Alkanes (OCR GCSE Chemistry A (Gateway))

Alkanes Structure & Formula

• Alkanes are a group of saturated hydrocarbons

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

• The general formula of the alkanes is C_nH_2n+2

• They are colourless compounds which have a gradual change in their physical properties as the number of carbon atoms in the chain increases

• Alkanes are generally unreactive compounds but they do undergo combustion reactions, can be cracked into smaller more useful molecules

• 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
	butane	C4H10
	pentane	C5H12

The first five members of the alkane homologous series

Reactions of Alkanes

• Alkanes are hydrocarbons, meaning they contain hydrogen and carbon only

• 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