Alkanes (Cambridge (CIE) IGCSE Chemistry)

Alkanes: properties & bonding

What is an alkane?

• Alkanes are a group of saturated hydrocarbons

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

• Alkanes have covalent bonds as they consist of non-metal atoms 

• The general formula of the alkanes is C_nH_2n+2

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

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

• They are generally unreactive compounds but they:

  • Undergo combustion

  • Can be cracked into smaller molecules

  • React with halogens in the presence of light in substitution reactions

Combustion of alkanes

• Alkanes undergo complete combustion to form carbon dioxide and water:

Alkane + oxygen → carbon dioxide + water

• Combustion of methane:

CH₄ (g) + 2O₂ (g) → CO₂ (g) + 2H₂O (l)

• Combustion of ethane:

C₂H₆ (g) + 3½O₂ (g) → 2CO₂ (g) + 3H₂O (l)

• Combustion of propane:

C₃H₈ (g) + 5O₂ (g) → 3CO₂ (g) + 4H₂O (l)

• Combustion of butane:

C₄H₁₀ (g) + 6½O₂ (g) → 4CO₂ (g) + 5H₂O (l)

Substitution reaction of alkanes with halogens

Extended tier only

• In a substitution reaction, one atom (or group of atoms) is swapped with another atom (or group of atoms)

• Alkanes undergo a substitution reaction with halogens in the presence of ultraviolet radiation (sunlight is a source of UV radiation)

• This is called a photochemical reaction

• The UV light provides the activation energy, E_a, for the reaction 

• A hydrogen atom is replaced with the halogen atom

• More than one hydrogen atom can be substituted depending on the amount of ultraviolet radiation there is

In the presence of ultraviolet (UV) radiation, methane reacts with chlorine to form chloromethane and hydrogen chloride