(O Level Chem) Atmosphere Teaching & Learning Notes

1. The atmosphere is a layer of air which is a mixture of several gases comprising mainly of nitrogen and oxygen. Suppose that in an experiment, a candle is burned in an enclosed 70.5 ml sample of dry air. The candle wax undergoes complete combustion with oxygen to give carbon dioxide and water. After the candle flame is extinguished, the air is shaken with sodium hydroxide solution to absorb the carbon dioxide produced. The volume of the air is found to be 55 ml. This shows that oxygen occupies approximately 21% in the air. In fact, the typical composition by volume of dry air is 78% nitrogen, 21% oxygen and 1% noble gases and carbon dioxide. Because the air is a mixture of gases of different boiling points, it can separated by fractional distillation after it is liquefied.

2. Oxygen exists as a diatomic gas, meaning that its molecules contains two atoms each. Oxygen is reactive and it is a strong oxidizing agent. It supports combustion. For example, oxygen oxidizes hydrocarbons such as methane to produce carbon dioxide and water. The burning of fuels is useful because it is exothermic. Oxygen is also responsible for the corrosion of metals such as the rusting of iron. In living things, oxygen is important for respiration which produces energy from food.

3. Nitrogen also exists as a diatomic gas but, unlike oxygen, it is chemically unreactive. This is because a large amount of energy is required to break its strong N≡N triple covalent bonds. Since nitrogen gas is unreactive, it is used in packing food and in providing an inert atmosphere to prevent oxidation of oil. Under extremely high temperature such as that during lightning and in combustion engines, however, nitrogen can undergo combustion with oxygen to form oxides of nitrogen. Note that nitrogen is important to living things to make proteins. The Haber Process is used to manufacture ammonia which is in turn used to make fertilizers. Plants need nitrogenous fertilizers to make proteins.

4. Only 0.03% of clean air comprises of carbon dioxide. This level of carbon dioxide is kept constant by the carbon cycle which includes photosynthesis, combustion, respiration and fermentation. Only during photosynthesis, carbon dioxide in the air is removed by plants and algae in the presence of sunlight to make food. Combustion, respiration and fermentation add carbon dioxide into the air. The rate of removing carbon dioxide from the air had been balanced by the rate of adding carbon dioxide into the air. We have, unfortunately, upset this balance as we increase the amount of carbon dioxide in the air. 

5. Polluted air contains air pollutants such as carbon monoxide, sulfur dioxide, oxides of nitrogen, methane, unburned hydrocarbons and ozone. Air pollutants are substances in the atmosphere that are harmful to living things and to the environment. Where do they come from and what consequences do they have?

6. Carbon monoxide, a colorless, odorless and poisonous gas comes from the incomplete combustion of fossil fuels in motor vehicles and fuel power stations. Carbon monoxide is poisonous because if it is inhaled, it combines with the haemoglobin in our red blood cells preventing haemoglobin from absorbing oxygen.

7. Sulfur dioxide is formed during the combustion of fuels such as coal which contain sulfur impurities. It is also given off by volcanic eruptions. Being an acidic gas, sulfur dioxide dissolves in rain water to form acid rain. Rain of pH less than 5.0 can kill plants and aquatic lives, and corrode limestone buildings. Sulfur dioxide gas is also a lung and eye irritant.

8. Oxides of nitrogen including nitrogen monoxide and nitrogen dioxide are formed when nitrogen is oxidized at high temperatures. Lightning activity and combustion engines provide high temperature for the oxidation of nitrogen to occur. Oxides of nitrogen damage the lungs while nitrogen dioxide causes acid rain.

9. Methane comes from natural gas, rice fields and cattle. The natural gas which contains mainly methane is a gaseous fossil fuel that is found together with crude oil. Cattle including cows, goats and sheep produce methane when bacteria in their stomachs digest food. Rice fields also give out methane that is produced from the decomposition of organic matter. Methane is one of the greenhouse gases that lead to global warming.   

10. Unburned hydrocarbons come from the incomplete combustion of hydrocarbon fuels in combustion engines, while ozone is created by the reaction between nitrogen oxides, carbon monoxide and unburned hydrocarbons in the presence of ultra-violet radiation from the Sun. Ozone and unburned hydrocarbons react together to form photochemical smog that is harmful to health.

11. Ozone O₃ is usually not present at sea level. It is, however, present as an ozone layer which is 10 km above us. This ozone layer absorbs ultraviolet radiation from the Sun and therefore protects us from this harmful radiation. Over exposure to ultraviolet radiation can cause skin cancer. Unfortunately, the ozone layer has been depleting creating a so called “ozone hole” allowing more ultraviolet radiation through. The reason for its depletion traces back to our use of aerosol sprays. We have been releasing compounds called chlorofluorocarbons (CFC) into the atmosphere when we use aerosol sprays. Over the years, these CFC compounds rise up to the ozone layer. At the ozone layer, the CFC molecules absorb ultraviolet radiation and release chlorine atoms which react with ozone.

12. We have mentioned that methane is a greenhouse gas which is responsible for global warming. Greenhouse gases are gases that trap heat from the Sun causing air temperature to increase. Another important greenhouse gas is carbon dioxide. Some consequences of global warming include melting of polar icebergs and ice caps, climatic change, floods and droughts.

13. How can we reduce the emission of atmospheric pollutants? All motor vehicles should be installed with catalytic converters which convert harmful gases to less harmful gases before they are released into the air. Catalytic converters make use of transition metals, rhodium, platinum and palladium, to speed up redox reactions. In the converters, oxides of nitrogen are reduced to nitrogen and oxygen, carbon monoxide is oxidized to carbon dioxide, and hydrocarbons are oxidized to carbon dioxide and water.

14. Fossil fuel power stations also reduce emissions of air pollutants such as sulfur dioxide. Flue-gas desulfurization refers to the technologies that remove sulfur dioxide from exhaust gases. Since sulfur dioxide is acidic, limestone, lime or any alkaline substance can be used to remove sulfur dioxide from the gases before they are released.

15. We have mentioned that acidic pollutants such as nitrogen dioxide and sulfur dioxide cause acid rain. Acid rain decreases the pH of soils and water bodies such as rivers and lakes and kills plants and aquatic lives. To neutralize acidic soil and water bodies, powdered limestone can be added.