Chemistry - EDEXCEL IGCSE - Chemistry in Society

CHEMISTRY IN SOCIETY

Extraction and uses of metals:

If the metal is below carbon in the reactivity series than carbon could be used to react with the metal oxide to create a pure version of the metal. Above zinc, for metals such as aluminium, electrolysis has to be used. For some metals, such as titanium, its oxide is reacted with a more reactive metal. 

Using carbon - cheapest

Using electrolysis - expensive

Using other metals - most expensive

Extraction of aluminium from aluminium oxide:

Electrolysis is used as aluminium does not react with carbon. For this to work the ions must be free to move. Molten aluminium oxide only occurs at high temperature so it is dissolved in molten cryolite, another aluminium compound that melts at a lower temperature of 100°C and is a better conductor of electricity. 

The electrodes used for the electrolysis is made of graphite (carbon) and needs to be replaced regularly to prevent CO₂ from being formed, this applies to the anode. 

The amount of electricity involved is huge as it needs to create enough heat to keep the cryolite molten. Therefore, it is very costly.

At cathode (negative) electrode :

Al³⁺ + 3e^- —> Al

At anode (postive) electrode:

2O^2- —> O₂ + 4e^-

Extraction of iron from iron ore:

1. Hematite (a type of iron ore), coke (impure carbon) and limestone are placed in a blast furnace
2. C + O₂ —> CO₂ occurs
3. CO₂ + C —> 2CO occurs as the CO₂ is reduced by the coke at such a high temperature
4. Fe₂O₃ + 3CO —> 2Fe + 3CO₂ - the CO (reducing agent) reacts with hematite to form iron
5. Fe₂O₃ + 3C —> 2Fe + 3CO - this also happens at the hot parts of the furnace
6. Iron flows to the bottom of the furnace and can be tapped off
7. The limestone is added to remove impurities. Limestone undergoes thermal decomposition - CaCO₃ —> CaO + CO₂ - the calcium oxide (a basic oxide) reacts with acidic oxides such as silicon oxide and forms a molten slag which floats on top of the iron and can be tapped off separately.

Uses for…

Aluminium:

1. Conducts heat and electricity - used in planes (so that lightning conducts through the body of the plane), also used in cooking pans.
2. Shiny appearance - used in pans
3. Light (low density) - planes, electric cables
4. Resists corrosion (because of the layer of Al₂O₃) - aeroplanes, pans, electric cables
5. Strong when made into an alloy

Iron:

• Cast iron (4% carbon) - doesn't shrink much when solidified and so it is used for making castings. It is also used for manhole covers, guttering and drainpipes as it is very hard and cheap.
• Mild iron (<0.75% carbon) - nails, car bodies, girders. Very hard and relatively expensive
• Wrought iron (0% carbon) - decorates gates and raising as it is fairly soft
• High ( carbon steel 0.25 - 1.5% carbon) - cutting tools, masonry nails. The difference between measuring nails and mild steel nails is that the later would bend if miss-struck while the former would break as more carbon, means more strength but also means the material is more brittle.
• Stainless steel - an alloy of iron with chromium and nickel. Forms an oxide layer like aluminium so does not corrode. Used in kitchen utensils.

Crude Oil:

Crude oil is a mixture of hydrocarbons.

Fractional distillation is used to separate crude oil. A fractionating column is cooler at the top than at the bottom. Say a hydrocarbon boils at 120°C, the temperature as it enters the bottom of the column is high enough to make it rise as a gas. It rises until the temperature is low enough for it to condense, it can then be tapped off.

Crude oil contains:

1. Refinery gases - contains ethanes and is used as LPG (liquefied petroleum gas) for domestic heating and cooking.
2. Gasoline (petrol) - used in cars as fuel
3. Kerosene - used as fuel for jet aircrafts, for domestic heating oil and as paraffin for small heaters
4. Diesel gas - buses, lorries, some cars, oil railway engines
5. Fuel oil - ship boilers, industrial heating
6. Bitumen - tarmac

Viscosity and boiling point increases from 1 - 6 as the crude oil fraction becomes large in length.

Incomplete combustion may occur when burning hydrocarbons.

e.g. CH₄ + 3O₂ —> CO + 4H₂O

Carbon monoxide is bad for humans as it combines with haemoglobin in the bloodstream, preventing it from carrying oxygen, which can cause death.

In car engines, temperatures are high enough to allow nitrogen and oxygen in the air to react, forming nitrogen oxide.

The problem with fractional distillation of crude oil is that too much of the long chain hydrocarbons such as fuel oil is produced and too little of small chain hydrocarbons such as those of petrol are contained in crude oil.

To solve this, catalytic cracking may be used where long chain hydrocarbons such as fuel oil using silicon dioxide or aluminium aside at 600-700°C undergoes thermal decomposition to produce a random group/mixture of shorter chained hydrocarbons which can then be separated by fractional distillation.

e.g. C₁₃H₁₈ —> C₂H₄ + C₃H₆ + C₈H₁₈

Synthetic polymers:

Monomers are the base units of a polymer. A polymer is a long chain molecule made up of many monomers. 

Nylon is a condensation polymer where small molecules from both monomers are lost to allow them to join together. This molecule lost is usually water or hydrogen chloride.

Monomers of nylon are basically hydrocarbons with either COOH (carboxylic acid) or Cl(chloride) at both ends.

The Industrial Manufacture of Chemicals:

Ammonia is made by reacting nitrogen from the air and hydrogen from natural gas or from the cracking of hydrocarbons.

The Haber process:

In the Haber process, nitrogen and 3 hydrogen atoms react at 450°C, a pressure of about 200 atmospheres and an iron catalyst. 15% of it turns into ammonia. The unused gas is then recycled and reused after the ammonia has turned to liquid via cooling and condensation.

Ammonia is used to create nitric acid and is put into fertiliser as is contains nitrate ions.

Sulphuric acid is made by reacting sulphur (found in rocks and some natural gases) and oxygen (from the air)

Contact process:

1. Burn sulphur in air - S + 2O₂ —> SO₂
2. React oxygen with sulphur dioxide at 450°C, a pressure of about 2 atmospheres and a Vanadium (V) oxide catalyst. SO₂ + O —> SO₃
3. Reacts with sulphuric acid - H₂SO₄ + SO₃ —> H₂S₂O₇
4. Then water is added - H₂S₂O₇ + H₂O —> 2H₂SO₄

Sulphuric acid is used in detergents, fertiliser and paint.

Electrolysis of sodium chloride solution creates the products of chloride at the anode, hydrogen at the cathode and sodium hydroxide in the solution. Chloride is prevented from coming into contact with sodium chloride which creates bleach by using a diaphragm cell.

At anode:

2Cl^- —> Cl₂ + 2e^-

At cathode:

2H⁺ + 2e^- —> H₂

Uses of sodium hydroxide:

1. Making bleach by reacting it with chlorine
2. Making paper by helping break the wood down into pulp
3. Making soap by reacting with vegetable fats and oils to make compounds such as sodium stearate, which is present in soap

Uses of chlorine:

1. Sterilised water
2. Making bleach (as above)
3. Making hydrochloric acid