Hydrocarbons (Oxford AQA IGCSE Combined Science Double Award)

What are Alkanes?

• Most of the hydrocarbons in crude oil are alkanes

  • These are saturated hydrocarbons 

• The term saturated means that they only have single carbon-carbon bonds, there are no double bonds

• The bonds are covalent bonds due to being between non-metal atoms 

• The general formula of the alkanes is C_nH_2n+2

  • This means that we can double the number of carbon atoms and add two to find the number of hydrogen atoms 

  • E.g. An alkane with 15 carbon atoms has (2 x 15) + 2 = 32 hydrogen atoms

  • Therefore, the formula of this alkane would be C₁₅H₃₂

• Methane is an alkane and is the major component of natural gas

Table of Alkanes

Displayed formula	Name	Molecular formula
	methane	CH4
	ethane	C2H6
	propane	C3H8

Properties of Hydrocarbons

• Some properties of hydrocarbons depend on the size of their molecules

• These properties include:

  • Boiling point

  • Viscosity (the ability of the substance to flow)

  • Flammability (how easily the substance burns)

• These properties influence how hydrocarbons are used as fuels

Boiling Point

• The bigger the hydrocarbon, the higher its 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

As the size of the hydrocarbon increases, the boiling point also increases

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