What is crude oil, how is it separated into fuels, and what pollution does burning fuels cause?
Crude oil as a mixture of hydrocarbons, fractional distillation and the main fractions, the alkanes and their combustion, the composition of the atmosphere, the pollutants from burning fuels, and the greenhouse effect and global warming.
A focused CCEA GCSE Single Award Science answer on organic chemistry and fuels, covering crude oil as a mixture of hydrocarbons, fractional distillation and the fractions, the alkanes and combustion, the composition of the atmosphere, the pollutants from burning fuels, and the greenhouse effect.
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What this dot point is asking
CCEA wants you to describe crude oil as a mixture of hydrocarbons, explain fractional distillation and name the fractions, describe the alkanes and their combustion, state the composition of the atmosphere, describe the pollutants from burning fuels, and explain the greenhouse effect.
Crude oil and the alkanes
The alkanes are a family of hydrocarbons with single bonds only, such as methane (), ethane and propane.
Fractional distillation
The main fractions, from the top (short chains) down, are refinery gases, petrol, kerosene, diesel and bitumen. As the chains get longer, the boiling point and viscosity increase and flammability decreases.
Combustion
The atmosphere and pollution
The greenhouse effect
Examples in context
Example 1. Why diesel is used in lorries and petrol in cars. Diesel is a longer-chain fraction than petrol, with a higher boiling point and higher viscosity. It releases a lot of energy and suits the heavy, steady work of a lorry engine, while the more volatile, easily ignited petrol suits a car's engine. This shows how the changing properties of the fractions with chain length decide their uses.
Example 2. Why acid rain damages limestone buildings. Sulfur dioxide and oxides of nitrogen from burning fuels dissolve in rain to make acids. When this acid rain falls on limestone (calcium carbonate), it reacts with it, slowly wearing away statues and buildings. This links the pollutants from combustion to a visible, real-world effect, a common CCEA discussion point.
Try this
Q1. Name the two products of the complete combustion of a hydrocarbon. [2 marks]
- Cue. Carbon dioxide and water.
Q2. Name the gas, increasing due to burning fossil fuels, that is the main cause of global warming. [1 mark]
- Cue. Carbon dioxide.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA SAS 20214 marksDescribe how fractional distillation separates crude oil into useful fractions.Show worked answer →
Four marks for the mixture, the column, the temperature gradient and the condensing.
Crude oil is a mixture of hydrocarbons of different chain lengths, which have different boiling points.
The oil is heated to a vapour and fed into a fractionating column that is hot at the bottom and cooler at the top.
The vapours rise and each fraction condenses when it reaches the part of the column at its boiling point.
Short-chain hydrocarbons (low boiling points) condense near the cool top; long-chain ones condense low down near the hot base. Markers reward the mixture of different chain lengths, the temperature gradient, and condensing at the boiling point.
CCEA SAS 20194 marksName two pollutants released when fuels burn, state the problem each causes, and explain why incomplete combustion is dangerous.Show worked answer →
Four marks for two pollutants with problems and the incomplete combustion point.
Sulfur dioxide, from sulfur impurities in the fuel, causes acid rain, which damages trees, lakes and buildings.
Oxides of nitrogen, formed at the high temperature of combustion, also cause acid rain and smog. (Particulates or soot causing breathing problems is an alternative.)
Incomplete combustion, in a limited oxygen supply, produces carbon monoxide, which is toxic because it combines with haemoglobin and stops the blood carrying oxygen.
Markers reward two pollutants each linked to a problem, and carbon monoxide from incomplete combustion being toxic.
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Sources & how we know this
- CCEA GCSE Science: Single Award specification — CCEA (2017)