CIE IGCSE Chemistry

Revision Notes

11.1.2 Air

The Composition of Air

  • The chart below shows the approximate percentages by volume of the main gases in unpolluted, dry air:

 

Pie chart showing composition of air, IGCSE & GCSE Chemistry revision notesPie chart showing the composition of clean air

 

Uses of air

  • The gases available in the air have many important applications
  • Oxygen is used in steel making, welding and in breathing apparatus
  • Nitrogen is used in food packaging, the production of ammonia and in the production of silicon chips
  • Both of these gases are separated from air by fractional distillation
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Fractional Distillation of Air

  • The air is first filtered to remove dust, and then cooled in stages until it reaches –200°C
  • At this temperature the air is in the liquid state
  • Water vapour and carbon dioxide freeze at higher temperatures and are removed using absorbent filters
  • The Noble gases are still in the gaseous state at -200ºC, leaving a mixture of liquid nitrogen and oxygen
  • The liquefied mixture is passed into the bottom of a fractionating column
  • Note that the column is warmer at the bottom than it is at the top
  • Oxygen liquefies at -183°C and nitrogen liquefies at -196°C
  • Nitrogen has a lower boiling point than oxygen so it vaporises first and is collected as it rises in the gaseous state to the top of the colum
  • The liquid O2 is then removed from the bottom of the column

 

Fractional Air Distillation, IGCSE & GCSE Chemistry revision notesDiagram showing the fractional distillation of liquid air to produce N2 gas and liquid O2

Air Pollution

Carbon monoxide

  • Sources: incomplete combustion of fossil fuels e.g: incomplete combustion of gasoline:
C8H18 + 9O2 → 5CO + 2CO2 + 9H2O
  • Adverse effects: poisonous, combining with hemoglobin in blood and prevents it from carrying oxygen

Sulfur dioxide

  • Sources: combustion of fuels, natural gas and sulfide ores e.g: zinc blende (ZnS) in the extraction of zinc:
2ZnS + 3O2 → 2ZnO + 2SO2
  • Adverse effects: acid rain which causes corrosion to metal structures, buildings and statues made of carbonate rocks, damage to aquatic organisms. Pollutes crops and water supplies, irritates lungs, throats and eyes

Oxides of nitrogen

  • Sources: reaction of nitrogen with oxygen in car engines and high-temperature furnaces and as a product of bacterial action in soil
  • Adverse effects: acid rain with similar effects as SO2 as well as producing photochemical smog and breathing difficulties, in particular for people suffering from asthma

Compounds of lead

  • Sources: old water pipes, old paints, petrol in some kinds of racing cars and from very old engines
  • Adverse effects: causes significant damage to the central nervous system, young infants are particularly susceptible to lead poisoning

Exam Tip

Complete and incomplete combustion of hydrocarbons produce different products. Complete combustion occurs in excess oxygen and produces CO2 and H2O. Incomplete combustion occurs in oxygen-deficient conditions and produces CO, H2O and sometimes carbon.

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Nitrogen Oxides in Car Engines

Nitrogen oxides

  • These compounds (NO and NO2) are formed when nitrogen and oxygen react in the high pressure and temperature conditions of internal combustion engines and blast furnaces
  • Exhaust gases also contain unburned hydrocarbons and carbon monoxide
  • Cars are fitted with catalytic converters which form a part of their exhaust systems
  • Their function is to render these exhaust gases harmless

Catalytic converters

  • They contain a series of transition metal catalysts including platinum and rhodium
  • The metal catalysts are in a honeycomb within the converter to increase the surface area available for reaction
  • A series of redox reactions occurs which neutralises the pollutant gases
  • Carbon monoxide is oxidised to carbon dioxide:
2CO + O2 → 2CO2
  • Nitrogen oxides are reduced to N2 gas:
2NO → N2 + O2
2NO2 → N2 + 2O2
  • Unburned hydrocarbons are oxidised to carbon dioxide and water:
C8H18 + 12½O2 → 8CO2 + 9H2O

 

Catalytic-Converters, IGCSE & GCSE Chemistry revision notesCatalytic converters are designed to reduce the polluting gases produced in car exhausts

The Rusting of Iron

  • Rust is a chemical reaction between iron, water and oxygen to form the compound iron (III) oxide
  • Oxygen and water must be present for rust to occur
  • Rusting is a redox process and it occurs faster in salty water since the presence of sodium chloride increases the electrical conductivity of the water
Iron +  Water + Oxygen  →  Hydrated Iron (III) Oxide 
4Fe(s) + 3O2(g) + xH2O(l) → 2Fe2O3.xH2O(s)

 

Conditions for rusting, IGCSE & GCSE Chemistry revision notesDiagram showing the requirements of oxygen and water for rust to occur: only the nail on the left rusts

 

Rust prevention methods

 Barrier methods

  • Rust can be prevented by coating iron with barriers that prevent the iron from coming into contact with water and oxygen
  • However, if the coatings are washed away or scratched, the iron is once again exposed to water and oxygen and will rust

Barrier Methods of Rust Prevention table, IGCSE & GCSE Chemistry revision notes

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Galvanising / Sacrificial Protection

  • Iron can be prevented from rusting using the reactivity series
  • Galvanising is a process where the iron to be protected is coated with a layer of zinc
  • ZnCO3 is formed when zinc reacts with oxygen and carbon dioxide in the air and protects the iron by the barrier metho
  • If the coating is damaged or scratched, the iron is still protected from rusting by the sacrificial method (magnesium can also be used)
  • This is because zinc is more reactive than iron and so it loses its electrons more readily:
Zn → Zn2+ + 2e-
  • The iron stays protected as it accepts the electrons released by zinc, remaining in the reduced state and thus it does not undergo oxidation
  • The electrons donated by the zinc react with hydrogen ions in the water producing hydrogen gas:
2H+ + 2e- → H2
  • Zinc therefore reacts with oxygen and water and corrodes instead of the iron

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.
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