Properties of Hydrocarbons (AQA GCSE Chemistry)

• 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

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

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 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

• 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