Fractional distillation of crude oil

Crude oil is a mixture of hydrocarbons formed from the remains of simple marine organisms over millions of years.

Fractional distillation separates a mixture into a number of different parts, called fractions. A fraction of crude oil is a mixture of chemicals in the crude oil that have similar boiling points.

A tall fractionating column is fitted above the mixture, with several condensers coming off at different heights. The column is hot at the bottom and cool at the top. Substances with high boiling points condense at the bottom and substances with lower boiling points condense on the way to the top.

The crude oil is evaporated inside a furnace before entering the fractionating column where its vapours condense at different temperatures. Each fraction contains hydrocarbon molecules with a similar number of carbon atoms and a similar range of boiling points (eg the chemicals in the fraction known as petroleum gases all have boiling points below 25°C).

Oil fractions

The diagram below summarises the main fractions from crude oil and their uses, and the trends in properties. Note that the gases leave at the top of the column, all other fractions of crude oil are extracted as liquids

As you go up the fractionating column, the hydrocarbons have:

• smaller chain lengths
• lower boiling points
• more volatility (easy to evaporate)
• higher flammability (they ignite more easily and burn more cleanly)
• lighter colour (the bottom fraction is black, the middle fractions range from brown to yellow and the fraction at the top is colourless)

Other fossil fuels

Crude oil is not the only fossil fuel.

Natural gas mainly consists of methane. It is used in domestic boilers, cookers and Bunsen burners, as well as in some power stations.

Coal was formed from the remains of ancient forests. It can be burned in power stations. Coal is mainly carbon but it may also contain sulfur compounds, which produce sulfur dioxide when the coal is burned. This gas is a cause of acid rain. Also, as all fossil fuels contain carbon, the burning of any fossil fuel will contribute to global warming due to the production of carbon dioxide.

Combustion of fuels

Complete combustion

Fuels are substances that react with oxygen to release useful energy (exothermic). Most of the energy is released as heat, but light energy is also released.

About 21 per cent of air is oxygen. When a fuel burns in plenty of air, it receives enough oxygen for complete combustion.

Complete combustion needs a plentiful supply of air so that the elements in the fuel react fully with oxygen.

Fuels such as natural gas and petrol contain hydrocarbons. These are compounds of hydrogen and carbon only. When they burn completely:

• the carbon oxidises to carbon dioxide
• the hydrogen oxidises to water (remember that water, H₂O, is an oxide of hydrogen)

In general, for complete combustion:

hydrocarbon + oxygen → carbon dioxide + water

Here is the equation for the complete combustion of methane, used in Bunsen burners:

methane + oxygen → carbon dioxide + water

CH₄ + 2O₂ → CO₂ + 2H₂O

Here is the equation for the complete combustion of propane, used in bottled gas:

propane + oxygen → carbon dioxide + water

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Ethanol, a simple alcohol, also combusts to form carbon dioxide and water:

ethanol + oxygen → carbon dioxide + water

C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O

Alkanes

The alkanes are a homologous series of hydrocarbons. This means that they have similar chemical properties to each other and they have trends in physical properties. For example, as the chain length increases, their boiling point increases.

The alkanes share the same general formula.

C_nH_2n+2

The general formula means that the number of hydrogen atoms in an alkane is double the number of carbon atoms, plus two. For example, methane is CH₄ and ethane is

C₂H₆.

Alkane molecules can be represented by displayed formulae in which each atom is shown as its symbol (C or H) and the covalent bonds between them by a straight line.

Here are the names and structures of five alkanes:

Alkanes are saturated hydrocarbons. This means that their carbon atoms are joined to each other by single bonds. This makes them relatively unreactive, apart from their reaction with oxygen in the air – which we call burning or combustion.

[Higher tier only]

Like other homologous series, the alkanes show isomerism. This means that their atoms can be arranged differently to make slightly different compounds with different properties. For example, an isomer of butane is methylpropane.

Alkenes

Alkenes are a homologous series of hydrocarbons that contain a carbon-carbon double bond. The number of hydrogen atoms in an alkene is double the number of carbon atoms, so they have the general formula:

C_nH_2n

For example, the molecular formula of ethene is C₂H₄, while for propene it is C₃H₆.

Here are the names and structures of four alkenes:

Butene has two straight-chained isomers, as the double bond can be placed in two different places or locations:

• but-1-ene (double bond with the first carbon)
• but-2-ene (double bond with the second carbon)

Alkenes are unsaturated, meaning they contain a double bond. This bond is why the alkenes are more reactive than the alkanes.

Testing for alkenes

The presence of the C=C double bond allows alkenes to react in ways that alkanes cannot. This allows us to distinguish alkenes from alkanes using a simple chemical test.

Bromine water is an orange solution of bromine. It becomes colourless when it is shaken with an alkene. Alkenes can decolourise bromine water, but alkanes cannot. The slideshow shows this process.

The reaction between bromine and alkenes is an example of a type of reaction called an addition reaction. The bromine is decolourised because a colourless dibromo compound forms. For example:

ethene + bromine → 1,2-dibromoethane

C₂H₄ + Br₂ → C₂H₄Br₂

CH₂=CH₂ + Br–Br → CH₂BrCH₂Br

Other addition reactions of alkenes:

• Hydrogen can be added to a C=C double bond. This has the effect of ‘saturating’ the molecule, and will turn an alkene into an alkane. For example: C₂H₄ + H₂ → C₂H₆.
• If steam (H₂O) is added to an alkene, an alcohol is made. For example: C₂H₄ + H₂O → C₂H₅OH.