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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.