IGCSE Science C2 - Gases in the Atmosphere

IGCSE Science - C2 Inorganic Chemistry - 2C Gases in the Atmosphere (Double)

• Learning Outcomes:
  • Know the approximate percentages by volume of the four most abundant gases in dry air.
  • Understand how to determine the percentage by volume of oxygen in air using experiments involving reactions of metals (e.g., iron) and non-metals (e.g., phosphorus) with air.
  • Describe the combustion of elements in oxygen, including magnesium, hydrogen, and sulfur.
  • Describe the formation of carbon dioxide from the thermal decomposition of metal carbonates, including copper(II) carbonate.
  • Know that carbon dioxide is a greenhouse gas and that increasing amounts in the atmosphere may contribute to climate change.
  • Practical: determine the approximate percentage by volume of oxygen in air using a metal or a non-metal.

Composition of Air

• The percentages (by volume) of the four most abundant gases in unpolluted, dry air are approximately:
  • Nitrogen (N₂): 78.1% (About 4/5)
  • Oxygen (O₂): 21.0% (About 1/5)
  • Argon (Ar): 0.9%
  • Carbon Dioxide (CO₂): 0.04%
• The remainder of the air is made up of other noble gases in small amounts (helium, neon, krypton, xenon, and radon).

Measuring the Percentage of Oxygen - Practical Methods

• There are three key experimental methods to determine the percentage of oxygen in air:
  • Using copper
  • Using the rusting of iron
  • Using phosphorus
• All methods depend on the reaction of a substance with oxygen in the air and measuring the decrease in volume.

Using Copper - Practical Details

• Apparatus:
  • Silica tube packed with copper filings
  • Gas syringe (initially set to 100 cm³)
  • Bunsen burner (for heating).
• Equation: 2Cu (s) + O₂ (g) → 2CuO (s)
• Procedure:
  • Heat the silica tube containing copper strongly using a Bunsen burner.
  • Pass air over copper, taking readings from the gas syringe over a week's time. The volume in the syringe decreases as oxygen is used up and the resulting copper(II) oxide will have turned black.
• Calculations:
  • Volume of oxygen = Initial volume - Final volume
  • Percent of oxygen = (Volume of oxygen / Initial volume) x 100

Using Copper - Key Points

• Ensure all oxygen reacts (copper must be in excess).
• Cool the apparatus before taking the final reading, as gases expand when hot.
• Values are typically between 18% and 24%, not precisely 21%.

Using Rusting of Iron - Practical Details

• Apparatus:
  • Conical flask
  • Connecting tube
  • Measuring cylinder
  • Wet iron filings
  • Gas syringe (to measure volume).
• Equation: 4Fe (s) + 3O₂ (g) + 6H₂O (l) → 4Fe(OH)₃ (s)
• Procedure:
  • Set up the apparatus shown, including adding iron filings to the flask.
  • Measure the initial and final volume in the gas syringe to calculate the volume of oxygen used up during the process.
• Calculations:
  • Total Initial Volume = sum of initial volumes of air in different components.
  • Volume of oxygen = Initial total volume - Final total volume
  • Percent of Oxygen = (Volume of oxygen / Total initial volume) x 100

Using Rusting of Iron - Key Points

• The total volume used in calculations includes the conical flask and connecting tube.
• Ensure the iron is in excess.
• The experiment usually takes at least a week.

Using Phosphorus - Practical Details

• Apparatus:
  • Evaporating basin
  • Bell jar
  • Trough (water)
  • Phosphorus sample
  • Measuring cylinder
• Equation: 4P(s) + 5O₂(g) → 2P₂O₅(s)
• Procedure:
  • Set up the apparatus and place phosphorus in the bell jar.
  • Measure and record initial and final water level to determine the volume of oxygen that has reacted.

Using Phosphorus - Key Points

• Phosphorus must be in excess to ensure all oxygen reacts.
• For safety, this experiment should be carried out by a teacher. Similar experiments can be performed by students using a candle, petri dish, and measuring cylinder.
• Values are typically between 18% and 24%, not 21%.

Combustion of Elements in Oxygen

• Some elements burn more brightly and react rapidly in pure oxygen than in air.
• Key elements include magnesium, sulfur, and hydrogen which form MgO, SO₂ and H₂O respectively

Thermal Decomposition

• Carbon dioxide can be formed from thermal decomposition of metal carbonates, such as copper carbonate or calcium carbonate.
• Metal carbonates decompose into metal oxide and carbon dioxide.
• Thermal decomposition is the process of breaking something down by heating.

Greenhouse Effect

• Carbon dioxide formed from burning fossil fuels contributes to the greenhouse effect, trapping heat in the atmosphere.

• The greenhouse effect is a natural process, but increasing CO₂ levels cause climate change.

• Impacts include melting polar ice, rising sea levels, and extreme weather.

Test your knowledge on the composition of air and the properties of gases found in the atmosphere with this quiz! Covering elements such as nitrogen, oxygen, and carbon dioxide, you'll explore their roles, reactions, and implications for climate change. Perfect for IGCSE students studying Inorganic Chemistry.