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Chemistry 30S. Unit 2: Gases and the Atmosphere. Learning Outcomes.

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chemistry 30s

Chemistry 30S

Unit 2: Gases and the Atmosphere

learning outcomes
Learning Outcomes
  • C11-2-01 Identify the abundances of the naturally occurring gases in the atmosphere and examine how these abundances have changed over geologic time. Include: oxygenation of Earth’s atmosphere, the role of biota in oxygenation, changes in carbon dioxide content over time
  • C11-2-02 Research Canadian and global initiatives to improve air quality.
present composition of the atmosphere
Present Composition of the Atmosphere
  • Earth’s air is composed of two different types of gases: permanent and variable.
  • Permanent Gases – amounts have not significantly changed in recent history
    • Nitrogen – 78.1%
    • Oxygen – 20.9%
    • Argon – 0.9%
    • Neon – 0.002%
    • Helium – 0.0005%
    • Krypton – 0.0001%
    • Hydrogen – 0.00005%
  • Variable Gases – amounts have shown significant variance in recent history
    • Water Vapour – 0 – 4%
    • Carbon Dioxide – 0.035%
    • Methane – 0.0002%
    • Ozone – 0.000004%
the origins of earth s atmosphere
The Origins of Earth’s Atmosphere
  • Scientists believe that before life began on the earth, the composition of the atmosphere was dramatically different than it is today.
    • Billions of years ago the atmosphere consisted mainly of helium, hydrogen, ammonia and methane. It is believed that little free oxygen existed.
  • Assuming that volcanic eruptions have the same composition as today, it is believed that volcanoes released gases into the developing atmosphere.
    • This volcanic outgassing released nitrogen, carbon dioxide and water vapour. Volcanic eruptions contain about 85% water vapour.
  • This water vapour accumulated in the atmosphere and eventually returned to the Earth in the form of rain.
    • The rain collected and created lakes, rivers and oceans.
    • Nitrogen gas is not very chemically reactive so it continued to accumulate in the atmosphere.
the origins of earth s atmosphere1
The Origins of Earth’s Atmosphere
  • Scientists believe that ultraviolet radiation from the sun penetrated the relatively dense atmosphere and sparked chemical reactions that eventually led to life on Earth.
  • Origin-of-life models have generally proposed that about 1 billion years after the first primitive organisms emerged, blue-green algae appeared on the Earth.
    • These algae converted the existing carbon dioxide and water to free oxygen gas and glucose through the process of photosynthesis.
      • These photo synthesizers were also responsible for helping to bind atmospheric hydrogen into carbonates and water.
the origins of earth s atmosphere2
The Origins of Earth’s Atmosphere
  • Another important source of oxygen was the photodecomposition of water vapour by ultraviolet light according to the equation below:

2 H2O(g) 2 H2(g) + O2(g)

  • As the amount of free oxygen increased, an ozone layer began to form filtering out ultraviolet radiation and allowing for the development of more complex species.
  • The nitrogen cycle maintains the present amount of nitrogen in the atmosphere.
    • Nitrogen is removed from the atmosphere by nitrogen-fixing bacteria and lightning.
    • The nitrogen is returned to the atmosphere through the decomposition of biological matter.
the origins of earth s atmosphere3
The Origins of Earth’s Atmosphere
  • Oxygen
    • Maintained by photosynthesis and photodecomposition reactions
  • Nitrogen
    • Maintained by the nitrogen cycle
the variable gases water vapour
The Variable Gases: Water Vapour
  • Water on the earth is present as solid, liquid and gas.
  • Water plays several roles in the atmosphere
    • Distributes heat
    • Provides fresh water for plant and animal life
    • Greenhouse gas
      • Traps heat energy within the atmosphere
  • Water vapour is present in largest amounts, about 4%, between the tropics and is in its lowest amounts, as low as 0%, in deserts and at the poles
the variable gases carbon dioxide
The Variable Gases: Carbon Dioxide
  • Removed from the atmosphere by photosynthesis
  • Returned to the atmosphere by respiration, decay of biological material, volcanic activity and burning fossil fuels
  • Some scientists believe that carbon dioxide is a greenhouse gas and increased amount of atmospheric CO2 is responsible for global warming
    • This is a topic of intense debate among scientists, environmentalists and politicians
  • Between 1840 and the year 2000, the average amount of atmospheric carbon dioxide steadily increased by 25%.
    • The increase in atmospheric CO2 is believed to be largely due to the increased burning of fossil fuels and deforestation.
the variable gases methane
The Variable Gases: Methane
  • Considered to be a greenhouse gas
  • Its levels have increased dramatically over the last 200 years due to the increased amounts of rice paddies, grazing animals and landfills.