed over a hot, powdered aluminium oxide (or silica) catalyst. Bonds break at random, giving a mixture of shorter alkanes and alkenes. Cracking is endothermic, so heat must be supplied.
Section 2 of 7Process & sequence
1
Crude oil is heated until most of it vaporises.
2
The vapour enters the bottom of a fractionating column.
3
The column is hot at the bottom and cooler at the top.
4
Vapour rises; each hydrocarbon condenses when it reaches a level below its boiling point.
5
Liquid fractions are drawn off at different heights.
6
Refinery gases leave at the top; bitumen collects as a liquid at the bottom.
Fractional distillation of crude oilwrite these steps in this order; each step is a marking point.
1
Heat crude oil to vapour
2
Vapour enters column bottom
3
Vapour rises and cools
4
Fractions condense at boiling points
5
Fractions drawn off at levels
Section 3 of 7Compare
| Fraction | Approx. carbon atoms | Use |
|---|---|---|
| Refinery gases | 1–4 | Bottled gas for heating and cooking |
| Gasoline (petrol) | 5–10 | Fuel for cars |
| Kerosene | 10–16 | Fuel for aircraft |
| Diesel oil | 15–25 | Fuel for lorries, buses, some cars and trains |
| Fuel oil | 20–40 | Fuel for ships and power stations |
| Bitumen | over 40 | Road surfaces and roofing |
Going down the column
- chain length, boiling point and viscosity increase;
- volatility and ease of ignition decrease.
| Pollutant | Formed by | Problem |
|---|---|---|
| Carbon monoxide | Incomplete combustion | Toxic: reduces oxygen carried by blood |
| Soot (carbon) | Incomplete combustion | Blackens buildings, breathing problems |
| Sulfur dioxide | Burning sulfur impurities in fuel | Acid rain |
| Nitrogen oxides | N2 and O2 react at high temperature in engines | Acid rain, breathing problems |
| Carbon dioxide | Complete combustion | Greenhouse gas, climate change |
Section 4 of 7Worked examples
Worked examples
Complete the cracking equation: C14H30 → C10H22 + ____
- Carbon
- 14 − 10 = 4.
- Hydrogen: 30 − 22 = 8.
- The missing product is C4H8 (butene), an alkene.
Explain why the fractions at the bottom of the column are more viscous than those at the top.
- They contain longer hydrocarbon chains.
- Longer molecules have stronger intermolecular forces and tangle more,
- so they do not flow as easily and have higher boiling points,
- which is why they condense low in the hot part of the column.
Section 5 of 7Key terms
Section 6 of 7Command terms
Command terms
Give / State / Name
Recall a fact; one idea per mark.
Define
Give the exact meaning using the specification keywords.
Describe
Give an account of what happens / the pattern in data, quoting values; no reasons needed.
Explain
Give reasons with linked statements (cause → effect). Each linked step is usually a separate marking point.
Suggest
Use knowledge in a new context; more than one answer may be accepted.
Compare
Similarities and differences; each point must refer to both items.
Calculate
Show working; include units; check significant figures. Method marks are given even if the final answer is wrong.
Discuss
Explore the issue from different angles and link the points.
Evaluate / Assess
Weigh strengths and weaknesses and reach a supported judgement.
Deduce
Draw a conclusion from the information given.
Justify
Support a statement/choice with evidence.
Section 7 of 7Exam technique
Keywords the mark scheme looks for in this lesson
Keywords the mark scheme looks for in this lesson
Crude oil · Fraction · Fractional distillation · Viscosity · Volatility · Fuel · Cracking · Acid rain · Catalyst
Learn each definition in the Key words tab exactly; a missing keyword in a definition usually loses the mark.
How to get full marks (examiner guidance)
How to get full marks (examiner guidance)
Use specification keywords exactly (e.g. electronegativity, nucleophile, dynamic equilibrium, activation energy).
Link ideas
Link ideas for “explain” each marking point is usually a cause → consequence step; unlinked facts often score nothing.
Use the information in the question
“discuss/evaluate” answers must use the data provided and match the number of marks.
Convert units carefully (mg → g, cm³ → dm³) and show working for calculations.
Avoid vague words
“it”, “stuff”, “things”, “goes up”, “energy is made”. Name the substance and the process.
Do not extrapolate a graph line beyond the data or to zero unless asked.
Six-mark extended responses are marked on content AND a logical, linked line of reasoning.
Marked practiceQuizzes for this lesson
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