Chemistry Module C1: Air Quality

The Atmosphere

The atmosphere is a thin layer of gases around the Earth made up from… •       78% nitrogen •       21% oxygen •       1% argon •       Small amounts of water vapour, carbon dioxide and other gases. Scientists believe it formed due to large amounts of volcanic activity on Earth a long time ago, therefore the atmosphere was mostly carbon dioxide and water vapour. Eventually the Earth cooled and water vapour became our oceans, allowing microscopic organisms to form which produced oxygen and removed carbon dioxide.

What the Earth might have looked like during volcanic activity which produced large amounts of carbon dioxide and water vapour.

Pollutants

Pollutants are chemicals which damage the environment and, indirectly, humans. Pollutants can be formed when we burn fossil fuels such as in our cars, and power stations. Other examples of pollutants are… •       Carbon dioxide – traps heat in the Earth’s atmosphere (a greenhouse gas) •       Nitrogen oxides – causes acid rain, creates breathing problems •       Sulphur dioxide – causes acid rain •       Carbon monoxide – prevents blood from carrying oxygen •       Particulates – makes things dirty, creates breathing problems Pollutants cannot just disappear from the atmosphere, they have to go somewhere. This is where problems start occurring. Particulate carbon is deposited on surfaces such as stone building which make them dirty. Carbon dioxide stops heat from leaving the atmosphere, which contributes to global warming. Sulphur dioxide and nitrogen dioxide cause acid rain after reacting with water. Acid rain damages trees, corrodes metals and upsets the pH level of rivers which causes animals and plants to die.

A dirty van, caused by particulates from incomplete combustion/

Chemicals

Elements are the ‘building blocks’ of everything, each is made up from the structure of tiny particles called atoms. Compounds are where elements chemically combine to form a new substance. For example, H2O is formed when 2 hydrogen (H) elements combine with an oxygen (O) atom. Like Hydrogen and Oxygen, each element has its own chemical symbol which is recognised worldwide. Reactions in these formulae can completely change the atom structure, depending on whether they separate or join other atoms, or even do both. Changes are usually not reversible. No atoms are lost or created in a reaction, so the mass stays the same. The mass is conversed. Combinations like H2O are shown in equations with an arrow from the reactants (starting substances) to the products. There are word equations which clearly show what has reacted and what the products are and there are formula equations which show more depth into why it happens. There will always be the same number of atoms on both sides of the equation because none are lost and none are created.

H    +   O    >     2H O 

2

2

2

A simple balanced equation for hydrogen reacting with oxygen to produce water.

Combustion

Combustion (burning) is a reaction between a fuel and oxygen that creates energy, such as coal and oxygen. Oxygen usually comes from the air, but pure oxygen will give a faster reaction. Coal is mostly carbon, so when one atom of carbon reacts with an atom of oxygen it creates CO2. There are two types of combustion, complete and incomplete. Complete is when there is plenty of oxygen for the fuel to completely burn. Hydrocarbons are formed from these reactions, which are mainly carbon and hydrogen. They produce carbon dioxide and water vapour only when burned in the air. Incomplete combustion occurs when there is not enough oxygen for all of the fuel to burn. It results in carbon particulates or carbon monoxide being produced. Incomplete combustion most commonly occurs in car engines; hence the exhaust is needed to remove particulates and other dangerous chemicals which have been created.  High temperatures, such as that of a car engine, can cause oxygen and nitrogen to react which produced nitrogen monoxide (NO). This is then oxidised to produce nitrogen dioxide (NO2). When they occur together, they’re called NOx.

Combustion

Reducing Pollution

Some Carbon Dioxide is removed by photosynthesis in plants and by dissolving in water. However, we produce too much carbon dioxide and cut down trees etc. so the levels of it rise each year. In power stations, emissions are reduced by, simply, using less electricity. Also, sulphur and other toxic chemicals are removed from coal, gas and oil before combustion. Sulphur dioxide can be removed from a chimney of a power station with a process called wet scrubbing with a spray of calcium oxide and water, or seawater. Alternatively, we should start using renewable energy sources more such as solar, wind and hydroelectricity.   Around half of the UK’s carbon monoxide emissions come from road transport. Emissions can be reduced by… •       Using low-sulphur fuel in cars with new, efficient engines •       Legal limits for exhaust emissions, tested for during MOT tests •       Installing catalytic converters in cars to reduce toxic gas emissions •       Using public transport Catalytic converters replace hazardous gases such as carbon monoxide and nitrogen monoxide with nitrogen and carbon dioxide by reacting them with oxygen. Electricity is an alternative way to power cars, so long as it is renewable. Biofuels could also be a solution, but it takes up lots of valuable farmland. Simply though, the only way to reduce emissions is to burn less fossil fuels. This is becoming more difficult as the ever-growing world population demands more electricity.

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The Atmosphere

Pollutants

Chemicals

Combustion

Reducing Pollution