This chemistry grades 9-10 lesson on crude oil and hydrocarbons notes gives the key ideas first, then worked examples and diagrams. After that, practise with the worksheet on crude oil and hydrocarbons and check yourself with the quiz.
Quick answer: Crude oil is a mixture of hydrocarbons separated by fractional distillation into fractions with different chain lengths and boiling points. Long chains can be cracked into shorter alkanes and reactive alkenes. Alkanes (CnH2n+2) are saturated, while alkenes (CnH2n) contain C=C and decolourise bromine water.
Level: Chemistry grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: GCSE C7
Crude oil and hydrocarbons notes: the key ideas
What is crude oil and how is it separated?
Crude oil is a thick, dark liquid that formed over millions of years from the remains of tiny sea creatures buried under mud and rock. It is a mixture of many hydrocarbons. A hydrocarbon is a compound made of carbon and hydrogen atoms only. Because the molecules differ in size, they have different boiling points, so the mixture can be separated.
In a refinery, crude oil is heated until most of it is vapour. The vapour enters a tall column that is hot at the bottom and cooler at the top. As the vapour rises it cools, and each hydrocarbon condenses where the temperature drops below its boiling point. Small molecules reach the top; large molecules condense near the bottom.
| Fraction | Carbon atoms (approx.) | Boiling range (approx.) | Typical use |
|---|---|---|---|
| Refinery gases | 1 to 4 | below 30 °C | heating and cooking gas |
| Petrol | 5 to 10 | 40 to 175 °C | car and motorbike fuel |
| Kerosene | 10 to 16 | 175 to 275 °C | jet fuel, lamps |
| Diesel | 14 to 20 | 250 to 350 °C | lorries, buses, generators |
| Fuel oil | 20 to 70 | 350 to 500 °C | ships, power stations |
| Bitumen | over 70 | above 500 °C | road surfaces, roofing |
Going down the column, the chains get longer. The boiling point rises, the liquid becomes more viscous (it flows less easily), it becomes harder to ignite and it burns with a smokier flame.
What are alkanes?
An alkane is a saturated hydrocarbon: every carbon–carbon bond is a single bond, so the molecule holds as many hydrogen atoms as possible. The general formula is CnH2n+2.
The first four alkanes are methane CH₄, ethane C₂H₆, propane C₃H₈ and butane C₄H₁₀. Alkanes are fairly unreactive, but they burn well. With plenty of oxygen they undergo complete combustion and form carbon dioxide and water. With too little oxygen they form carbon monoxide or soot (carbon), and carbon monoxide is a toxic gas.
Burning 15 g of ethane (M = 30 g/mol) completely. Equation: 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O. Moles of ethane = 15 ÷ 30 = 0.50 mol. The ratio ethane : CO₂ is 2 : 4, so CO₂ = 1.0 mol. Mass = 1.0 × 44 = 44 g of carbon dioxide.
What is cracking and why do refineries do it?
Crude oil contains more long-chain molecules than the market needs, and fewer short, useful ones. Cracking breaks long alkane molecules into shorter alkanes and alkenes. It is a thermal decomposition reaction. In catalytic cracking, hot vapour passes over a catalyst such as aluminium oxide or a zeolite at about 550 °C. In steam cracking, the vapour is mixed with steam and heated to a much higher temperature.
Balance the cracking equation C₁₂H₂₆ → C₈H₁₈ + ?C₂H₄. Carbon: 12 − 8 = 4 carbon atoms left, so two ethene molecules. Check hydrogen: 18 + 2 × 4 = 26. The equation is C₁₂H₂₆ → C₈H₁₈ + 2C₂H₄.
The shorter alkane is a better fuel. The alkene is a valuable starting material, because alkene molecules can join together to make plastics.
What are alkenes and how do we test for them?
An alkene is an unsaturated hydrocarbon that contains one carbon–carbon double bond (C=C). The general formula is CnH2n.
| Property | Alkanes | Alkenes |
|---|---|---|
| General formula | CnH2n+2 | CnH2n |
| Bonding | single bonds only | one C=C double bond |
| Example | propane C₃H₈ | propene C₃H₆ |
| Reactivity | low | higher, because of C=C |
| Flame | mostly clean | smoky, yellow |
To tell them apart, shake the liquid or gas with orange bromine water. An alkene adds the bromine across the double bond, so the orange colour disappears at once. An alkane does not react, so the water stays orange. For ethene: C₂H₄ + Br₂ → C₂H₄Br₂.
Alkenes are unsaturated, not saturated: they hold fewer hydrogen atoms than the alkane with the same number of carbons. Do not confuse the two processes either. Fractional distillation is a physical separation and breaks no bonds, but cracking breaks carbon–carbon bonds.
To get the alkane formula, double the carbon number and add 2. For an alkene, just double the carbon number. Seven carbons: alkane C₇H₁₆, alkene C₇H₁₄.
- Fractional distillation separates crude oil by boiling point: short chains at the top, long chains at the bottom.
- Alkanes: CnH2n+2, saturated, single bonds only; alkenes: CnH2n, one C=C bond.
- Longer chain: higher boiling point, higher viscosity, lower flammability.
- Cracking turns long alkanes into shorter alkanes plus alkenes, using heat and a catalyst or steam.
- Bromine water changes from orange to colourless with an alkene but not with an alkane.
Splitting crude oil into alkanes and alkenes: frequently asked questions
Why can crude oil be separated by fractional distillation?
Crude oil contains molecules of many different sizes, and each size boils at a different temperature. In the column each hydrocarbon condenses at the height where it is cooler than its boiling point, so similar chain lengths collect together.
What is the difference between saturated and unsaturated hydrocarbons?
A saturated hydrocarbon has only single carbon–carbon bonds and the most hydrogen atoms possible. An unsaturated hydrocarbon has at least one C=C double bond, so it holds fewer hydrogen atoms and is more reactive.
Why are long hydrocarbons cracked?
Demand for short fuels such as petrol is higher than the supply from crude oil. Cracking converts surplus long chains into useful shorter alkanes and alkenes, which are raw materials for plastics.
What is the general formula of the alkenes?
It is CnH2n, so an alkene has twice as many hydrogen atoms as carbon atoms. Ethene is C₂H₄ and butene is C₄H₈.
Why does a smoky flame show incomplete combustion?
A smoky flame means carbon particles formed because there was not enough oxygen. Incomplete combustion may also make carbon monoxide, which is toxic and has no smell.
