Air (Cambridge O Level Chemistry)

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

Pie chart showing the composition of clean dry air

 Uses of air

• The gases available in the air have many important applications
• The noble gases are used in many applications, e.g. helium is used to fill balloons, argon is used in tungsten light bulbs, krypton is used in lasers for eye surgery
• Oxygen is used in steel making, welding and breathing apparatus
• Nitrogen is used in food packaging, the production of ammonia and in the production of silicon chips
• Oxygen and nitrogen are separated from the air by fractional distillation

Investigating the percentage of oxygen in air

• The percentage of oxygen in the air can be investigated by passing a known quantity of air over a metal
• The oxygen in the air will react with the metal, forming a metal oxide
• The oxygen will be removed from the air and the volume of the air with the oxygen removed can be measured
• An example of the apparatus that can be used to investigate this is shown below:

Method:

• Heat the copper using a Bunsen burner
• Push the plunger of the syringe containing air, forcing the air into the other plunger until all of the air has transferred
• Push the air back from the now filled plunger to the other plunger
• Repeat this several times for about 3 minutes
• The copper will turn black as copper reacts with the oxygen in the air and copper oxide is produced
• Allow the apparatus to cool
• Ensure all the gas is in one syringe and record the volume of gas

• The percentage of oxygen in the air can be calculated from the results

Step 1 - calculate the volume of oxygen in 50.0 cm³ of air

            volume of oxygen in air = starting volume - final volume 

                         = 50.0 - 39.5 = 10.5 cm³

Step 2 - calculate the percentage of oxygen in air

Percentage of oxygen in air = 

                                                                                =     

                                                                                = 21.0 %

• In addition to the gases present naturally in our atmosphere, other gases are present due to human activities and are classed as air pollutants

Carbon dioxide

• Sources: complete combustion of carbon-containing fuels such as fossil fuels, e.g. the complete combustion of methane:

CH₄ + O₂ → CO₂ + 2H₂O

• Adverse effects: increases global warming, which leads to climate change

Carbon monoxide

• Sources: incomplete combustion of carbon-containing fuels such as fossil fuels, e.g. incomplete combustion of gasoline:

C₈H₁₈ + 9O₂ → 5CO + 2CO₂ + 9H₂O

• Adverse effects: toxic, combining with haemoglobin in the blood and prevents it from carrying oxygen

Particulates

• Sources: incomplete combustion of carbon-containing fuels such as fossil fuels can also produce particulates of carbon (soot), e.g the incomplete combustion of methane can produce CO and C:

2CH₄ + 3O₂→ 2CO + 4H₂O

CH₄ + O₂→ C + 2H₂O

• Adverse effects: respiratory problems and cancer

Methane

• Sources: waste gases from digestive processes of animals, decomposition of vegetation, bacterial action in swamps, rice paddy fields and landfill sites
• Adverse effects: increases global warming, which leads to climate change

Oxides of nitrogen

• Sources: reaction of nitrogen with oxygen in the presence of high temperatures, e.g. in car engines, high-temperature furnaces and when lightning occurs. It is also a product of bacterial action in the soil
• Adverse effects:
  • Produces photochemical smog 
  • Dissolves in rain to form 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 and causes respiratory problems 

Sulfur dioxide

• Sources: combustion of fossil fuels containing sulfur compounds. Power stations are a major source of sulfur dioxide
• Adverse effects: dissolves in rain to form acid rain with similar effects as the acid rain caused by oxides of nitrogen

How acid rain is produced