Beatty 18.1 Air WS2 - Carbon Cycle and Ozone Answers

1 Beatty Secondary School Science Department (Chemistry Unit) Chemistry 6092 Name: ___________________________________ ( ) Date: _______________ Class: 4E1 TOPIC: AIR – ATMOSPHERE & ENVIRONMENT (WORKSHEET 2) Learning Objectives: (a) Discuss the importance of the ozone layer and the problems involved with the depletion of ozone by reaction with chlorine -containing compounds, chlorofluorocarbons (CFCs). (b) Describe the carbon cycle in simple terms, to include (i) the processes of combustion, respiration and photosynthesis, (ii) how the carbon cycle regulates the amount of carbon dioxide in the atmosphere. (c) State that carbon dioxide and methane are greenhouse gases and may contribute to global warming, give the sources of these gases and discuss the possible consequences of an increase in global warming. Multiple-Choice Questions 1 Which statement about the ozone layer is true? A It is a pollutant in the stratosphere. B It absorbs some of the ultraviolet radiation from the Sun. C Excess greenhouse gases can cause its depletion. D It is destroyed when fluorine atoms react with the ozone molecules. ( B ) 2 In the past, CFC compounds were used as aerosol propellants. The structure of one CFC compound is shown. Which element in this compound can cause the depletion of ozone? A carbon B chlorine C fluorine D hydrogen ( B ) 3 Which substance is the main cause of the greenhouse effect? A carbon dioxide B chlorofluorocarbons (CFCs) C ozone D sulfur dioxide ( A ) 4 The global atmospheric concentration of carbo n dioxide has increased in the last 200 years. What could be causing this increase? 1 emissions from motor vehicles 2 photosynthesis 3 power stations using coal and oil A 1 and 2 only B 1 and 3 only C 2 and 3 only D 1, 2 and 3 ( B )

2 5 The diagram shows part of the carbon cycle. What are processes P, Q and R? P Q R A combustion photosynthesis respiration B photosynthesis combustion respiration C respiration combustion photosynthesis D respiration photosynthesis combustion ( D ) 6 Which row shows both the correct source and the correct effect of the named pollutant? pollutant source effect A carbon monoxide incomplete combustion of carbon-containing materials global warming B oxides of nitrogen decaying vegetable matter global warming C ozone photochemical reactions acid rain D sulfur dioxide volcanoes acid rain ( D ) 7 In wh at w ay do chlorof lurocarbons, methane and nitrogen dioxide a ffect the atmosphere and the environment? chloroflurocarbons methane nitrogen dioxide A acid rain depletion of the ozone layer greenhouse gas B depletion of the ozone layer acid rain global warming C depletion of the ozone layer greenhouse gas acid rain D global warming depletion of the ozone layer acid rain ( C ) Structured Questions 8 The Mars at mosphere contains small amounts of methane. Scientists think th is may be evidence of life. Methane on Earth is a pollutant gas. (a) What is the main source of methane in the Earth’s atmosphere? Bacterial decay of plant / vegetative and animal matter (b) What environmental problem does methane cause on Earth? Methane contributes to global warming.

3 9 Cars have cat alytic converters fitted to reduce problems caused by some of the exhaust gases. The diagram shows some of the gases that enter and leave a catalytic converter. (a) When catalytic converters were first available, some advertisements claimed they ‘solved all car pollution problems’. Explain why this is not true. Cars also produce sulfur dioxide which is not removed by catalytic converters. Sulfur dioxide causes acid rain. Carbon dioxide gas produced from the catalytic converter is a greenhouse gas which causes global warming. (b) What environmental problems do carbon monoxide and nitrogen oxides cause? Carbon monoxide combines with haemoglobin in the blood thereby preventing the transport of oxygen by the blood . This causes death of organisms. Nitrogen oxides r eact with rainwater and oxygen to form acid rain , which corrodes limestone buildings. 10 Hydrogen is a f uel that may be used on a large scale in the future for cars. Ethanol , C2H5OH, can also be used as a car fuel. In some countries, it is produced from the sugars in sugar cane. An environmentalist claims that ethanol as a fuel is ‘carbon neutral’ because u sing it does not add to the amount of carbon dioxide in the atmospher e. Explain why this is true. The combustion of ethanol releases carbon dioxide into the air. Sugar canes take in the carbon dioxide from the air through the process of photosynthesis. Thus the carbon is removed from the atmosphere. 11 Nitrogen monoxide damages the ozone layer by reacting with ozone is two steps. NO + O3 → NO2 + O2 step 1 NO2 + O3 → NO + 2O2 step 2 (a) Give one natural source of nitrogen oxides in the atmosphere. lightning activity (b) Use oxidation states to identify which element is oxidised in step 1. element nitrogen change in oxidation state +2 in NO to +4 in NO2

4 (c) One NO molecule can destroy thousands of ozone molecu les. Use equations for step 1 and 2 to explain why. Every o ne nitrogen monoxide molecule is able to break down two ozone molecules and regenerate another nitrogen monoxide molecule in the two step reaction. The newly generated nitrogen monoxide molecule is able to take part in the reaction again. Hence, a continuous cycle of nitrogen monoxide molecules is being regenerated and this can destroy thousands of ozone molecules 12 Chlorofluorocarbons, commonly known as CFCs, damage the ozone layer in the presence of sunlight. This is illustrated by trichlorofluoromethane, CCl3F, a component of CFC, in a three step reaction shown below. CCl3F → CCl2F + Cl step 1 Cl + O3 → ClO + O2 step 2 ClO + O → Cl + O2 step 3 (a) The table below shows the bond energies for some of the bonds in CFCs. bond bond energy in kJ/mol C−F 485 C−Cl 327 Use the table to explain why the ozone layer contains many more chlorine atom than fluorine atoms. Breaking a C-F bond (485 kJ/mol) requires more energy than breaking a C-Cl bond (327 kJ/mol). When energy from sunlight is absorbed by CFCs molecules, the probability of breaking C -Cl bond is therefore higher , r esulting in many more chlorine atoms than fluorine atoms. (b) State one source of chlorofluorocarbons. propellants in aerosols / coolant in refrigerators and air-conditioners (c) Write a balanced chemical equation for the overall conversio n from ozone to oxygen without reference to the three steps of equation. 2O3 → 3O2 (d) Explain the importance of the ozone layer. Ozone layer reduces / filters out / absorbs the amount of ultra -violet rays reaching the earth. This reduces the chances of cataracts, skin cancer a nd genetic mutation in humans.

5 (e) Ozone is a polluta nt at ground level beca use it produces photochemical smog which is harmful to health. State how photochemical smog is formed. It is formed fr om the reaction of nitrogen oxides with unburnt hydrocarbons in the presence of sunlight. (f) One CFC mo lecule containing CC l3F can destroy thousands of ozone molecules. Use the equations to explain why. One molecule of CCl3F produces one Cl atom that can destroy one O 3 molecule. However, as shown in step 3, the Cl atom is regenerated. Hence, the Cl atom can continue to react with another O3 molecule as shown in step 2 . Thus, there is a continuous cycle in which Cl atoms are used up and regenerated. 13 The diagram shows some of the processes that happen in the carbon cycle. (a) Name process X. complete combusti