[I] Chemistry - C1

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  • Created by: Olivia
  • Created on: 21-02-14 09:29

The air around us

Air is a gas. It is a mixture of lots of different substances, which are all gases.The most common gases in air are nitrogen (78%), oxygen (21%) and argon (1%). There is also water vapour, carbon dioxide and many other substances. Water vapour is a gas that is part of the air. Clouds are droplets of liquid water or ice

A gas spreads out to take up all the space available. When a gas is inside a container, such as a balloon, it can be squeezed into a smaller volume. The explanation for these properties is that a gas is made up of particles. The particles are very small and move around. Compared with the size of the particles, there is a lot of space between them.

In air - most of the particles are small molecules, mainly the elements nitrogen (N₂), oxygen (O₂), and argon (Ar). There are also small molecules that are compounds, such as water (H₂O) and carbon dioxide (CO₂).

Scientific data tells us that nearly all the air is nitrogen, oxygen and argon. Less than 0.1% of dry air is made up of all the other gases. These percentages are almost the same all over Earth. Only the percentage of water vapour varies, between 0% and 1%, because the water can condense to form clouds and rain.

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How we know what's in the air

There are specific apparatus to find out how much oxygen is in the air. Between two syringes, a boiling tube with copper inside is placed. Air is being passed backwards and forwards from one syringe to the other while the copper in the tube is heated. The oxygen from the air joins with the copper, so the volume of the air gets smaller. The amount of oxygen in the air is the difference in volume before and after the air has been passed over the copper.

Measurements always vary from the true value because of errors. The RANGE of a set of data is from the smallest measurement to the largest. The MEAN is usually closest to the true value.

The Earth's atmosphere is just right. There's plenty of oxygen for us to breathe but not so much that fires would spread uncontrollably. The temperature over most of the Earth is suitable for life. The Earth is the only planet in the solar system which has these conditions.

Sixty years ago, many scientists thought that the air in the early atmosphere was largely ammonia and methane. New discoveries of the composition of very old rocks convinced most scientists that this idea was not correct and that the atmosphere was largely CO₂. Today, many scientists think that humans are causing changes in the atmosphere by burning fossil fuels and releasing carbon dioxide back into the air. 

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Changing air

Scientists look at the what comes out of a volcano when it erupts. Apart from lava and tiny particles of dust, they find that volcanoes also give out a lot of water vapour and carbon dioxide. 

By examining very old rocks, scientists have discovered that the Earth was formed about 4 billion years ago. There were many more volcanoes then, so scientists think that the Earth's early atmosphere probably consisted of carbon dioxide and water vapour, given out by the volcanoes.

Four billion years ago, the Earth's atmosphere was hot and much thicker than it is now. Gradually the temperature fell and most of the water vapour condensed. It rained for thousands of years and the water became the oceans. About 3 billion years ago, life began in the form of tiny bacteria-like creatures. They began to use the carbon dioxide in the air and the light from the Sun to grow. This process of photosynthesis removed carbon dioxide from the atmosphere and produced oxygen as a waste product. Gradually the amount of oxygen in the air increased. Humans and animals evolved to make use of it.

Carbon that was once in the atmosphere became part of the bodies of the animals and plants living on Earth. When these animals and plants died, their bodies sometimes became buried and eventually became the FOSSIL FUELS we use today. 

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Humans and the air

Carbon dioxide dissolves in the oceans. Some of it reacts with salts in the seawater to form insoluble calcium carbonate. This forms SEDIMENT and eventually turns into SEDIMENTARY rocks. Sea creatures also use the carbon dioxide to build shells of calcium carbonate. When they die, the shells sink to the bottom of the sea and become part of the sediment. Over billions of years thick layers of limestone and chalk have built up.

These different processes have removd almost all of the carbon dioxide that was in the early atmosphere, leaving the air with the composition we have today. Scientists have found evidence in rocks and fossils in different parts of the Earth to support these ideas. Most scientists agree that this was how the atmosphere changed.

Farms, factories, power stations, shops and offices, homes, lorries, buses, cars, trains, ships, aircraft - all these places and types of transport produce gases that find their way into the atmosphere. Since humans started farming, burning fuels and using machines, we have changed the amount of some gases such as carbon monoxide, nitrogen oxides and sulphur dioxide in the air and added some extra substances. As the population increases, we are making more changes to the atmosphere.

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Measuring air quality

Burning fuels releases carbon dioxide. In the last 50 years, since Charles Keeling began his measurements, the amount of carbon dioxide in the air has increased by about 25%.

Modern agriculture makes a lot of use of machines that give out gases, but ploughing land and rearing livestock also releases gases into the atmosphere. For the last 200 years the increasing amount of industry, electricity generation and transport has also produced gases. 

Particulate matter is tiny bits of solid that float in the air. Dust from volcanoes, desert sand storms and soot from forest fires are natural sources of PARTICULATES. Soot is made up of tiny bits of carbon. In cities there can be four times the normal amount of particulates due to the soot and other dust produced by burning fuels. Burning down forests to make more farmland also produces particulates and carbon dioxide. 

The amount of carbon dioxide in the air is measured in parts per million (ppm). Air quality montioring stations across the UK measure automatically the amount of particular substances in the air at regular intervals.

The data is transmitted to a central computer for recording and analysis. The data can be accessed on the internet.

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Air quality and health

A POLLUTANT is a substance that is harmful to health or the environment and which is produced by human activity. Air quality is "good" if there are very few pollutants in the air. If there are large amounts of polluntants in the air then the air quality is "bad" or "poor". When air quality is poor, people with asthma and other lung diseases have difficulty breathing and may die. Carbon monoxide in the air is one cause of severe breathing difficulties. Inhaling it reduces the amount of oxygen that the blood can carry to the organs. If it is present in large quantities, it can quickly lead to death. 

Many pollutants are released into the atmosphere by human activities. Where the amount of pollutants in the air from burning fossil fuels is high there are more deaths from asthma, and heart and lung diseases. Scientists say there is a CORRELATION between air quality and health.

Mexico city has grown from about 2 million people to 20 million. In the 1960s, when there was little fuel burned, the air was clear and air quality was good. Now there are many days where this is SMOG and air quality is poor. Many more young children and older people die from heart and lung diseases today than in the 1950s. Sulphur dioxide and nitrogen oxides make rain acidic. ACID RAIN damages plants and kills fish in rivers and lakes. The increasing amount of carbon dioxide in the air is causing climate change and this is a factor in the extinction of many plants and animals.

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Correlation and cause

Breathing air containing a little sulphur dioxide will not give you asthma or heart disease, but if you have these diseases already then the pollutant may make it more difficult for you to breathe. Many studies have shown that as the amount of pollutants breathed in increases so does the number of people dying from various heart and lung diseases.

Poor air quality is therefore linked to a high death rate from these diseases. This kind of connection between one factor and another is called a correlation even though the pollutants are not a cause of the disease.

If acid rain or climate change damages crops then people will go hungry. Damage to the environment caused by air pollution will therefore affect health indirectly.

Some pollutants are waste materials but others are useful substances that have escaped into the atmosphere. 

London is allowed by the European Union to have 35 "bad" air quality days a year but usually it has many more than this, possibly leading to over 4000 extra deaths a year.

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Burning fuels

Coal is a fossil fuel. It is mainly carbon atoms. When coal burns, the carbon combines with oxygen in the air to make carbon dioxide gas and energy (heat) is released. Other fossil fuels such as petrol, diesel, fuel oil and natural gas are made of compounds called HYDROCARBONS. A hydrocarbon is made up of carbon and hydrogen atoms joined together. When a hydrocarbon burns it joins with oxygen. The carbon in the hydrocarbon produces carbon dioxide and the hydrogen produces hydrogen oxide, which is water. 

Hydrocarbon fuel + oxygen ---> carbon dioxide + water (+energy)

We give the name OXIDATION to any reaction where substances combine with oxygen. Reactions where oxygen is lost are called REDUCTION. Burning or combustion of a fuel is an oxidation reaction. Gradually more and more people began to agree with Lavoisier because more observations could be explained by his ideas than by the old descriptions of burning. The English chemist, Joseph Priestley never agreed with Lavoisier's theory of combustion. However he did show that fuels burn more rapidly and more brightly in pure oxygen than in air.

Acetylene + oxygen ---> carbon dioxide + water (+energy)

hydrogen (2H₂) + oxygen (O₂) ---> water (H₂O) (+energy)

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Rearranging atoms

When a piece of coal or a drop of petrol burns, the atoms do not change. Instead there is a change in the way the atoms are joined together. New substances are formed with the atoms joined up in different molecules. These new substanes are often gases. This is how atoms that were ina  fuel can become part of molecules in the air. The new gases in the air can affect air quality.

In a chemical reaction the atoms from the REACTANTS always rearrange themselves in a particular way to make the PRODUCTS. For example, when methane burns completely the carbon and hydrogen atoms from the methane molecules separate and join with oxygen atoms to make carbon dioxide and water. The number of carbon, hydrogen and oxygen atoms in the products is the same as in the reactants. The mass of the reactants and products is made up of all the atoms. We can weigh a sample of the reactants before the reaction and then weight the products after the reaction. The mass stays the same because the number of atoms of each element has not change. We say that the mass has been CONSERVED.

If we carry out a reaction in a closed container so that no gases can escape, we can show that the mass is conserved and no atoms are lost. That is why all the carbon atoms in the fossil fuels that have ever been burned are still present in the environment, as CO₂, carbon monoxide or particulates. The law of conversion mass is the reason why air quality has changed.

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Reactants and products

When you watch a chemical reaction you can often see things happening. This is because the substances are changing. The products which have formed have different properties from the reactants taking part in the reaction. If you burn some sulphur it changes from a yellow solid into a colourless gas. The product is sulphur dioxide. Sulphur dioxide does dissolve in water and makes a solution that is an acid.

New substances are formed by chemical reactions. Elements may join together to form compounds, or compounds may have split up. The products have a different arrangement of atoms to the reactants and so their properties are different.

Fossil fuels are formed from the remains of dead animals and plants. The sulphur which was in the organism is trapped in the fossil fuel. When the fuel is burned the sulphur burns too and forms sulphur dioxide. Each atom of sulphur joins with two atoms of oxygen to make sulphur dioxide.

Sulphur (S) + oxygen (O₂) ---> sulphur dioxide (SO₂)

Fossil fuels include coal, oil, petrol and natural gas. Coal often contains the most sulphur, so burning coal can give off a lot of sulphur dioxide gas.

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Sources of pollutants

Power stations and transport produce most of the pollutants because they use most of the fossil fuel that is burned. Burning fossil fuels produces pollutants in various ways. 

Sulphur dioxide is produced if there is sulphur in the fuel. Carbon dioxide is always formed. Carbon monoside is formed when there i snot enough air to burn the fuel completely to carbon dioxide and it is poisonous. Particles of carbon, called particulate carbon, are produced when there is not enough air to burn all of the fuel. Nitrogen oxides are formed when nitrogen and oxygen in the air react together. Nitrogen is not a fuel but the high temperature in furnaces and engines makes the gases react.

As the use of electricity and transport increased, people did not realise that damage was being done to health and the environment by burning fossil fuels. They did not know what all the chemicals being produced were, nor how they interacted to form damaging pollutants. The effects of the combnations of pollutants are very complex. It is only in the last 50 years that scientists have discovered how the different pollutants are formed in furnaces and engines and also how they react with the air to produce smog, acid rain and climate change.

N₂ + O₂ ---> 2NO nitrogen monoxide                               2NO + O₂ ---> nitrogen dioxide

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Removing pollutants

Scientists measure air quality in cities every day. They repeat measurements many times because the results may vary. A gust of wind may blow more or less pollutant over the sensor. 

Climate scientists take the mean of many measurements for each pollutant. The mean is a good estimate of the "true" value. They also take the range result (this is the difference between the highest and lowest results).

Pollutants may be removed from the air but they have not disappeared completely. Particulates settle on surfaces such as the leaves of plants, the ground or on buildings. Leaves that are coated with soot and dust cannot carry out photosynthesis properly so the plants may grow more slowly. Buildings covered with soot look dirty.

Carbon dioxide is used in photosynthesis but plants cannot use all the carbon dioxide added to the atmosphere. Some dissolves in rain water and in the oceans. Carbon dioxide solution is a weak acid. Sometimes a measurement may seem to be very different from the others. If it can be shown that this reading is an error then it is labelled an OUTLIER. Sulphur dioxide and nitrogen dioxide react with oxygen and water in the atmosphere to produce a mixture of sulfuric acid and nitric acid in rain water. This acid rain can have a pH as low as 3. Acid rain can damage plants that it falls on and it also makes rivers and lakes too acidic for plants/fish to survive.

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Improving power stations

If we use less electricty, then power stations will burn less fossil fuels. Scientists have found ways of reducing the electricity used by new fridges, washing machines and other electrical goods. Low energy lights have replaced the older, wasteful types of light bulb. 

Even if we reduce electricity consumption we will still need fossil fuel power stations. In the last 30 years scientists have developed ways of making power stations produce less pollutants. Some power stations are burning low sulphur fuels. In Europe and America, fuel oil and natural gas have replaced a lot of the coal used. Before oil and natural gas is burned, sulphur can be removed and sold to other industries. It is difficult to remove sulphur from coal. Power stations burning coal produce the most sulphur dioxide and particulates but these can be removed before the waste gases are released.

Solid particulates are removed by an electrostatic filter. This gives the particles an electric charge, which makes them collect on a charged metal grid. They can then be easily removed.

Sulphur dioxide can be removed from power station waste gases by FLUE GAS DESULPHURISATION. This is a chemical process that relies on sulphur dioxide being acidic.

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Removing the sulphur dioxide

There are two methods of flue gas desulphurisation. The most common method uses calcium oxide (lime) which is obtained by heating limestone. Carbon dioxide gas is given off. The lime is mixed with water to make an alkaline slurry. Sulphur dioxide is washed or "scrubbed" out of the flue gases with water and reacts with the lime. The compound formed is reacted with oxygen in the air to make calcium sulphate. This can be sold to make plaster for walls and this helps to pay for the process.

The second method is used in coastal power stations where seawater is used as a coolant. Seawater is naturally slightly alkaline and will react with the sulphur dioxide. Air is then pumped through to oxidise the product to sulphate. Carbon dioxide is given off and the seawater is returned to the sea. A large volume of seawater has to be used for this process, otherwise it would become acidic.

When we burn fossil fuels, the carbon that was trapped in the fuels is released into the air as carbon dioxide. If we want to reduce the amount of carbon dioxide we put into the air we should burn less fossil fuels. We can do this by using alternative ways to produce electricity that do not involve burning fossil fuels, improve insulation in buildings so that we use less fuel or electricity to keep warm and walking, cycling or using public transport instead of using private cars.

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Reducing carbon dioxide

Instead of burning fossil fuels we can use BIOFUELS. Biofuels are made from plants. As plants grow, they remove carbon dioxide from the air through the process of photosynthesis. The carbon becomes part of the plant. When the biofuel is burned the same amount of carbon dioxide is released again. Biofuels include wood chips, palm oil and alcohol made from sugar.

A very large area of land would be needed to grow enough biofuel to replace the fossil fuels that we use today. 

One part of the solution to reducing carbon dioxide emissions in developed countries like the UK has been to reduce the amount of coal burned in power stations. Other fossil fuels have a lower carbon content and so produce less carbon dioxide for the same amount of energy released. The fuel with the least carbon is natural gas (methane). Many gas-fired power stations have been built in recent years.

Scientists realise that stocks of natural gas will not last long and it is not renewable. Also, although it is cleaner than coal, it still produces carbon dioxide when burned. It is not SUSTAINABLE. Hydrogen gas is an alternative fuel that provides no carbon dioxide when it is burned, but there are difficulties with its production and supply. 

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Improving transport

Air pollution from vehicles is reduced by:

1. Encouraging people to drive cars which use less fuel

2. Using cleaner fuels 

3. Removing pollutants from the exhausts of vehicles

4. Encouraging people to use public transport such as buses and trains by increasing parking charges and charging for using some roads.

Modern vehicles have been designed with more efficient engines, which use less fuel to cover the same distance. The fuel is burned more completely so less carbon monoxide and particulates are formed. In the UK, the Government encourages people to use cars that burn less fuel by increasing the tax on fuel, and by increasing taxes on vehicles that use more fuel. Vehicles are only allowed on the roads if their exhaust emissions are within the limit allowed by the law. This legal limit is enforced by the MOT vehicle test.

Carbon monoxide (CO) + nitrogen monoxide (NO) ---> nitrogen (N₂) + carbon dioxide (CO₂).

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Catalytic convertors

Since 1980, new cars have been fitted with a CATALYTIC CONVERTOR. This contains a platinum catalyst that makes the pollutant gases react with each other. In this reaction, carbon monoxide gains an oxygen atom and is oxidised. The nitrogen monoxide loses an oxygen atom and is reduced. 

Sulphur damages the platinum catalyst, so low sulphur fuels are used. This also reduces the amount of sulphur dioxide produced. 

Since 1980, the amount of carbon monoxide in vehicle exhausts has fallen in developed countries. It could be that the use of the catalytic convertors has caused the fall in carbon monoxide emissions but there may be other factors such as improved efficiency of engines. 

We can only say that there is a correlation between the use of catalaytic convertors and reduced carbon monoxide emissions. 

Carbon monoxide + nitrogen monoxide ---> nitrogen + carbon dioxide

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Transport without fossil fuels

There are a growning number of electric cars which do not give out any pollutant gases at all. Unfortunately, electric cars cannot travel far and it takes a long time to charge up the batteries. Electric vehicles are only zero carbon if the electricity used to charge them is produced using renewable energy sources or nuclear power.

Biofuels are an alternative to fossil fuels for powering vehicles.

These include plant oils which replace diesel fuel, and alcohol which is fermented from sugar and replaces petrol. Burning biofuels does not contribute to global warming because the carbon released was taken in when the plants were grown. Vehicle engines do not have to be modified and vehicles can fill up with fuel at service stations in the normal way.

Unfortunately, to replace all the fossil fuels used in transport with biofuels, far more farmland would be needed to grow the fuel plants that we have available.

Already vast areas of natural forest have been turned into plantations of crops such as palm oil, destroying habitats.

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