Chemistry 30S

1. Chemistry 30S Unit 2: Gases and the Atmosphere

2. 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.

3. 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%

4. 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.

5. 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.

6. 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.

7. The Origins of Earth’s Atmosphere • IN SUMMARY: • Oxygen • Maintained by photosynthesis and photodecomposition reactions • Nitrogen • Maintained by the nitrogen cycle

8. 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

9. 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.

10. 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.