Properties of Hydrocarbons (AQA GCSE Chemistry: Combined Science)

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Trends in physical properties

  • Some properties of hydrocarbons depend on the size of their molecules
  • These properties include:
    • Viscosity (the ability of the substance to flow)
    • Flammability (how easily the substance burns)
    • Colour
    • Cleanliness of burn 
  • These properties influence how hydrocarbons are used as fuels

Boiling Point

  • The bigger the hydrocarbon, the higher it's boiling point
  • This is because the size of the intermolecular forces increases requiring more energy to overcome them

Graph to show the trend in boiling points of hydrocarbons

A graph to show how the boiling points of the alkanes increases with carbon length

A graph of the boiling points of the first eight alkanes showing a gradually increasing trend

Examiner Tip

The boiling points of some of the alkanes are below zero meaning they are gases at room temperature.

Viscosity

  • Viscosity refers to the ease of flow of a liquid
    • The opposite of high viscosity is to say a liquid is runny
    • High viscosity liquids are thick and flow less easily
  • Viscosity also increases with increasing chain length
    • This is also due to the increased intermolecular forces of attraction as molecular size increases
  • Longer chain hydrocarbons would not be as useful as fuels for cars as they would be too thick and clog the engine
  • Increased viscosity means that longer chained alkanes are useful as lubricants in machinery as they are less likely to burn and function to reduce friction between moving parts

Flammability

  • Molecular size again influences the ease of ignition or flammability of hydrocarbons
  • Smaller hydrocarbon molecules are more flammable and are easier to ignite than larger molecules
  • This makes them very useful as fuels, releasing large amounts of energy when they burn

Combustion of hydrocarbons

Products of complete combustion

  • Hydrocarbons undergo combustion in the presence of air
  • Complete combustion occurs to form water and carbon dioxide gas
  • Carbon and hydrogen from the fuel are oxidised / gain oxygen: 
    • Hydrogen oxidises to form water
    • Carbon oxidises to form carbon dioxide
  • For example, the simplest alkane, methane burns as follows:

CH4 +  2O2 → CO2 +  2H2O

  • Petrol is largely composed of isomers of octane, C8H18 , which requires large amounts of oxygen to combust fully

2C8H18 + 25O2 → 16CO2 + 18H2O

  • The efficiency of car engines does not usually enable all the petrol to burn, so car exhaust will contain small amounts of unburnt hydrocarbons as well as other products such as carbon monoxide and soot which lead to environmental problems
  • The carbon dioxide produced is a major contributor to global warming and the replacement of combustion engines with electric vehicles is a major ongoing challenge for all countries

Examiner Tip

  • You need to know how to write balanced symbol equations for the complete combustion of hydrocarbons.
  • When balancing these equations, balance the elements in this order:
    1. Carbon
    2. Hydrogen
    3. Oxygen.
  • If you get a fraction when you balance the number of oxygen atoms then multiply everything by 2.

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Stewart

Author: Stewart

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Exam Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.