Crude Oil, Hydrocarbons & Alkanes (AQA GCSE Combined Science: Trilogy: Chemistry): Revision Note

What is organic chemistry?

• Organic chemistry is the chemistry of carbon compounds

• Carbon forms a vast number of compounds because it can form strong covalent bonds with itself

• This enables it to form long chains of carbon atoms, and hence an almost infinite variety of carbon compounds are known

• Carbon always forms four covalent bonds which can be single, double or triple bonds

• A functional group is a specific atom or group of atoms which confer certain physical and chemical properties onto the molecule

• Organic molecules are classified by the dominant functional group on the molecule

  • Organic compounds with the same functional group, but a different number of carbon atoms, are said to belong to the same homologous series

  • Every time a carbon atom is added to the chain, two hydrogen atoms are also added

• Organic compounds can be represented in a number of ways

Types of Formulae

General Formulae

• This type of formula tells you the composition of any member of a whole homologous series of organic compound

• For example, all of the alkanes have the general formula C_nH_2n+2

• This tells you that however many carbon atoms there are in the alkane, doubling this number and adding two will give you the number of hydrogen atoms present in the alkane

Displayed Formulae

• This shows the spatial arrangement of all the atoms and bonds in a molecule

Molecular Formulae

• This shows the actual number of each atom in a molecule, one molecule at a time

Displayed and molecular formulae 

The displayed formulae and molecular formulae of butane and butene

Structural Formulae

• This gives enough information to make the structure clear, but most of the actual covalent bonds are omitted

• Only important bonds are shown, such as double and triple bonds

• Identical groups can be bracketed together

Example of a structural formula

Representing structural formulae of pentane and two alkenes

What is a homologous series?

• Things we can say about a homologous series:

  • each member has the same functional group

  • each member has the same general formula

  • each member has similar chemical properties

  • each subsequent member differs by  -CH₂ -

  • members have gradually changing physical properties, for example, boiling point, melting point and density

• As a homologous series is ascended, the size of the molecule increases

• This has an effect on the physical properties, such as boiling point and density

Homologous Series of Alkanes Table

Composition of crude oil

• Crude oil is a finite resource which we find in the Earth's crust

• Crude oil is a complex mixture of lots of different hydrocarbon compounds of different sizes

• Hydrocarbons are compounds that contain hydrogen and carbon atoms only

• It is a thick, sticky, black liquid that is found in porous rock (under the ground and under the sea)

• Crude oil formed over millions of years from the effects of high pressures and temperatures on the remains of biomass (plants and animals), mainly plankton that was buried in mud

• It is being used up much faster than it is being formed, which is why we say crude oil is a finite resource

Diagram to show the location of crude oil

Crude oil found under the sea

Alkanes

• 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 for 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 molecules and can react with halogens in the presence of light

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

The first four alkanes

• The first four members of the alkanes are:

• Methane

  • Molecular formula: CH₄

• Ethane

  • Molecular formula: C₂H₆

• Propane

  • Molecular formula: C₃H₈

• Butane

  • Molecular formula: C₄H₁₀