AQA GCSE Chemistry

Revision Notes

7.1.3 Properties of Hydrocarbons

Trends in Physical Properties

  • Some properties of hydrocarbons depend on the size of their molecules, including boiling point, viscosity and flammability
  • These properties influence how hydrocarbons are used as fuels

Boiling Point

  • The hydrocarbons are grouped together into homologous series according to their functional group and common formula
  • Gradation in the physical properties of a homologous series can be seen in the trend in boiling points of the alkanes
  • Each alkane has a boiling point that is higher than the one before it
  • As the molecules get larger, the intermolecular forces of attraction between the molecules becomes greater as there are more electrons in the molecules and greater surface area contact between them
  • This means that more heat is needed to separate the molecules, hence with increasing molecular size there is an increase in boiling point

Alkanes - Boiling Point Graph, downloadable IB Chemistry revision notes

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

Exam 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
  • High viscosity liquids are thick and flow less easily
    • The opposite of high viscosity is to say a liquid is runny
  • Viscosity also increases with increasing chain length
  • This is also due to the increased intermolecular forces of attraction as molecular size increases
  • Increased viscosity means that higher 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

  • Hydrocarbons undergo combustion in the presence of air
  • Complete combustion occurs to form water and carbon dioxide gas
  • For example, the simplest alkane, methane burns as follows:

CH4 +  2O2 → CO2 +  2H2O

  • Gasoline 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 gasoline 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 on-going challenge for all countries

Author: Francesca

Fran has taught A level Chemistry in the UK for over 10 years. As head of science, she used her passion for education to drive improvement for staff and students, supporting them to achieve their full potential. Fran has also co-written science textbooks and worked as an examiner for UK exam boards.
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