Fractional distillation separates a mixture into a number of different parts, called fractions.

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.

Crude oil is a mixture of hydrocarbons. The crude oil is evaporated and its vapours condense at different temperatures in the fractionating column. Each fraction contains hydrocarbon molecules with a similar number of carbon atoms and a similar range of boiling points.

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.

Cracking

Fuels made from oil mixtures containing large hydrocarbon molecules are not efficient as they do not flow easily and are difficult to ignite. Crude oil often contains too many large hydrocarbon molecules and not enough small hydrocarbon molecules to meet demand. This is where cracking comes in.

Cracking allows large hydrocarbon molecules to be broken down into smaller, more useful hydrocarbon molecules. Fractions containing large hydrocarbon molecules are heated to vaporise them. They are then either:

• heated to 600-700°C
• passed over a catalyst of silica or alumina

These processes break covalent bonds in the molecules, causing thermal decomposition reactions. Cracking produces smaller alkanes and alkenes (hydrocarbons that contain carbon-carbon double bonds). For example:

hexane → butane + ethene

C₆H₁₄ → C₄H₁₀ + C₂H₄

Some of the smaller hydrocarbons formed by cracking are used as fuels, and the alkenes are used to make polymers in plastics manufacture. Sometimes, hydrogen is also produced during cracking.

Combustion of fuels

Complete combustion

Fuels are substances that react with oxygen to release useful energy. 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 are the equations for the complete combustion of propane, used in bottled gas:

propane + oxygen → carbon dioxide + water

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Incomplete combustion

Incomplete combustion occurs when the supply of air or oxygen is poor. Water is still produced, but carbon monoxide and carbon are produced instead of carbon dioxide.

In general for incomplete combustion:

hydrocarbon + oxygen → carbon monoxide + carbon + water

The carbon is released as soot. Carbon monoxide is a poisonous gas, which is one reason why complete combustion is preferred to incomplete combustion. Gas fires and boilers must be serviced regularly to ensure they do not produce carbon monoxide.

Carbon monoxide is absorbed in the lungs and binds with the haemoglobin in our red blood cells. This reduces the capacity of the blood to carry oxygen.

Here are the equations for the incomplete combustion of propane, where carbon is produced rather than carbon monoxide:

propane + oxygen → carbon + water

C₃H₈ + 2O₂ → 3C + 4H₂O

Nitrogen oxides

When fuels are burned in vehicle engines, high temperatures are reached. At these high temperatures, nitrogen and oxygen from the air combine to produce nitrogen monoxide.

nitrogen + oxygen → nitrogen monoxide

N₂(g) + O₂(g) → 2NO(g)

When this nitrogen monoxide is released from vehicle exhaust systems, it combines with oxygen in the air to form nitrogen dioxide.

nitrogen monoxide + oxygen → nitrogen dioxide

2NO(g) + O₂(g) → 2NO₂(g)

Nitrogen dioxide is a cause of acid rain.

Nitrogen monoxide and nitrogen dioxide are jointly referred to as NO_x.

Sulfur dioxide and acid rain

Many fossil fuels contain sulfur impurities. When these fuels are burned, the sulfur is oxidised to form sulfur dioxide.

S(s) + O₂(g) → SO₂(g)

This sulfur dioxide then dissolves in droplets of rainwater to form sulfurous acid.

SO₂(g) + H₂O(l) → H₂SO₃(aq)

Effects of acid rain

Acid rain reacts with metals and rocks such as limestone. Buildings and statues are damaged as a result.

Acid rain damages the waxy layer on the leaves of trees and makes it more difficult for trees to absorb the minerals they need for healthy growth. They may die as a result.

Acid rain also makes rivers and lakes too acidic for some aquatic life to survive.

The carbon cycle

Most of the chemicals that make up living tissue contain carbon. When organisms die, the carbon is recycled so that it can be used by other organisms. The model that describes the processes involved is called the carbon cycle.

Stages in the carbon cycle

1. Carbon enters the atmosphere as carbon dioxide from respiration and combustion.
2. Carbon dioxide is absorbed by producers to make carbohydrates during the process of photosynthesis.
3. Animals feed on the plant passing the carbon compounds along the food chain. Most of the carbon they consume is exhaled as carbon dioxide (formed during respiration). The animals and plants eventually die.
4. The dead organisms are eaten by decomposers and the carbon is returned to the atmosphere as carbon dioxide. In some conditions decomposition is blocked. The plant and animal material may then be available as fossil fuel in the future for combustion.

Note that throughout the processes, carbon is always being recycled.

Methane

Methane, CH₄, is a gas that can be produced by:

• decomposition of vegetation
• waste gases from digestion in animals

Methane is a powerful greenhouse gas and therefore contributes to global warming.

Addition polymers

Alkenes can be used to make polymers.

Polymers are very large molecules made when many smaller molecules join together, end to end. The smaller molecules are called monomers.

In general:

lots of monomer molecules → a polymer molecule

The polymers formed are called addition polymers.

This slideshow shows how several chloroethene monomers can join end to end to make poly(chloroethene), also called PVC:

Alkenes can act as monomers because they are unsaturated:

• ethene can polymerise to form poly(ethene), also called polythene
• propene can polymerise to form poly(propene), also called polypropylene
• chloroethene can polymerise to form poly(chloroethene), also called PVC

Repeating units

Polymer molecules are very large compared with most other molecules, so the idea of a repeat unit is used when drawing a displayed formula. When drawing one, you need to:

1. change the double bond in the monomer to a single bond in the repeat unit
2. add a bond to each end of the repeat unit

Addition polymerisation

It can be tricky to draw the repeat unit of poly(propene). Propene is usually drawn like this:

It is easier to construct the repeat unit for poly(propene) if you redraw the monomer like this:

You can then see how to convert this into the repeat unit.

Condensation polymers

Some polymers are made via condensation polymerisation.

In condensation polymerisation, a small molecule is formed as a by-product each time a bond is formed between two monomers. This small molecule is often water.

An example of a condensation polymer is nylon.

Uses of polymers

Different polymers have different properties, so they have different uses. The table gives some examples:

PolymerTypical use Poly(ethene) Plastic bags and bottles Poly(propene) Crates and ropes Poly(chloroethene) Water pipes and insulation on electricity cables

Polymers have properties that depend on the chemicals they are made from and the conditions in which they are made.

For example, there are two main types of poly(ethene) - LDPE, low-density poly(ethene), and HDPE, high-density poly(ethene). LDPE is weaker than HDPE and becomes softer at lower temperatures.

Modern polymers are very useful. For instance, they can be used as:

• new packaging materials
• waterproof coatings for fabrics (eg for outdoor clothing)
• fillings for teeth
• dressings for cuts
• hydrogels (eg for soft contact lenses and disposable nappy liners)
• smart materials (eg shape memory polymers for shrink-wrap packaging)