C1.2 What chemical reactions produce air pollutants? What happens to these pollutants in the atmosphere?
Many atmospheric pollutants are made by the burning of fossil fuels. This happens in power stations and in the engines of vehicles.
POWER STATIONS: Most power stations are fuelled by either coal or natural gas. Fuel and air go into the furnace and waste chemicals come out of the chimney.
COAL IS MAINLY CARBON
CAR ENGINES: Vehicle engines burn petrol or diesel. Petrol, diesel fuel and fuel oil are mainly compounds of hydrogen and carbon (HYDROCARBONS).
When fuels burn, atoms of carbon and/or hydrogen from the fuel combine with atoms of oxygen to produce carbon dioxide and/or water (hydrogen oxide)
Any change that forms a new chemical is called CHEMICAL CHANGE or a CHEMICAL REACTION.
REACTANTS → PRODUCTS
A substance chemically combing with oxygen is an example of OXIDATION
However the loss of oxygen from a substance is an example of REDUCTION
COMBUSTION is a chemical reaction that occurs when fuels burn, releasing energy as heat. For combustion to take place, oxygen must be present. Combustion is an example of oxidation.
*Fuels burn more rapidly in pure oxygen than in air because fire needs four things: fuel, heat, chemical reaction and OXYGEN. Burning is an oxidation of a material. Pure oxygen = 100% oxygen however air = 21% oxygen which means there is a faster oxidation rate of reaction.
An example of oxidation is the reaction between nitrogen and oxygen to form nitrogen dioxide:
The number of atoms of each element in the reactants and in the products is the same. As the atoms are conserved and all atoms have mass, the mass of the reactants will be the same as the mass of the products. This is known as CONSERVATION OF MASS.
Incomplete combustion occurs in car engines, so exhaust emissions contain carbon particulates and carbon monoxide, as well as carbon dioxide. Many samples of coal contain sulfur, so is released into the atmosphere when fuels that contain sulfur compounds burn to create SULFUR DIOXIDE.
Burning fossil fuels in power stations and for transport pollutes the atmosphere with:
- Carbon dioxide and sulfur dioxide
- Carbon monoxide and particulate (from incomplete burning)
- Nitrogen oxides (from the reaction between atmospheric nitrogen and oxygen at the high temperatures inside engines)
However atmospheric pollutants cannot just disappear, they have to go somewhere:
- Particulate carbon is deposited on surfaces, making them dirty
- Sulfur dioxide and nitrogen dioxide react with water and oxygen to produce acid rain which is harmful to the environment
- Carbon dioxide is by plants in photosynthesis
- Carbon dioxide dissolves in rain water and in sea water.