Chemistry

The Earth is surrounded by a mixture of gases called the atmosphere. The main gases in the atmosphere and their approximate percentages in the air are below.

Nitrogen ~ 78%

Oxygen ~ 21%

Argon ~ 1%

Water ~ 0 – 5 depending on where you are

Carbon Dioxide ~ 0.03-0.04

Human activities produce a number of gases that are released into the atmosphere. Most of these pollutant gases are produced by the burning of fuels.

The burning of fuels releases a large amount of carbon dioxide into the atmosphere. This is thought to cause global warming.

Other pollutant gases that are released when fuels are burned include carbon monoxide, nitrogen oxides and sulfur dioxide.

The incomplete combustion of fuels also releases small particles of solids, such as carbon, into the air. This makes buildings dirty and affects the lungs.

Some of these pollutants are directly harmful to humans. For example, carbon monoxide is toxic and, if breathed in, can cause death.

Others pollutants are harmful to the environment, so cause harm to humans indirectly. For example, sulfur dioxide causes acid rain that can damage or kill trees and crops.

The composition of the present day atmosphere is very different from that of the early Earth. Evidence can be interpreted in different ways leading to a number of explanations as to how the atmosphere has changed.

The atmosphere of the early Earth was largely made up of carbon dioxide and water vapour, probably coming from volcanoes. When the Earth cooled the water vapour condensed to form the oceans.

Carbon dioxide dissolved in the oceans and began to form sedimentary rocks. A lot of carbon dioxide was trapped underground and eventually formed fossil fuels.

The early atmosphere cannot be measured directly so scientists look for indirect evidence. This includes the chemical make-up of rocks, examining air bubbles in ice cores and looking at fossils.

Some fossils suggest early organisms underwent the process of photosynthesis, using the carbon dioxide and releasing oxygen. This meant carbon dioxide levels in the atmosphere went down and oxygen levels went up.

Burning fuels releases harmful pollutants into the air. Pollutants can kill plants, harm our lungs, cause acid rain and are thought to contribute to global warming.

A combustion reaction is when a chemical reacts with oxygen and releases energy.

We burn fuels to produce energy. When these fuels burn, the atoms in the fuels combine with oxygen from the air to make new molecules.

Coal is made mainly of carbon. This burns to produce carbon dioxide.

Petrol, diesel fuel and fuel oil are hydrocarbons. Their molecules are made of carbon and hydrogen atoms. When these fuels burn, the carbon and hydrogen atoms combine with oxygen atoms to produce carbon dioxide and water vapour.

Fuels are burned in the vehicles we use for transport, and in power stations to make electricity.

Properties ~ Many fuels contain small amounts of sulfur compounds. When these fuels are burned sulfur dioxide is released into the air. Sulfur dioxide causes acid rain that can damage buildings and kill plants.

Incomplete combustion ~ If there is not enough oxygen present to burn the fuels completely, incomplete combustion takes place. Carbon monoxide, a very poisonous gas, is formed.Incomplete combustion also releases very small particles of carbon into the air. This particulate carbon makes buildings dirty and can cause breathing difficulties.

Nitrogen oxides ~ In the high temperatures of vehicle engines, nitrogen and oxygen react to form nitrogen oxides. These gases irritate the lungs and cause acid rain.

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.

Coal is made mainly of carbon. This burns to produce carbon dioxide.

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

Nitrogen dioxide causes acid rain. It also causes breathing problems and can worsen the effects of asthma.

Nitrogen monoxide and nitrogen dioxide are jointly referred to as NOx

Particulates ~ Particulates are very small pieces of solids, mainly carbon, that are released into the air during incomplete combustion. Since they are solids, they will stick to other solid materials that they come into contact with. They coat the walls of buildings making them look dirty.

Sulfur dioxide ~ Sulfur dioxide is formed when fuels containing sulfur compounds are burned. Sulfur dioxide reacts with water and oxygen in the air to produce acid rain.This removes sulfur dioxide from the air, but the acid rain corrodes buildings and kills plants.

Nitrogen monoxide ~ Nitrogen monoxide reacts in the atmosphere to form nitrogen dioxide.

Nitrogen dioxide ~ Nitrogen dioxide also reacts with water and oxygen in the air to produce acid rain.

Carbon dioxide ~ Carbon dioxide is produced by burning fossil fuels, such as like coal, oil, petrol and natural gas.

Plants remove some of the carbon dioxide from the atmosphere, because they use it in photosynthesis.

Carbon dioxide is also removed from the atmosphere when it dissolves in both rain water and sea water. As a result rain water becomes slightly acidic, and the oceans are a huge reservoir of dissolved carbon dioxide.

Not all the carbon dioxide we produce is removed from the atmosphere. The level of CO₂ is steadily increasing, and this contributes to global warming.

Carbon dioxide

The fossil fuels include coal, natural gas and the products from the fractional distillation of crude oil, such as petrol and diesel. When any fossil fuel is burned, one of the combustion products is carbon dioxide.

Over the past 100 years, increasing amounts of fossil fuels have been burned each year. This has led to an increase in the percentage of carbon dioxide in the air. Many scientists believe this is causing global warming.

Reducing carbon dioxide in the air

One way to reduce the amount of carbon dioxide in the air is to burn less fossil fuels. Unfortunately, we depend on the burning of fossil fuels for heating, electricity generation and transport. If we are to burn less fossil fuels, we will need alternatives for these essential activities.

Power stations ~ Power stations that burn fossil fuels to produce electricity are one of the main contributors of carbon dioxide in the atmosphere.One way to reduce the air pollution caused by power stations is to use less electricity.

Sulfur dioxide ~ Power stations give out sulfur dioxide, which is thought to be a cause of acid rain. The amount of sulfur dioxide given off by a power station can be reduced by:

• removing sulfur from natural gas and fuel oil
•  removing sulfur dioxide from the flue gases emitted by coal-burning power stations.

Wet scrubbing involves the removal of sulfur dioxide from flue gases before it escapes from the power station chimney. Two methods are employed:

• spraying the flue gases with seawater droplets (naturally alkaline) which react with the sulfur dioxide
•  using powdered lime (calcium oxide) mixed with water to form an alkaline slurry. When mixed with air and sprayed with the slurry the sulfur dioxide in the flue gases reacts and forms solid calcium sulphate. This solid is removed allowing cleaned gases to escape.

Coal-powered stations give off solid particulates. These can also be removed from the flue gases. Waste gases pass through an electrostatic precipitator allowing particulates to be collected and removed.

Different materials behave in different ways. They have different properties.

Solid materials can differ in one or more of the following ways:

• melting point
• strength in tension (when pulled)
• strength in compression (when pushed)
• stiffness
• hardness
• density.

Many of the products that we use are made using plastics such as polythene and rubbers; and fibres such as cotton. The suitability of a material to a particular job depends on its properties.

• a washing-up bowl may be made from the plastic poly(ethene), commonly called polythene, because this material is waterproof and can be melted and reformed into the desired shape
•  a bicycle tyre is made from rubber because it needs to be flexible and tough
• a pair of tights may be made from nylon fibres because these are flexible and elastic.

The materials we use are chemicals or mixtures of chemicals. For example, sugar is a chemical, but chocolate is a mixture of chemicals, one of which is sugar.

Materials can be obtained or made from living things. Examples include cotton, paper, silk and wool. These are sometimes called natural materials.

Materials can also be made from chemicals. Examples of these include plastics such as polythene used in plastic shopping bags. These are called synthetic materials.

Crude oil is a raw material obtained from the Earth’s crust. It is a mixture of many different chemicals, most of which are hydrocarbons. These are chain molecules of varying length that are made from hydrogen and carbon atoms only. Crude oil is not very useful until it has been processed at an oil refinery. The process of refining involves separating the hydrocarbons into fractions or batches using a technique called fractional distillation. Each fraction separates as they have different boiling points. The crude oil is heated in a furnace to around 400°C. This allows all of the hydrocarbons in the crude oil to move into the bottom of the fractionating tower. The tower is hottest at the bottom and coolest at the top.

The smallest molecules contained in the crude oil have lower boiling points and so move to the top of the tower. This is because the forces between these molecules are weak, so little energy is required to vaporise them. Larger molecules will remain lower down the tower as they have higher boiling points. This is because forces between the molecules are stronger. The fractions gathered still contain a mixture of hydrocarbons but they are now useful mixtures. The use depends on the properties of the fraction.

Size and properties of molecules ~ The size of a molecule has an effect on the properties that molecule shows. For example, the boiling point of hydrocarbons increases as the number of carbon atoms in the molecule increases. This is because forces between molecules increase as the size of the molecule increases. Hydrocarbons with up to four carbon atoms are gases, five to 16 carbon atoms are liquids and hydrocarbons with more than 16 carbon atoms are solids.

Some small molecules (monomers) can join together to make very long molecules called polymers. This process is called polymerisation.

Many polymers are made from chemicals that are obtained from crude oil. For example, molecules of ethene join together to make poly(ethene), commonly known as polythene.

By polymerising other small molecules, a wide variety of different polymers can be made. These synthetic materials have many uses.

In a chemical reaction, the substances that react together are called the reactants, while the substances that are formed are called the products.

The products have different properties from the reactants.

As a chemical reaction takes place, atoms in the reactants are rearranged to make the products. The number of atoms of each element in the reactants is the same in the products. No atoms are added or taken away. This is called the conservation of atoms.

When polythene is made from ethene, the polymer has the same number of carbon and hydrogen atoms as the ethene molecules it is made from.

New synthetic materials are often used to make items that at one time were made from natural materials. The new material often has better properties, and may be cheaper to make.

Window frames used to be made from wood. This has to be painted regularly, and in time may rot. Many new window frames are made from uPVC, a synthetic polymer. The ‘u’ means it is unplasticised so is different from regular PVC. This can be made in a variety of colours, never needs to be painted, and does not rot.

The properties of solid materials depend on how the particles in these materials are arranged, and how they are held together.

A solid melts when the forces between its particles are broken. This requires energy.

The stronger the forces between the particles, the more energy is needed to break the particles out of the solid structure. The more energy that is needed, the higher the melting point of the solid.

Different polymers have different properties, depending on the small molecules they were made from. But the properties of a polymer can also be changed.

Polymer chains

Many polymers, such as poly(ethene), contain long molecules that lie side by side. These can uncoil and slide past each other, making the material flexible.

Long polymer chains have stronger forces of attraction than shorter ones. By making the chains of a polymer longer, a stronger and less flexible material is produced.

Plasticisers

Plasticisers are small molecules that can be added to polymers during their manufacture. They push the polymer molecules slightly further apart, weakening the forces between them and making the material softer and more flexible.

Plasticisers hold the polymer chains apart making it easier for them to slide past each other

Unplasticised PVC, usually called uPVC, is hard. It is used for pipes and window frames. Plasticised PVC is soft. It is used for clothing and flooring.

Chemical bonds can be formed to link together the chains of some polymers. These cross-links make the material tougher and less flexible.

Some polymer chains have side branches.  The side branches stop the polymer molecules from lining up regularly. Its structure is not crystalline.

This means that:

• the material has a lower density than HDPE
•  the forces of attraction between polymer molecules are weakened
• the material is less strong, and has a lower melting point, than HDPE.

High-density polyethene (HDPE) does not have side branches.

The polymer molecules line up regularly to give a crystalline structure.

This means that:

• the material has a higher density than LDPE
•  the forces of attraction between polymer molecules are strong
• the material is stronger, and has a higher melting point, than LDPE.

Various amounts of crystallinity

By carefully controlling the amount of branching, it is possible to make polymers with various amounts of crystallinity. This means it is possible to make a polymer with the exact properties that are required for a particular purpose.

The use and control of tiny matter is called nanotechnology. The tiny matter is referred to as nanoparticles. These particles are measured in nanometres (nm). A nanometre is one billionth of a metre (0.000 000 001m). Nanotechnology is concerned with the use and control of structures that are 1-100 nanometres in size.Some of these nanoparticles occur naturally, for example in volcanic ash. Some occur by accident, for example during the combustion of fuels. Many occur by design.

Properties of nanoparticles ~ Nanoparticles of a material show different properties compared to larger particles of the same material. Forces of attraction between surfaces can appear to be weak on a larger scale, but on a nanoscale they are strong. One reason for this is the surface area to volume ratio. In nanoparticles this is very large. Atoms on the surface of a material are often more reactive than those in the centre, so a larger surface area means the material is more reactive.

Nanoparticles are used in products that are currently available.

• sports equipment: nanoparticles are added to materials to make them stronger whilst often being lighter. They have been used in tennis rackets, golf clubs and shoes
•  clothing: silver nanoparticles have been added to socks. This stops them from absorbing the smell of sweaty feet as the nanoparticles have antibacterial properties
• healthcare: nanoparticles are used in sunscreens. They offer protection and can be rubbed in so there are no white marks.

Harmful effects ~ There are some concerns that nanoparticles may be toxic to people. They may be able to enter the brain from the bloodstream and cause harm. Some people think more tests should take place before nanoparticles of a material are used on a wider scale.