Wednesday, 8 April 2009

iGCSE Chemistry - Chemistry in Society

Extraction and uses of metals

 

  • Metals are found in ores containing minerals mixed with rock
  • The mineral is a compound of the metal and needs to be purified
  • Aluminium is extracted from the ore bauxite
  • The aluminium oxide in the bauxite needs to be purified, and it is melted so that electrolysis can be carried out.
  • The anode is made from carbon
  • The cathode is the steel-lined case.
  • Aluminium forms at the cathode
  • Oxygen forms at the anode
  • The oxygen formed reacts with the carbon anode to form CO2, and this causes the anode to be worn down
  • This process requires large amounts of electricity
  • Copper is obtained by heating the ore malachite with carbon
  • All metals below carbon in the reactivity series can be obtained this way
  • Iron is produced on a large scale by this process in a blast furnace:
    1. Iron ore (source of iron), coke (source of carbon) and limestone are added at the top of the blast furnace
    2. Hot air is blasted from the bottom of the furnace to the top
    3. Oxygen in the air reacts with coke to form CO2, which in turn reacts with the coke again to from carbon monoxide
    4. Carbon monoxide is a reducing agent and thus turns iron oxide into iron.
    5. Molten iron runs to the bottom of the furnace and is run off
    6. The limestone is broken down by heat into calcium oxide, which in turn reacts with impurities to form slag

·        Zinc can be extracted from zinc blende:

1.       Zinc sulphide is turned into zinc oxide by heating

2.     Zinc oxide is either reduced by carbon monoxide or

3.     it is dissolved in sulphuric acid and then electrolysed

  • Uses of metals:

Metal

Properties

Uses

Aluminium

Very light

Aircraft construction

Iron

Strong + malleable

Car bodies, cutlery

Zinc

Low melting point

Galvanising

Chromium

Strong and non-corrosive

Stainless steel production

Copper

Good electrical conductor

Electrical wiring

 

 

Natural Gas and Oil

 

  • Crude oil is a mixture of hydrocarbons
  • Separate parts, or fractions, of crude oil can be separated using fractional distillation.
  • The crude oil is heated in a furnace and passed into the bottom fo the fractionating column
  • The mixture evaporates and the different fractions condense at different levels.
  • The fractions are:

                   Gases

                   Gasoline

                   Kerosene

                   Gas oil

                   Fuel oil

                   Bitumen

  • Fractional distillation produces more long-chain hydrocarbons than required, and these are turned into short-chain hydrocarbons by cracking
  • When hydrocarbons are burnt (combusted) they release CO2, and if they do not combust completely, they release CO (carbon monoxide)
  • Carbon monoxide molecules bind with haemoglobin in the blood more strongly than oxygen, and thus are dangerous
  • Hydrocarbons are damaging to the environment because when burnt they release CO2 which is a greenhouse gas
  • If crude oil is spilled, it can destroy habitats and kill animals.

 

 

Synthetic Polymers

 

  • Polymers are large molecules made up of many small molecules called monomers
  • Polymers can be made in tow ways: addition and condensation
  • Addition polymerisation consists of alkenes reacting with each other to form long chains
  • Condensation polymerisation consists of joining two different monomers, and when they react they expel a small molecule, often water
  • Ethene is used to make polyethene through addition polymerisation
  • Nylon is manufactured using condensation polymerisation, and its starting monomers are: hexanedioic acid and 1,6-diaminohexane

 

The Manufacture of some Important Chemicals

 

  • Ammonia is produced in the Haber Process from nitrogen (from the air) and hydrogen (from natural gas)
  • The Haber Process involves an iron catalyst, a temperature of 450°C and 200 atmospheres
  • Any unused hydrogen and nitrogen is recycled.
  • Ammonia is used for the manufacture of nitric acid and fertilisers
  • Sulphuric acid is manufactured in the contact process
  • The raw materials needed are sulphur and air
  • The conditions are as follows: 450°C, 2 atmospheres and a vanadium oxide catalyst.
  • Four steps in process:
    1. Sulphur + Oxygen -> Sulphur Dioxide
    2. Sulphur Dioxide + Oxygen -> Sulphur trioxide
    3. Sulphur Trioxide + Sulphuric Acid -> Oleum
    4. Oleum + Water -> Sulphuric Acid

·        Sulphuric acid is used in the manufacture of chemicals

·        Sulphur dioxide and nitrogen oxides are pollutant gases which, when mixed with water, form acid rain

·        Acid rain kills plants and animals and depletes the soil of minerals

·        Sodium hydroxide is manufactured through the electrolysis of brine (sodium chloride solution)

·        At the cathode, hydrogen ions are turned into atoms, and at the anode, chloride ions are turned into atoms.

·        What is left is sodium hydroxide solution.

·        Sodium hydroxide is used for the manufacture of soap.

·        Chlorine is used in the manufacture of bleach.

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