Chemistry Matters for GCE ‘O’ Level [2
nd
Edition]: Full Solutions to Textbook Questions Chapter 20 © 2013 Marshall Cavendish International [Singapore] Private Limited 20.1
Chapter 20 The Atmosphere and Environment
Test Yourself 20.1 [page 391]
1. Clean air contains carbon dioxide, which dissolves in water to form carbonic acid. Hence, the pH of the water decreases. 2. [a] Droplets of a colourless liquid will be formed in the U-tube. [b] A white precipitate will be formed in the limewater. Conclusion: Air contains water vapour and carbon dioxide. 3. Nitrogen is distilled over first because it has the lowest boiling point.
Test Yourself 20.2 [page 396]
1. Hydrocarbons and oxides of nitrogen are found in the exhaust fumes of cars. Thus, their concentration levels increase when there are more vehicles on the road. Between 6 a.m. to 12 p.m., more vehicles are present on the road as people drive to work. Hence, there is a higher concentration of pollutants. 2. [a] 2SO
2
[g] + 2H
2
O[l] + O
2
[g]
!
2H
2
SO
4
[aq] [b] Acid rain reacts with metals and with carbonates in marble and limestone. Thus, metal bridges and stone buildings are corroded and damaged by acid rain. Acid rain also leaches essential nutrients from the soil and hence causes farmland to become less fertile.
Test Yourself 20.3–20.5 [page 406]
1. Carbon monoxide and oxides of nitrogen are converted into carbon dioxide and nitrogen; unburnt hydrocarbons are converted into carbon dioxide and water. 2. [a] The carbon cycle is the mechanism that maintains the level of carbon dioxide in the atmosphere. [b] Removal of carbon dioxide: Photosynthesis and ocean uptake Addition of carbon dioxide: Respiration, combustion of fuels or decay and bacterial decomposition [Any two] 3. [a] Water vapour and carbon dioxide [or methane, nitrous oxide] [b] Greenhouse gases trap heat, which warms the Earth’s surface. A rapid build-up of greenhouse gases or an excess of these gases in the atmosphere may lead to global warming.
Get it Right [page 408]
[a] False. Nitrogen is the most abundant gas in air. [b] False. Not all air pollutants are acidic in nature. For example, carbon monoxide is a neutral gas. [c] False. Acid rain contains nitric acid and sulfuric acid. [d] True [e] False. Calcium carbonate or calcium oxide is used in flue gas desulfurisation. [f] True [g] False. Chlorofluorocarbons [CFCs] react with ozone in the stratosphere and cause the depletion of the ozone layer. [h] True [i] True [j] True
Chemistry Matters for GCE ‘O’ Level [2
nd
Edition]: Full Solutions to Textbook Questions Chapter 20 © 2013 Marshall Cavendish International [Singapore] Private Limited 20.2
Let’s Review [pages 408–410] Section A: Multiple-Choice Questions
1. C 2. B 3. C
Section B: Structured Questions
1. [a] [i] Carbon monoxide, oxides of nitrogen and unburnt hydrocarbons [ii] Carbon dioxide, water and nitrogen [b] Advantage: Greenhouse gases keep the Earth warm enough to support life. Disadvantage: When human activities cause the greenhouse gases to build up rapidly in the atmosphere, more heat gets trapped in the atmosphere. The excess heat will have harmful effects on the environment like global warming. 2. [a] Acid rain is rainwater with a pH value that is less than 4. [b] [i] Sulfur dioxide and nitrogen dioxide [ii] They are acidic oxides. [iii] They irritate the eyes and lungs and cause breathing difficulties. [iv] The pollutants dissolve in water to form acidic solutions: 4NO
2
[g] + 2H
2
O[
l
] + O
2
[g]
!
4HNO
3
[aq] SO
2
[g] + H
2
O[
l
]
!
H
2
SO
3
[aq] 3. [a] Desulfurisation is the process of removing sulfur dioxide from flue gases. Flue gases are waste gases produced during the combustion of fossil fuels. [b] Calcium carbonate is used in the following reaction during desulfurisation: CaCO
3
[aq] + SO
2
[g]
!
CaSO
3
[s] + CO
2
[g] [c] Desulfurisation helps to reduce air pollution by removing sulfur dioxide from the air. 4. [a] [i]
X
is platinum or rhodium. [ii]
X
is used to catalyse the reactions in the catalytic converter. [b] [i] Carbon monoxide [CO], nitrogen monoxide [NO] or nitrogen dioxide [NO
2
] and unburnt hydrocarbons such as octane [ii] Oxygen [O
2
], carbon dioxide [CO
2
] and nitrogen [N
2
] [from the air] [c] Carbon dioxide [CO
2
], nitrogen [N
2
] and water vapour [H
2
- [d] [i] oxides of nitrogen + carbon monoxide
!
nitrogen + carbon dioxide unburnt hydrocarbons + oxygen
!
carbon dioxide + water vapour carbon monoxide + oxygen
!
carbon dioxide [Any two] [ii] Redox reactions occur in the catalytic converter. [e] The lead from leaded petrol will be deposited on the catalytic converter. This will poison the catalyst and reduce its efficiency.
Section C: Free-Response Questions
1. [a] Nitrogen oxides are produced when nitrogen and oxygen combine at high temperatures in the car engine. Carbon monoxide is produced in the car engine by the incomplete combustion of petrol. [b] When the car engine is idling, the supply of air is insufficient for the complete combustion of petrol. Hence, the concentrations of unburnt hydrocarbons and carbon monoxide are higher in the idling mode than in the accelerating mode. When the engine is accelerating, the temperature of the car engine increases. Hence, a higher concentration of nitrogen oxides is produced. [c] Fix a catalytic converter onto the car exhaust system. Oxides of nitrogen and carbon monoxide will be reduced to nitrogen and carbon dioxide respectively and unburnt hydrocarbons will be oxidised to carbon dioxide and water.