What is crude oil, how is it separated into fuels, and how does cracking help?
Hydrocarbons and fuels: crude oil and the alkanes, fractional distillation into useful fractions, the trends in the properties of the fractions, complete combustion, and cracking.
A focused answer to Edexcel GCSE Chemistry topic 8, covering crude oil as a mixture of hydrocarbons, the alkane homologous series, how fractional distillation separates crude oil into fractions, the trends in boiling point, viscosity and flammability, complete combustion, and cracking to make more useful smaller molecules and alkenes.
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What this dot point is asking
Edexcel wants you to describe crude oil as a mixture of hydrocarbons (mostly alkanes), recall the alkane homologous series and general formula, explain how fractional distillation separates crude oil into fractions, describe the trends in the properties of the fractions, write equations for complete combustion, and explain cracking. Fractional distillation, combustion and cracking are the central marks.
Crude oil and the alkanes
The alkanes are a homologous series with the general formula and only single carbon-carbon bonds (they are saturated). The first four are methane , ethane , propane and butane .
Fractional distillation
Crude oil is separated into fractions (groups of hydrocarbons with similar boiling points) by fractional distillation:
- The oil is heated until most of it evaporates.
- The vapours rise up a fractionating column that is hot at the bottom and cooler at the top.
- Each fraction rises until it reaches the level where the temperature equals its boiling point, where it condenses and is collected.
The fractions include (from the top down) refinery gases, petrol, kerosene, diesel, fuel oil and bitumen, used as fuels and for surfacing roads.
Trends in the properties of the fractions
The properties of the fractions change with the size of the molecules:
Complete combustion
When a hydrocarbon burns in plenty of oxygen, it undergoes complete combustion to produce carbon dioxide and water, releasing energy:
For example, . Both carbon and hydrogen are oxidised.
Cracking
Cracking is carried out because:
- There is greater demand for short-chain hydrocarbons (such as petrol) than fractional distillation supplies, while there is a surplus of long-chain fractions.
- It produces alkenes, which are reactive and used to make polymers and other chemicals.
For example, (octane plus ethene).
Try this
Q1. State the general formula of the alkanes. [1 mark]
- Cue. .
Q2. Write the balanced equation for the complete combustion of methane. [2 marks]
- Cue. .
Q3. Give two reasons why long-chain hydrocarbons are cracked. [2 marks]
- Cue. To meet the greater demand for short-chain fuels such as petrol, and to make alkenes for polymers.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20194 marksCrude oil is separated into fractions by fractional distillation. Describe how fractional distillation separates crude oil, and explain why the fractions collected near the top of the column have lower boiling points than those near the bottom.Show worked answer →
A 4-mark fractional-distillation question.
The crude oil is heated until most of it evaporates, and the vapours rise up the fractionating column, which is hot at the bottom and cooler at the top (1 mark). Each fraction condenses and is collected at the level where the temperature matches its boiling point (1 mark). Fractions with smaller molecules have lower boiling points, so they condense higher up where it is cooler (1 mark), while fractions with larger molecules have higher boiling points and condense lower down where it is hotter (1 mark).
Markers reward the temperature gradient up the column and linking boiling point to molecule size.
Edexcel 20214 marksA long-chain alkane is cracked to produce octane and one other product. State what type of substance the other product is, write a balanced equation for the reaction, and explain why cracking is carried out.Show worked answer →
A 4-mark cracking question.
The other product is an alkene (it has a carbon-carbon double bond) (1 mark). A balanced equation is (1 mark, atoms balance: 10 C and 22 H on each side). Cracking is carried out because there is a greater demand for shorter-chain hydrocarbons such as petrol than the supply from distillation (1 mark), and because it also produces alkenes, which are useful for making polymers (1 mark).
Markers reward identifying the alkene, a correctly balanced equation, and the supply-and-demand reason for cracking.
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Sources & how we know this
- Pearson Edexcel GCSE (9-1) Chemistry (1CH0) specification — Pearson Edexcel (2016)