GCSE & A Level Revision Notes

Subject: Chemistry
Level: IGCSE
Exam Boards: EDEXCEL


Section 5 (5.1 – 5.31)

Chemistry in Industry – Extraction and use of metals

5.1

The method used to extract metals depends on their position in the reactivity series. The least reactive metals (gold, silver, platinum) are found in their natural forms and therefore can simply be extracted. Anything less reactive than carbon can be displaced from its ore by carbon, but those at the top of the reactivity series (magnesium, aluminium etc.) must be extracted by electrolysis.

5.2

Aluminium is extracted from purified aluminium oxide, using electrolysis.

See illustration above

Bauxite is purified into aluminium oxide, which is dissolved into cryolite. Molten cryolite to bring down the boiling point, as it is much lower than that of aluminium and therefore reduces the energy needed for extraction. The walls of the tank make up the cathode, where aluminium is collected.

It sinks to to the bottom and is tapped off.

Oxygen is formed at the positive electrode, which reacts with the graphite anode to from carbon dioxide. This means that the anode has to be replaced regularly.

5.3

At the anode, two oxygen ions lose four electrons: 2O2- → O2 + 4e- At the cathode, each aluminium ion gains three electrons: Al3+ + 3e- →Al

Iron extration inside a blast furnace

5.4

Extraction of iron takes place inside a blast furnace.

  • Coke (impure carbon) reacts with oxygen, which produces great amounts of heat in the furnace: C (s) + O2 (g) → CO2 (g).
  • Carbon dioxide is reduced by more carbon to carbon monoxide: CO2 (g) + C (s) → 2CO (g)
  • Carbon monoxide is the main reducing agent in the furnace. Iron ore (haematite) reacts with the carbon monoxide to give iron and carbon dioxide, especially in the cooler parts of the furnace: Fe2O3 (s) + 3CO (g) → 2 Fe (l) +3CO2 (g)
  • Limestone is added to the furnace to remove impurities. It is hot enough to thermally decompose. Calcium oxide (basic) then reacts with silicon dioxide which is acidic, to make slag, a waste product that can be tapped off: CaCO3 (s) → CaO (s) + CO2 (g) CaO (s) + SiO2 (s) → CaSiO3 (l)

5.5

Aluminium and iron have many different uses industrially.

Aluminium

  • Aluminium isn’t very strong so alloys are normally used instead, which are strengthened by adding copper, silicon or magnesium.
  • Used for airplanes as it resists corrosion (has a very thin layer of aluminium oxide around the outside to prevent anything getting through to the aluminium), has a low density and is strong.
  • Used for cooking as it has a shiny appearance, low density and is a good conductor of heat.
  • Used for cables due to its low density and is a good conductor of electricity. A core of steel strengthens it.

Cast Iron

  • Iron tapped directly from a furnace is known as pig iron, which is then re-melted and cooled under controlled conditions to form cast iron. It is very impure and contains around 4% carbon.
  • This is used for manhole covers, drainpipes and guttering and cylinders in engines, because it is very hard, but brittle as well.

Mild Steel

  • Mild steel is iron containing around 0.25% carbon, which increases the hardness and strength of the iron.
  • It is mostly used for wire, nails, car and ship bodies, girders and bridges, due to its properties.

Wrought Iron

  • Pure iron is know as wrought iron
  • It used to be used for decorative purposes, but has mostly been replaced by mild steel. It is soft, which makes it easy to work with, but not good for structural purposes.

High Carbon Steel

  • Iron containing up to 1.5% carbon. The increased carbon makes the iron harder, but more brittle.
  • Used for cutting tools and masonry nails.

Stainless Steel

  • Alloy of iron with nickel and chromium, which form strong oxide layers that prevent corrosion of the iron.
  • Used commercially for kitchen sinks, fridges, cutlery and gardening tools.
  • Used industrially to make corrosion-resistant vessels, such as in the production of dairy or chemicals.

Crude Oil

5.6

Crude oil is made up of a mixture of hydrocarbons of different chain lengths.

5.7

Industrially, crude oil is separated using fractional distillation. Crude oil is heated until it boils, and rises up the fractionating column. As it rises, the temperature decreases and the longer chained hydrocarbons re-condense into liquids. At this point, they are collected to be used.

5.8

Refinery Gases – bottled gas

Gasoline – Fuel for cars and other vehicles

Naphtha – used to make chemicals

Kerosene – aircraft fuel

Diesel – Fuel for cars and other larger vehicles

Fuel Oil – fuel for ships, power stations

Bitumen – used to make roads and roofs

5.9

As the chain length gets longer, the boiling point gets higher, so bitumen is found as a liquid/solid, whereas refinery gases are found as gases. The fractions also get less viscous as the chain length decreases.

5.10

Incomplete combustion of hydrocarbon fuels may result in the production of carbon monoxide, which is poisonous, because it reacts with haemoglobin, the red pigment in the blood that carries oxygen, more readily than O2 and reduces the capacity of the blood to carry oxygen.

5.11

In car engines, the temperatures get very high from the combustion of fuel, which results in nitrogen and oxygen from the air reacting, which forms nitrous oxides.

5.12

Nitrogen oxides and sulphur dioxides are pollutant gases, which contribute to acid rain. Sulphur dioxide causes acidic ‘water’ that mixes with rain clouds and nitrous oxides to form acid rain. This can lead to minerals leaching out of the soil, which results in infertile soil, and also kills aquatic animals.

5.13

Fractional distillation of crude oil produces more long-chain hydrocarbons than short chain hydrocarbons. Short chain hydrocarbons are much more useful than long chain hydrocarbons, which means that the process of cracking – breaking long chain hydrocarbons into shorter chain hydrocarbons – is necessary.

5.14

Long-chain alkanes are converted into shorter-chain alkanes and alkenes by the process of catalytic cracking. Silica or alumina are used a catalyst, and a temperature of around 600-700˚C is used.

Synthetic Polymers

5.15

Addition polymers are formed by joining together many small molecules, which are called monomers.

5.16

Polymer Structures

5.17

You can deduce the structure of a monomer from the repeat unit of an addition polymer. For example, with poly (ethene) the monomer that it is made from is ethene. The arms, which form the link between the polymers, come from a double bond in the monomer.

5.18

Different polymers have different uses: Poly (ethene)

  • Poly (ethene) comes in two types, low density (LDPE) and high density, (HDPE).
  • LDPE is mainly used as thin film in things like polythene bags, as it is flexible and not very dense.
  • HDPE is used for things such as plastic bottles, which are more rigid and also stronger. They are also recyclable.

Poly (propene)

  • Poly (propene) is used to make ropes and crates, among other things.
  • It is recyclable.

Poly (chloroethene)

  • Chloroethene is an ethene molecule where one of the hydrogen atoms has been replaced by chlorine, it is also known as polyvinylchloride or PVC.
  • PVC is quite strong and rigid which means it can be used for drainpipes or windows.
  • It can be made flexible by adding ‘plasticisers’, which makes it useful for floor coverings and clothing.
  • They are electrical insulators, and used to insulate wires.

5.19

Addition polymers can be hard to dispose of, because their inertness means that they do not easily biodegrade.

5.20

Some polymers, such as nylon 6-6, do not form by addition, but by a process called condensation polymerisation.

Condensation polymerisation

5.21

Condensation polymerisation produces a small molecule, such as water, which drops out of the chain, as well as the polymer.

The industrial manufacture of chemicals

5.22

Ammonia is industrially manufactured in the Haber process. It takes hydrogen from natural gas, such as ethane, and nitrogen from the air.

5.23

In the Haber process, there are certain essential conditions used to give a good yield:

  • A temperature of about 450˚C is used, because although the forward reaction is exothermic, a high temperature must be used for a good rate of reaction.
  • A pressure of around 200 atmospheres is used- the higher the pressure, the better the yield, but it is expensive to keep the system too highly pressurised.
  • An iron catalyst is used to speed up the reaction.

5.24

Ammonia has a much higher melting point than nitrogen or hydrogen and so the system is cooled enough for the ammonium to liquefy and be pumped off whilst the hydrogen and nitrogen are re-circulated back into the system so they can react again. This shifts the equilibrium to the side of the products.

5.25

Ammonia is used in the manufacture of fertilisers and nitric acid. Fertilisers contain high levels of nitrates, whilst nitric acid is made by adding oxygen to ammonia: 4NH3 + 8O2 → 4HNO3 + 4H2O

5.26

Sulphuric acid is manufactured in a reversible reaction known as the contact process. The raw materials are sulphur and oxygen that are reacted together to give sulphur dioxide. The sulphur dioxide is then reacted with more oxygen (in excess air.) S (s) + O2 (g) → SO2 (g) 2SO2 (g) + O2 (g) → 2SO3 (g)

5.27

In the contact process, the conditions are engineered to give a good reaction speed and a good yield:

  • Temperature of around 450˚C gives a good reaction speed, and the forward reaction is exothermic, so it is not too high to try and encourage that reaction.
  • The equilibrium would be pushed even further to the right with a higher pressure as there are only two moles on the right, as opposed to three on the left, however the reaction is good enough at low pressure that it is not economically viable.
  • A catalyst of vanadium (V) oxide is used in order to speed up the reaction, which would be too slow otherwise.

Reacting sulphur trioxide with water produces a mist of concentrated sulphuric acid, and so sulphur trioxide is absorbed into concentrated sulphuric acid to give oleum (or oleic acid.)

Manufacturing sulphuric acid (H2SO4)

H2SO4 (l) + SO3 (g) → H2S2O7 (l) H2S2O7 (l) + H2O (l) → 2H2SO4

5.28

Sulphuric acid has many practical uses, such as in detergents, fertilisers and paints. Sulphuric acid is used in extracting white pigment, titanium dioxide, from titanium ores.

5.29

The electrolysis of brine (NaCl) produces many useful products: sodium hydroxide, chlorine and hydrogen. This takes place in a diaphragm cell. Water provides OH- and H+ ions, whilst the ionic sodium chloride salt provides Na+ and Cl- ions.

5.30

The equation at the anode is: 2Cl- → Cl2 + 2e- The equation at the cathode is 2H+ + 2e- → H2

5.31

Sodium Hydroxide has many important uses:

  • Purification of Bauxite – part of the process of extraction + manufacture of aluminium.
  • Papermaking – Sodium hydroxide helps to break down the wood into pulp so that it can be used to make paper.
  • Soap making – NaOH reacts with animal and vegetable fats and oils to make compounds such as sodium stearate, which are present in soap.
  • Making bleach – Sodium hydroxide reacts with chlorine in cold to form bleach.

Chlorine also has many uses, which are important to human life:

  • Sterilising water – chlorine is used to make water safe to drink, as it kills the microbes that may be living in it.
  • Making hydrochloric acid – When chlorine has a controlled reaction with hydrogen, HCl is produced, which can be dissolved in water to get hydrochloric acid.
  • Making bleach – Chlorine reacts with sodium hydroxide to make bleach, which is a mixture of sodium chloride and sodium chlorate.


Select an option from the dropdown.

    Bright Young Things 2010-2015. All rights reserved.

    `