(b) Crude oil

4:08 describe how the industrial process of fractional distillation separates crude oil into fractions

  • Crude oil is separated by fractional distillation.
  • Crude oil is heated and the oil evaporates.
  • It then goes into the tower. As the vapours rise up the tower the temperature falls.
  • Different sized fractions condense at different heights because they have different boiling points.
  • Smaller molecules condense high up the tower. Larger molecules condense low down in the tower.
  • The fractions are collected.


4:09 know the names and uses of the main fractions obtained from crude oil: refinery gases, gasoline, kerosene, diesel, fuel oil and bitumen

Crude oil is separated into fractions by the process of fractional distillation.

Refinery gasesBottled gas
GasolineFuel for cars
KeroseneFuel for aeroplanes
Diesel OilFuel for lorries
Fuel OilFuel for ships
BitumenRoad Surfacing

4:10 know the trend in colour, boiling point and viscosity of the main fractions

The boiling point increases as the number of carbon atoms (chain length) increases.

The viscosity increases as the number of carbon atoms (chain length) increases.

The viscosity of a fluid describes how easily it flows. Water has a low viscosity, it flows very easily. Crude oil has a higher viscosity than water, it does not flow very easily.

Fractions (in order)Properties
Refinery gasesSmallest molecules. Lowest boiling point. Lowest viscosity.
Fuel oil
BitumenLargest molecules. Highest boiling point. Highest viscosity.

4:12 know the possible products of complete and incomplete combustion of hydrocarbons with oxygen in the air

Complete Combustion happens when there is enough oxygen available, producing carbon dioxide (CO2) and water (H2O)


Incomplete Combustion happens when there is not enough oxygen available, with possible products being carbon monoxide (CO), carbon (C, soot), carbon dioxide (CO2) and water (H2O)

4:15 explain how the combustion of some impurities in hydrocarbon fuels results in the formation of sulfur dioxide

Fossil fuels such as coal, gas and oil are derived from crude oil.

These fuels are hydrocarbons, but also include impurities such as sulfur.

When the fuels are burned, sulfur dioxide is produced which can escape into the atmosphere:

S (s)         +         O₂ (g)         →         SO₂ (g)


4:16 understand how sulfur dioxide and oxides of nitrogen oxides contribute to acid rain

Acids formed in the atmosphere can fall as acid rain. This can be a major problem, killing trees and fish in lakes. The acid rain also corrodes limestone buildings and marble statues since these are both made of calcium carbonate (CaCO₃). Some metals such as iron are also attacked by acid rain.


Sulfur dioxide released into the atmosphere from the burning of fossil fuels can react with water and oxygen to make sulfuric acid (H₂SO₄):

2SO₂ (g)         +         2H₂O (l)         +         O₂ (g)         →         2H₂SO₄ (aq)


Also, if sulfur dioxide in the atmosphere reacts with just water, a weaker acid called sulfurous acid (H₂SO₃) is formed:

SO₂ (g)         +         H₂O (l)         →         H₂SO₃ (aq)


In car engines the temperature is high enough for the nitrogen in the air to react with oxygen to produce oxides of nitrogen, e.g:

N₂ (g)         +         O₂ (g)         →         NO₂ (g)

In the atmosphere these nitrogen oxides can produce nitric acid (HNO₃).

4:17 describe how long-chain alkanes are converted to alkenes and shorter-chain alkanes by catalytic cracking (using silica or alumina as the catalyst and a temperature in the range of 600–700⁰C)

Cracking involves the thermal decomposition of long-chain alkanes into shorter-chain alkanes and alkenes:


Temperature: 600oC

Catalyst: aluminium oxide, Al2O3

4:18 explain why cracking is necessary, in terms of the balance between supply and demand for different fractions

Cracking converts long chain hydrocarbons into short chain hydrocarbons.

Long-chain alkanes are broken down into alkanes and alkenes of shorter length.

Crude oil contains a surplus long chains.

Shorter chain hydrocarbons are in greater demand, e.g. petrol.

Cracking also produces alkenes which are used in making polymers and ethanol.

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Section 1: Principles of chemistry

      a) States of matter

      b) Atoms

      c) Atomic structure

     d) Relative formula masses and molar volumes of gases

     e) Chemical formulae and chemical equations

     f) Ionic compounds

     g) Covalent substances

     h) Metallic crystals

     i) Electrolysis

 Section 2: Chemistry of the elements

     a) The Periodic Table

     b) Group 1 elements: lithium, sodium and potassium

     c) Group 7 elements: chlorine, bromine and iodine

     d) Oxygen and oxides

     e) Hydrogen and water

     f) Reactivity series

     g) Tests for ions and gases

Section 3: Organic chemistry

     a) Introduction

     b) Alkanes

     c) Alkenes

     d) Ethanol

Section 4: Physical chemistry

     a) Acids, alkalis and salts

     b) Energetics

     c) Rates of reaction

     d) Equilibria

Section 5: Chemistry in industry

     a) Extraction and uses of metals

     b) Crude oil

     c) Synthetic polymers

     d) The industrial manufacture of chemicals