Crude oil is a mixture of hydrocarbons. These are separated into useful products, such as fuels, using a process called fractional distillation.
The demand for short hydrocarbon molecules is greater than their supply in crude oil, so a reaction called cracking is used. Cracking converts long alkane molecules into shorter alkanes and alkenes, which are more useful. The exploitation of oil can damage the environment - for example, through oil spills.
Coal was formed from dead plant material
Crude oil and gas were formed from dead marine organisms.
Fossil fuels are non-renewable. They took a very long time to form and we are using them up faster than they can be renewed. Fossil fuels are also finite resources. They are no longer being made or are being made extremely slowly. Once they have all been used up, they cannot be replaced.
Formation of Crude Oil
Crude oil is found trapped in some of the sedimentary rocks of the Earth's crust.
Millions of years ago, huge numbers of microscopic animals and plants - plankton - died and fell to the bottom of the sea. Their remains were covered by mud.
As the mud sediment was buried by more sediment, it started to change into rock as the temperature and pressure increased. The plant and animal remains were ‘cooked’ by this process, and slowly changed into crude oil.
Oil is less dense than the water in the rocks and will rise as a result of pressure from below (as can be seen in the animation above). often the oil will escape altogether if the rocks are permeable (liquids can pass through them).
If some of the rocks above the oil are impermeable the oil cannot rise through them, so it gets trapped underneath.
Problems with Oil
Oil slicks travel across the sea, far from the original spill
Beaches and wildlife are harmed when they are coated with oil.
The oil damages feathers and birds may die. Detergents are often used to help clean up oil slicks, but these in turn may harm wildlife.
Heating → evaporating → cooling → condensing
Hydrocarbons have different boiling points. They can be solid, liquid or gas at room temperature,
Small hydrocarbons with only a few carbon atoms have low boiling points and are gases
Hydrocarbons with between five and 12 carbon atoms are usually liquids
Large hydrocarbons with many carbon atoms have high boiling points and are solids.
Fractional distillation is different from distillation in that it separates a mixture into a number of different parts, called fractions. A tall 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 and its vapours condense at different temperatures in the fractionating column. Each fraction contains hydrocarbon molecules with a similar number of carbon atoms.
Fuels made from oil mixtures containing large hydrocarbon (a group of compounds which only contain the elements hydrogen and carbon) molecules are not efficient. 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.
Cracking allows large hydrocarbon molecules to be broken down into smaller alkane and alkene molecules,
Smaller hydrocarbons are more useful as fuels, such as petrols
Alkenes are useful, because they are used to make polymers.
Fractions containing large hydrocarbon molecules are vaporised and passed over a hot catalyst. This breaks chemical bonds in the molecules and forms smaller hydrocarbon molecules.
Fuels react with oxygen to release energy. Complete combustion happens in a plentiful supply of air and incomplete combustion occurs when the supply of air is limited.
Complete combustion releases more energy than incomplete combustion. Incomplete combustion also creates carbon monoxide, and more soot.
Fuels such as natural gas and petrol contain hydrocarbons. These are compounds of hydrogen and carbon only.
The carbon oxidises to carbon dioxide
The hydrogen oxidises to water (remember that water, H2O, is an oxide of hydrogen).
hydrocarbon + oxygen → carbon dioxide + water (Complete combustion)
hydrocarbon + oxygen → carbon monoxide + carbon + water (Incomplete combustion)
The amount of carbon dioxide in the atmosphere is maintained through a balance between processes such as photosynthesis, respiration and combustion. But human activities are polluting the atmosphere.
Photosynthesis by plants is thought to be a key process in the evolution of the Earth’s atmosphere.
Photosynthesis is the process that plants use to produce their food. For photosynthesis, plants absorb carbon dioxide from the atmosphere and release oxygen as a by-product. Photosynthesis increased the proportion of oxygen in the atmosphere until it reached today’s level, 21 per cent.
The level of carbon dioxide in the atmosphere is maintained by several processes, including photosynthesis, respiration and combustion.
Plants remove carbon dioxide from the atmosphere by photosynthesis. Living organisms, including all plants and animals, release energy from their food using respiration. Respiration and combustion both release carbon dioxide into the atmosphere.
These processes form a carbon cycle in which the proportion of carbon dioxide in the atmosphere remains about the same.
Carbon Monoxide - incomplete combustion of the fuel in car engines
Oxides of Nitrogen - formed from the heat and pressures found in a car engine
Sulfur Dioxide - sulfur impurities in the fuel burn
Carbon monoxide is a poisonous gas. Oxides of nitrogen (NOx) react with other pollutants in sunlight to form a photochemical smog, which causes breathing difficulties. NOx and sulphur dioxide also form acid rain.
Car exhaust systems have catalytic converters. These convert carbon monoxide into carbon dioxide:
carbon monoxide + nitrogen oxide → nitrogen + carbon dioxide
2CO + 2NO → N2 + 2CO2
Note that nitrogen oxide is also converted into harmless nitrogen in the process.
Hydrocarbons are compounds made from carbon and hydrogen atoms joined by covalent bonds. Alkanes are saturated - they have only single bonds. Alkenes have a double bond - they are unsaturated. Alkenes react with brown bromine water and decolourise it, but alkanes do not.
Alkenes can act as monomers. Under high pressure and in the presence of a catalyst many monomer molecules join together to make polymer molecules. These polymer molecules are saturated.
A covalent bond is a shared pair of electrons.
The number of hydrogen atoms in an alkane is double the number of carbon atoms, plus two. The number of hydrogen atoms in an alkene is double the number of carbon atoms.
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.
Polymers - plastics - are very large molecules made from many smaller molecules called monomers. Alkenes are able to act as monomers because they contain a double bond. They can join end-to-end in a reaction called addition polymerisation. The polymers they form are called addition polymers.
- ethene → polyethene
- propene → polypropene
- chloroethene → polychloroethene (also called polyvinylchloride or PVC).
Gore-Tex contains layers of nylon, PTFE and polyurethane. The PTFE contains a lot of tiny holes called pores - there are around 14 million per square millimetre. Each one is too small for water droplets to pass through, but big enough to let water molecules from sweat out. Without the nylon, the layers would be too fragile to be useful.
- New substances are made
- The process is irreversible
- An energy change occurs
The protein molecules change shape as a result of the heat energy they absorb. This is called denaturing and it is permanent. Denaturing causes changes in the appearance and texture of the meat and eggs when they are cooked.
- Meat becomes firmer and turns from red to brown
- Egg white solidifies and becomes white instead of transparent
Raw potato is hard and has an unpleasant taste but it becomes softer and easier to digest when is cooked.
- The cell walls break, leading to a softer texture
- The starch grains in the cells swell and spread out
Antioxidants - stop food from reacting with oxygen
Emulsifiers - help oil and water mix, and not separate out
Emulsifiers are molecules that have two different ends:
- a hydrophilic end - 'water-loving' - that forms chemical bonds with water but not with oils
- a hydrophobic end - 'water-hating' - that forms chemical bonds with oils but not with water.
The hydrophilic 'head' dissolves in the water and the hydrophobic 'tail' dissolves in the oil. In this way, the water and oil droplets become unable to separate out. The emulsion is stabilised.
Esters (chemicals with pleasant smells) are made by reacting an alcohol with an organic acid. They are used in perfumes, and as solvents. Nail varnish dissolves in nail varnish remover, but not in water.
Perfumes have a pleasant smell and they stimulate receptors in the nose. Some perfumes are obtained from natural sources, such as lavender oil or sandalwood oil. Others are made synthetically.
Non-toxic - does not poison the wearer
Does not irritate skin - prevents the wearer from suffering rashes
Very volatile - perfume molecules reach the nose easily
Insoluble in water - it is not washed off easily
Does not react with water - avoids the perfume reacting with perspiration
Volatile liquids evaporate easily. They readily change from a liquid to a gas. This is because there are only weak attractive forces between particles in the substance. These forces are overcome easily, so particles with enough energy can escape from the liquid.
alcohol + organic acid → ester + water
A solvent is a liquid that dissolves substances. The substance that dissolves is called the solute and the mixture formed by a solvent and solute is called a solution. The components of a solution are mixed together completely and do not separate out.
Whether or not a substance will dissolve in a particular solvent depends on the relative strengths of the attractive forces:
- between the solute particles
- between the solvent particles
- between the solute particles and solvent particles.
The attraction between water and nail varnish particles is weaker than the attraction that joins water to water and the attraction that joins nail varnish to nail varnish. The attractive force between water and nail varnish particles is too weak to break those other bonds, so nail varnish will not dissolve in water.
- Paints are a type of mixture called a colloid. They contain:
- a pigment - gives the paint its colour
- a binding medium - a liquid polymer that hardens to form a continuous layer when the paint dries
- a solvent - dissolves the binding medium and makes the paint more fluid.
Emulsion paints are water-based. Their solvent is water and they dry when the water evaporates.
The pigments in oil paints are dispersed in oil, which may itself be dissolved in a solvent.
The solvent evaporates away when the paint dries. This leaves the pigment and oil behind. The oil oxidises to form a hard film. This happens because the oil reacts with oxygen in the air.
Paints are a type of mixture called a colloid. In a colloid, particles of one substance are mixed and dispersed with particles of another substance - but they are not dissolved in it. The components do not separate out because their particles are small enough not to settle at the bottom.