Where do fuels come from, and what happens chemically when we burn them?
Describe crude oil as a source of fuels, recognise alkanes as a family of hydrocarbons, and write the products of complete and incomplete combustion
A focused N(A)-Level answer on organic chemistry. Crude oil and fractional distillation, alkanes as a hydrocarbon family, and the products of complete and incomplete combustion.
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What this dot point is asking
SEAB wants you to describe crude oil as the source of many fuels, to recognise the alkanes as a family of hydrocarbons, and to write the products of complete and incomplete combustion. The central idea is that fuels are hydrocarbons that release energy when they burn in oxygen.
The answer
Crude oil
Crude oil is a mixture of many different hydrocarbons formed underground over millions of years. Because it is a mixture, it is separated by fractional distillation: the oil is heated and the different hydrocarbons boil off and are collected at different temperatures as fractions. Useful fractions include petrol, diesel, kerosene (jet fuel) and bottled gas. Each fraction has its own uses.
Hydrocarbons and alkanes
A hydrocarbon is a compound made of hydrogen and carbon atoms only. The alkanes are the simplest family of hydrocarbons. The first few are:
- methane, ,
- ethane, ,
- propane, ,
- butane, .
They form a family (a homologous series) where each member differs from the next by one carbon and two hydrogen atoms. Alkanes are widely used as fuels because they burn to release a lot of energy.
Complete combustion
When a hydrocarbon burns in plenty of oxygen, it undergoes complete combustion, giving carbon dioxide and water:
This releases the most energy and is the clean way to burn a fuel.
Incomplete combustion
When there is not enough oxygen, incomplete combustion happens. It releases less energy and produces carbon monoxide (a toxic gas) and carbon (soot) as well as water. This is why a poorly ventilated flame is dangerous and sooty.
Examples in context
Example 1. Why a Bunsen burner has an air hole. Opening the air hole lets more oxygen mix with the gas, giving complete combustion and a hot, clean blue flame. Closing it starves the flame of oxygen, giving an incomplete, sooty yellow flame that is cooler and deposits soot on glassware.
Example 2. Why aeroplanes and cars use different fuels. Fractional distillation splits crude oil into fractions with different properties. Kerosene is collected for jet engines, while petrol and diesel fractions are collected for cars, each chosen because its boiling range and properties suit the engine.
Try this
- Cue. Define a hydrocarbon. A compound containing only hydrogen and carbon atoms.
- Cue. Write the word equation for the complete combustion of ethane. Ethane plus oxygen gives carbon dioxide plus water.
- Cue. State the extra harmful product of incomplete combustion. Carbon monoxide (a toxic gas), along with soot.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original4 marksMethane, , is burned. (a) Write the word equation for its complete combustion. (b) State the two products and how you would test for each.Show worked answer →
(a) Methane plus oxygen gives carbon dioxide plus water.
(b) Carbon dioxide turns limewater milky. Water turns white (anhydrous) copper(II) sulfate blue (or blue cobalt chloride paper pink).
What markers reward: the correct word equation, naming carbon dioxide and water, and a correct test for each product.
Original3 marks(a) State what is meant by a hydrocarbon. (b) Explain why crude oil is separated by fractional distillation rather than used as it is.Show worked answer →
(a) A hydrocarbon is a compound made of hydrogen and carbon atoms only.
(b) Crude oil is a mixture of many different hydrocarbons with different boiling points. Fractional distillation separates them into useful fractions (such as petrol and diesel) that each have their own uses.
What markers reward: hydrocarbon defined as hydrogen and carbon only, and crude oil as a mixture separated by boiling point into useful fractions.
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