21 — maintaining air quality

Question 1

Volume composition of gases in dry air

Answer 1

Nitrogen: appx 78%
O2: appx 21%
CO2: appx 0.04%
Noble gases (mainly argon): appx 0.96%

Question 2

Common pollutants

Answer 2

CO
nitrogen oxides (NO, NO2)
Sulfur dioxide
Methane
Unburnt hydrocarbons
Ozone

Question 3

Carbon monoxide property, sources and harmful effects (PSH)

Answer 3

P: a toxic, colourless and odourless gas

S: incomplete combustion of carbon-based fuels such as petrol in car engines

Harmful effects:
CO is toxic and binds irreversibly with haemoglobin in RBCs, reducing the ability of haemoglobin to transport O2 to the rest of the body, may result in death

Question 4

Nitrogen oxides property, sources and harmful effects (PSH)

Answer 4

P:
NO -> colourless, odourless
NO2 -> red-brown, pungent smell

S:
1. Lightning activity
2. Internal combustion engines of vehicles (O2 and N2 reacts at high tempts)

H:
1. SO2 and nitrogen oxides react w oxygen in air to form acidic compounds, which dissolve in rain water, forming acid rain that corrodes limestone buildings/harms aquatic life and plants/reduced crop yield
2. NO can be regenerated at the end of step2 [1] and react with more ozone [1], acting as a catalyst which breaks down many ozone molecules. More UV rays can reach the earth to cause skin cancer.

Question 5

Sulfur dioxide property, sources and harmful effects (PSH)

Answer 5

P: colourless gas w pungent smell

S:
1. Volcanic eruptions
2. Combustion of fossil fuels

H:
1. Causes respiratory difficulties
2. SO2 and nitrogen oxides react w oxygen in air to form acidic compounds, which dissolve in rain water, forming acid rain

Question 6

Methane property, sources and harmful effects (PSH)

Answer 6

P: highly flammable colourless and odourless gas

S:
1. Anaerobic bacteria decay of organic substances
2. Waste gases from cattle
3. Leakage from methane reservoirs under the arctic tundra and from oceanic methane ice.

H:
1. CH4 is a major greenhouse gas that leads to global warming

Question 7

Unburnt hydrocarbons property, sources and harmful effects (PSH)

Answer 7

P: Colourless and odourless gas; may be pungent due to fuel additives

S: vehicle combustion engines

H:
1. It causes eye and respiratory tract irritation
2. It reacts w nitrogen oxides in the presence of sunlight to form photochemical smog which appears as brown haze. Formation of smog also produces O2, SO2 and NO2

Question 8

Ozone property, sources and harmful effects (PSH)

Answer 8

P: pale blue gas w pungent smell

S:
1. Lightning activity
2. Reactions betw O2 and sunlight in the upper atmosphere
3. Reactions betw Unburnt hydrocarbons and nitrogen oxides in the presence of sunlight in the lower atmosphere

H:
1. Causes eye and respiratory tract irritation, leading to respiratory difficulties
2. When inhaled, causes chest pains and headaches
3. Ozone slows down photosynthesis in plants, which can dmg crops.

Question 9

Solutions to problems arising from air pollutants

Answer 9

1. Catalytic converters
2. Flue gas desulfurisation

Question 10

Redox reactions in catalytic converters

Answer 10

Removal:
1. CO is oxidised to CO2
2CO (g) + O2 (g) ->. 2CO2 (g)

2. Nitrogen oxides r reduced to nitrogen
   2NO (g) + 2CO (g) -> N2(g) + CO2(g)
   2NO2(g). -> N2 (g) + 2O2(g)
3. Unburnt hydrocarbons r oxidised to CO2 and H2O
   2C8H18 (g) + 25O2 (g) -> 16CO2(g) + 18H2O (g)

CO is oxidised to CO2
Oxides of nitrogen are reduced to nitrogen
Unburied hydrocarbons are oxidised to CO2 and water

Question 11

Flue gas desulfurisation

Answer 11

Fossil fuels have significant Sulfur content. Combustion of fuels produce SO2, known as flue gas.

*- can remove a significant proportion of SO2 from the flue gas before it is released into the atmosphere. In wet scrubbing, calcium carbonate slurry (limestone + water) can remove SO2 from flue gas, reducing the effect of acid rain.

*- Challenge:
1.Set-up requires large amount of space and can be quite costly to run.
2. Using low sulfur fuels will go a long way in reducing amt of Sulfur dioxide in atmosphere -> Solution: Removing excess sulfur from fossil fuels by treating it w H2 gas, producing hydrogen sulfate (H2S) as by-product

CaCO3 is basic and reacts w SO2 which is acidic

Question 12

Redox reactions in flue gas desulfurisation

Answer 12

1. Calcium carbonate reacts w SO2 to form calcium sulfite and CO2
   CaCO3 (s) + SO2 (g) -> CaSO3 (s) + CO2 (g)
2. Calcium sulfite is further oxidised to form calcium sulfate
   2CaCO3 (s) + O2 (g) -> 2CaSO4 (s)
3. Calcium sulfate can then be hydrated to form hydrated calcium sulfate aka gypsum
   CaSO4 (s) + 2H2O (l) -> 2CaSO4.2H2O (s)

CaCO3 is basic and reacts with SO2 which is acidic.

Question 13

Acid rain

Answer 13

pH around 4.0; Formed when acidic pollutants eg sulfur dioxide and nitrogen dioxide dissolve in rainwater

1. Sulfur dioxide dissolves in water to form sulfurous acid
   SO2 (g) + H2O (l) -> H2SO3 (aq)
   In presence of oxygen in air, sulfurous acid is slowly oxidised to nitric acid
2. In presence of O2 and H2O, nitrogen dioxide is converted to nitric acid
   4NO2 (g) + 2H2O (l) + O2 (g) -> 4HNO3 (aq)

Question 14

Effects of acid rain

Answer 14

1. Leaches essential nutrients like Mg and Ca from the soil and causes trees/plants to wither and die
2. Lowers pH of soil/water bodies, which may be too acidic for many plant species/aquatic life to survive, hence killing them
3. Corrodes buildings and structures made from limestone, marble and metals

Question 15

Benefits of ozone

Answer 15

Pollutant at ground level, beneficial at stratosphere.

1. Absorb UV radiation, thus reducing amt of UV radiation that reaches the Earth’s surface
   - Ozone has a pale blue colour + pungent smell.
   - O2 molecules interact w UV radiation from sun to form ozone molecules. They break up to reform oxygen when they absorb UV radiation.

Question 16

Carbon cycle

Answer 16

Carbon cycle describes a set of processes that regulates the amount of carbon dioxide in the atmosphere. The carbon cycle describes how carbon-containing compounds r converted from 1 form to another. The main processes which produce CO2 are respiration, combustion and decomposition. The main processes which removes CO2 are photosynthesis and ocean uptake.

Question 17

Main processes that produce CO2 in carbon cycle

Answer 17

1. Respiration
   Energy is released during respiration thus process is exothermic. Respiration converts glucose in food we eat into CO2 and H2O

C6H12O6 + 6O2 -> 6CO2 + 6H2O

2. Combustion
   An exothermic reaction occurs when fossil fuels r burnt in the presence of oxygen to release energy. Carbon dioxide and water vapour r byproducts
3. Decomposition
   Aerobic decomposition (in presence of O2) releases CO2 in the atmosphere

Question 18

Main processes in carbon cycle which removes CO2

Answer 18

1. Photosynthesis
   Plants help to remove CO2 from the atmosphere thru photosynthesis. Energy is absorbed during photosynthesis in the endothermic reaction
   6CO2 + 6H2O -> C6H12O6 + 6O2
2. Ocean uptake
   Oceans and other large bodies of water absorb CO2. Dissolved CO2 is used by marine plants to photosynthesise and convert to carbonic acid and calcium carbonate that makes up the shells of marine organisms. When organisms die, they accumulate on seabed.

Question 19

Greenhouse gases and their major sources

Answer 19

methane is a stronger GHG than CO2

Sources of GHG:
CO2:
1. Combustion of fossil fuels in power plants, industries and vehicles
2. Deforestation
Why cut down CO2 emissions -> CO2 is a greenhouse gas which contributes to global warming. [1]

Methane:
1. Waste gas from cattle
2. Leakage from methane reservoirs under the arctic tundra and from oceanic methane ice.

Question 20

Global warming

Answer 20

Global warming is the increase in the average temperature of the Earth’s surface due to the increasing amount of greenhouse gases in the atmosphere.

Question 21

Climate change phenomenas

Answer 21

1. Desertification of fertile land
2. More frequent and severe heat waves
3. Increase in frequency and intensity of extreme weather
4. Ocean warming and acidification
5. Glacial retreat and melting of polar ice caps

Question 22

Desertification of fertile land Effects

Answer 22

1. Change in rainfall pattern results in desertification in areas receiving too little rain, or flooding in areas receiving too much rain
2. Leads to lower crop Imelda and thus the amt of food that can be reproduced globally would decrease

Question 23

More frequent and severe heat waves Effect

Answer 23

1. Some parts of the world become too hot for humans to live in. High tempts can be fatal for frail nd elderly
2. Higher global tempts lead to wildfires which could destroy whole ecosystem

Question 24

Increase in frequency and intensity of extreme weather effect

Answer 24

1. Tropical storms occur more
2. Property is destroyed and lives may be lost
3. Cyclonic storms can permanently decrease the fertility of farmland near the coast due to saltwater deposition

Question 25

Ocean warming and acidification Effect

Answer 25

1. Coral reefs r bleached, marine biodiversity decreases w habitats lost
2. Commercially-impt fish populations depleted
3. Acidified waters dissolve shells of crustaceans

Question 26

Glacial retreat and melting of polar ice caps

Answer 26

1. Glacier-fed river levels would change
2. Sea levels would rise and permanently flood coastal areas
3. Large amt of fresh water disrupt ocean currents which distribute heat around the globe and stabilise global climate. Such disruption would amplify effects of climate change

Question 27

Characteristics of SO2

Answer 27

It is an acidic oxide. It is a pollutant which dissolved in rainwater to form sulfurous acid, causing acid rain which leads to the corrosion of limestone buildings and metal bridges/reduced crop yield/harms aquatic life and plants [1]
SO2 is also removed as it causes respiratory difficulties. [1]
It is released from volcanoes
It is an Oxidising agent which changes purple acidified KMnO4 solution to colourless.

Question 28

Equation to form nitric acid from NO2

Answer 28

4 NO2 + 2H2O + O2 —> 4HNO3

Question 29

The DEF tank can cause an explosion when too hot. Explain why [1]

Answer 29

When temperature of the tank is high, 3 moles of gases are produced which increases the number of gas particles per unit volume hence increasing the pressure built up in the tank, causing an explosion.

Question 30

Benefits of using DEF to treat exhaust gases [2]

Answer 30

Reduces nitrogen monoxide to nitrogen, preventing nitrogen monoxide from being released into the environment which can cause respiratory problems.

Nitrogen monoxide can be reduced to nitrogen, preventing nitrogen dioxide from dissolving in rainwater to cause acid rain.

Ammonia which is highly soluble in water and may leech into water bodies causing an increase in the pH of water, harming aquatic plants and life, is removed.

Formation of nitrogen and water as products do not pollute the environment. [1]

Question 31

Explain why CO is collected and burned.

Answer 31

CO combines irreversibly with haemoglobin in blood to reduce oxygen carrying capacity of haemoglobin, causing death. Burning CO converts CO to less harmful CO2.

Question 32

Give reasons why it is important to use scrap steel to make new steel

Answer 32

Reduced the need for landfills required to dispose waste from metal extraction

Reduce energy required to extract metals from their ores hence reducing the burning of fossil fuels that provides energy for extraction

Enables the conservation of resources as metal ores are finite and non-renewable resources.

Question 33

Why winter blend petrol is not sold in tropical climate countries

Answer 33

Lower flashpoints of substances in winter blend causes particles to gain heat energy to overcome the forces of attraction holding them together, turning petrol into gaseous state.

Question 34

Why CFCs are considered to be harmful to the environment. [2]

Answer 34

CFCs break down in the presence of UV light to produce chlorine atoms, which decompose ozone into oxygen. [1]
Cl atoms are regenerated, reacts with MORE OZONE, catalysing the breakdown of ozone.
The decomposition of ozone causes the ozone layer to be depleted and less effective in filtering UV light. More exposure to harmful UV radiation can cause SKIN CANCER/GENETIC MUTATION/EYE DAMAGE/CATARACTS

Question 35

Explain how the processes of photosynthesis and respiration helps to regulate the amount of CO2 in the atmosphere. (1)

Answer 35

Photosynthesis involves taking in of CO2 while respiration involves release of CO2.
Thus, there is no net change in the amount of CO2

Question 36

Suggest why planting more trees is not a long term solution to the increase in the amount of CO2 [2]

Answer 36

In the long run, trees will need to be cut down to clear land for development. In the process of cutting down trees, they will be left to decay, contributing CO2 to the atmosphere.
OR
Trees take years to grow thus absorption of CO2 thru photosynthesis is much slower than the release of CO2 to the atmosphere from exhaust fumes of motor vehicles.

Question 37

Why recycling is important [1]

Answer 37

Recycling conserve finite metal ores and reduces the need for landfills.

Question 38

Explain why the CO level decreases and nitrogen oxides level increases with increased air: fuel ratio [4]

Answer 38

As air: fuel ratio increases, more oxygen present to increase rate of complete combustion. Lesser CO produced due to incomplete combustion of fossil fuels.

As air: fuel ratio increases, more complete combustion of fuels occurs. Since the reaction is exothermic, thermal energy is released, increasing the temperature of combustion engine which allows more N2 and O2 molecules to react together to form nitrogen oxides.

As amount of air increases, carbon monoxide will react with more oxygen to form
carbon dioxide [1].
Lesser CO present to react with NO [1] to form N2 gas and hence lesser NO will be successfully
removed from catalytic converter.

Question 39

For millions of years the concentration of CO2 has been constant. Explain why. Refer to processes A and B. [2]

Answer 39

Phytoplankton absorbs CO2 from the air in process A and CO2 from air is absorbed by ea water in process B. [1]
CO2 is also released back to the atmosphere in process B. [1]

Question 40

Explain why plant life changed the percentages of oxygen and CO2 on earth [2]

Answer 40

Plant takes in CO2 for photosynthesis and gives off oxygen. Hence plant life increases increases the percentage of oxygen and decreases the percentage of CO2.

Question 41

Explain in terms of processes involved, why the percentage of CO2 in Earth is now increasing rapidly. [1]

Answer 41

The percentage of CO2 increases due to the increased combustion of fossil fuels from rapid development of economies and industrialisation and lesser photosynthesis due to the large-scale deforestation.

Question 42

Explain whether the processes in the flowchart either follow or do not follow each of these principles. [3]
1. Using minimum amounts of energy
2. Using raw materials which are renewable or use waste from other processes
3. Making no harmful waste products

Answer 42

Does not follow 1st principle as it involves large amount of energy to heat to a high temperature of 1000dgC [1]

Ammonium sulfate used in the manufacture of SO2 was obtained as a waste product from the manufacture of other chemicals [1]

Both processes do not produce harmful waste product as process 1 produces water and process 2 produces water and nitrogen, all of which are not harmful substances [1]

Question 43

Catalytic converters

Answer 43

Contain a coating of catalysts made up of platinum, palladium and rhodium that speed up the conversion of harmful substances in the vehicle exhaust into less harmful substances. The catalysts are arranged in a honeycomb structure that maximise the surface area for gases to interact with.

Question 44

State how nitrogen monoxide, NO is formed in combustion engines of vehicles. [2]

Answer 44

Oxides of nitrogen are formed at high temperatures (✓) when large amount of
energy (✓) is absorbed to break the N-N triple bonds (✓) in N2.

Question 45

Hence, explain with the aid of a chemical equation how nitrogen monoxide is
removed by catalytic converters fitted in cars. [2]

Answer 45

Oxides of nitrogen react with carbon monoxide to form nitrogen gas and
carbon dioxide. [1]
2NO + 2CO → N2 + 2CO2 [1]