- Created by: rebecca dowell
- Created on: 13-10-09 16:45
Atoms, Elements and Compounds
All substances are made of atoms, which have a tiny central nucleus surrounded by electrons.
The simplest substances contain only one type of atom, and are called elements.
Atoms of each element have their own chemical symbol in the periodic table.
Elements join together to make compounds, by giving, taking or sharing electrons.
The formula of a substance shows the types of atoms it contains, and how many have combined together;
eg Water, H20 - two hydrogen atoms and one water atom
Carbon Dioxide CO2 - one carbon atom and two oxygen atoms
In a chemical reaction, there are always the same number of atoms on each side of the equation.
Limestone and its uses, in concrete, cement and gl
Limestone is a rock which is used as a building material and to make 'quicklime', cement, concrete and glass.
Limestone is mainly made of the compound, Calcium Carbonate, formula CaCO3.
When you heat limestone, you make quicklime (CaO) and carbon dioxide (CO2)
This is done on a large scale in lime kilns.
Water is added to the quicklime to make 'slaked lime' , or calcium hydroxide. This can be mixed with sand and used as 'lime mortar' in building. This is like a natural cement.
To make cement - limestone is mixed with clay, heated strongly and then powdered.
A mixture of cement, sand and gravel is used to make concrete, which is used a lot in building, as it is cheap, strong and can be poured into a mould to make different shapes.
Glass is made up of sand, limestone and sodium carbonate. This mixture is transparent, so good for windows!
Extracting Metals from Rocks called 'Ores'
Very few metals are found 'native', ie as just the metal. These are the unreactive ones, like gold. Most metals are found as compounds, and have to be extracted by chemical reactions.
The method we use to extract the metal depends on the reactivity.
If a metal is less reactive than carbon, we can extract the metal by heating with carbon in a blast furnace. This works for metals below carbon in the reactivity series, eg Iron.
Iron ore contains Iron Oxide, which is reduced in a blast furnace by carbon. The iron that is produces is hard and brittle, as it contains 4% impurities. We then turn it into steel, an alloy.
If a metal is more reactive than carbon, we have to use another method, called electrolysis, or by using a more reactive metal. This is more difficult and expensive.
Properties of Iron And Steels
Many pure metals are very soft, and lose their shape easily, so we use ALLOYS, which are a mixture of metals which makes them harder, stronger and more useful. 'Smart Alloys' have special properties such as returning to their original shape if bent, and are used in things like orthodontic braces.
Pure Iron - Regular arrangement of atoms, in layers which slide over each other so that its shape can easily be changed.
Steels - an ALLOY of iron, ie a mixture that contains other elements as well, these change the regular structure which changes the properties.
Carbon Steels - Contain up to 1.5% carbon, making it harder, but brittle and likely to snap
Low-Alloy Steels - Up to 5% other metals
High-Alloy Steels - Up to 25% other metals, eg stainless steel contains 25% chromium and nickel, and does not rust.
Transition Metals and the Extraction of Copper
The elements between group 2 and 3 of the periodic table are all metals and are called the transition metals.
They have many similar properties which makes them very useful. Eg They are strong, hard and dense, but malleable (bendy!) They are good conductors of heat and electricity and have high melting points.
Copper is really useful for electrical wires. Copper is more reactive than carbon, so must be extracted from its ore by mining and electrolysis. Copper ore is very impure, ie a chunk of rock may only contact about 2% copper, so copper mines are HUGE and affect the landscape.
New methods of extracting copper are being developed, including using bacteria, funghi and plants.
Aluminium and Titanium
Aluminium and Titanium are LOW DENSITY Metals. They resist corrosion (ie do not rust) and are light but very strong.
They are reactive metals, so must be extracted through expensive electrolysis, which uses a lot of energy, so is bad for the environment.
Recycling of these metals saves resources and the energy needed to extract the metal from its ore.
Aluminium - Soft, with a low melting point, but can be hardened by alloying.
Aluminium is used for buildings, aeroplanes, cooking foil, cans, and electricity cables
Titanium is used in jet engines, nuclear reactors and bicycles.
Fuels from Crude Oil
Crude oil (mined in the north sea!) is a mixture of many different compounds.
Fractional Distillation separates it into different liquids with different boiling points.
Most of the 'fractions' in crude oil are hydrocarbons (ie contain hydrogen and carbon only), and many of these are alkanes. The boiling point depends on the size of the hydrocarbons, ie the length of the carbon chain. Smaller hydrocarbons have a lower boiling point.
Alkanes are 'saturated hydrocarbons', made of H and C, eg ethane, propane
C - C - C
Alkenes are 'unsaturated hydrocarbons', eg ethene and propene. They have a double bond.
C = C - C
Fossil Fuels and Cleaner Fuels
Burning hydrocarbons (fuels) in plenty of air produces carbon dioxide and water. Carbon dioxide is a greenhouse gas and causes global warming. It does this by creating a 'blanket' around the planet which stops heat leaving the earth.
In a limited supply of air, carbon monoxide and other particles may be produced. These particles cause health problems eg asthma and cause global dimming by reflecting sunlight away from the Earth.
Any sulphur compounds in the fuel burn to make sulphur dioxide, which causes acid rain. This can be reduced by removing sulphur dioxide from waste gases after burning fuels in a power station.
Fossil fuels are being used up fast and will soon run out.
Plants can provide sugar to make BIOETHANOL fuels, or make oils for BIODIESEL. These fuels burn more cleanly, producing less greenhouse gases.