Chapter 13

Air & Atmosphere

Joseph Priestley • Conducted experiments and

observations of different kinds of air

• First person to isolate oxygen• Made the first carbonated soft

drink!

What does air consist of? Air is a mixture of several gases.

As air is a mixture, its composition varies from time to time and from place to place.

Gas Composition by volume

Nitrogen 79%

Oxygen 20%

Noble gases (mainly argon)

&carbon dioxide

1%

Separation of Air into its Components

Air is first cooled and compressed into liquid.

Liquid air is then separated into its constituents (or fractions) by fractional distillation.

Common air pollutants & its effects

Air Pollution

Air Pollution

Air pollution is the condition in which air contains a high concentration of certain chemicals that may harm living things or damage non-living things.

Air pollution is caused by solid particles and poisonous gases in the air.

These air pollutants include carbon monoxide, oxides of nitrogen and sulphur dioxide.

Click on the video on Air Pollution.

Pollutants Major Sources Harmful Effects(a) Carbon monoxide

Incomplete combustion of carbon-containing substances eg. charcoal, wood, petrol, etc.

- Carbon monoxide reacts with haemoglobin in the blood to form carboxyhaemoglobin so the haemoglobin cannot

transport blood to the rest of the body.

- Cause headaches, fatigue breathing difficulties or even death

Pollutants Major Sources Harmful Effects(b) Oxides of nitrogen(eg. nitrogen monoxide, NO & nitrogen dioxide, NO2.

1) Reactions that occur in car engines (internal combustion engines)

2) From lightning

Nitrogen reacts with oxygen in the air to form nitrogen monoxide.N2 (g) + O2 (g) 2NO (g)

Nitrogen monoxide reacts with oxygen to form nitrogen dioxide (NO2).2NO (g) + O2 (g) 2NO2 (g)

1) Eye irritation2) Breathingdifficulties3) Lung inflammation (bronchitis)4) Acid rain

Pollutants Major Sources Harmful Effects(c) Sulfur dioxide

1) combustion of sulfur containing fossil fuels (especially coal)

2) volcano eruptionsSulfur reacts with oxygen

in the air to form sulfur dioxide.

S (g) + O2 (g) 2SO2 (g)

1) Eye irritation2) Breathingdifficulties3) Lung inflammation (bronchitis)4) Acid rain

Other air pollutants include - unburnt hydrocarbons- methane - ozone

Other Pollutants

Unburnt hydrocarbons are released in car exhaust.

Methane is a colourless and odourless gas produced when plant and animal matter decay.

It is also produced from the decay of rubbish in landfills.

Methane is a greenhouse gas.

Acid Rain

Effects of Air Pollution

• Rain usually has a pH that is slightly below 7.

• Why?

Carbon dioxide in the air dissolves in rainwater to form carbonic acid, which is a weak acid.

• However, acid rain has a pH value of 4 or less.

Click on the animation on Acid Rain

Acid Rain

• Acid rain is formed when acidic air pollutants such as sulfur dioxide (SO2) and nitrogen dioxide (NO2) dissolve in rainwater.

• Sulfur dioxide dissolves in water to form sulphurous acid (H2SO3).

Chemical equation: SO2(g) + H2O(l) H2SO3(aq)

In the presence of oxygen in the air, acid is slowly oxidised to sulphuric acid (H2SO4).

• In the presence of oxygen and water, nitrogen dioxide is converted to nitric acid.

4NO2(g) + 2H2O(l) + O2(g) 4HNO3(aq)

Effects of acid rain

• Corrodes metal structures such as fences and bridges

• Reacts with carbonates in marble and limestone.

• Reduces pH value of natural water bodies to below 4, which will kill fish and other aquatic life.

• Leaches important nutrients from the soil and destroys plants.

Reducing effects of Acid Rain

Catalytic Converters

• Attached to the exhaust system of a car.

• Contains catalysts - platinum and rhodium.

Catalytic converters involve the use of redox reaction:

1. Carbon monoxide is oxidised to carbon dioxide. Oxides

of nitrogen are reduced to nitrogen.

Eg. Nitrogen monoxide + carbon monoxide nitrogen + carbon dioxide

2 NO (g) + 2 CO (g) N2 (g) + 2CO2 (g)

2. Unburnt hydrocarbons are oxidised to carbon dioxide

and water.

Eg. Octane + oxygen carbon dioxide + water vapour

2 C8H18 (g) + 25 O2 16 CO2 (g) + 18 H2O (g)

Catalytic Converters

In the long run…

• The best way to control air pollution is to …

burn less fossil fuels.

2 ways of minimising the effects of sulfur dioxide pollution:

Flue Gas Desulfurisation

1. Remove sulfur from fossil fuels before burning. However, this method is too expensive and difficult.

2. Remove sulfur dioxide from the waste gases (flue gases) from burning.- Process known as desulfurisation

What Happens in Desulfurisation?

Sulfur dioxide reacts with an aqueous suspension of calcium carbonate to form solid calcium sulfite.

CaCO3(s) + SO2(g) CaSO3(s) + CO2(g)

The calcium sulfite is further oxidised to calcium sulfate by atmospheric oxygen.

CaSO3(s) + O2(g) 2CaSO4(s)

Control of Air Pollution in Singapore

• Prohibition of the use of open fires for the disposal

of domestic and industrial wastes

• Introduction of unleaded petrol and phasing out of

leaded petrol

• Reduction of the permissible level of sulfur in diesel

• Fitting of all petrol-driven vehicles with catalytic

converters

Depletion of the Ozone Layer

Is our ozone layer being depleted?

What is ozone?Ozone is a bluish, very reactive gas, whose molecule is made by three oxygen atoms .

At low altitudes, it can cause breathing difficulties and photochemical smog.

However, in the stratosphere, the ozone layer shields us from ultraviolet radiation from the sun.

Note: UV radiation can cause skin cancer, genetic mutations and cataracts.

Click on the video on Ozone.

What is causing the depletion of ozone in the stratosphere?

Chlorofluorocarbons (CFCs).

- compounds containing the elements carbon, fluorine and chlorine.

- widely used as propellants for aerosols and as coolants in refrigerators and air conditioners.

The Carbon Cycle

Click on the video on Carbon cycle

The mechanism that maintains the level of carbon dioxide in the atmosphere is called the carbon cycle.

What processes produce carbon dioxide?

1. Respiration During respiration, carbon and hydrogen compounds in food are converted into carbon dioxide and water.

C6H12O6(aq) + 6O2(g) 6CO2(g) + 6H2O(l) + energy

2. Combustion of fuelsMost fuels contain both carbon and hydrogen. When these fuels are burnt, carbon dioxide and water are produced.

CH4(g) + 2O2(g) CO2(g) + 2H2O(l)Heat energy is also given off.

1. Photosynthesis During photosynthesis, green plants convert carbon dioxide and water into glucose and oxygen in the presence of sunlight.

6CO2(g) + 6H2O(l) C6H12O6(aq) + 6O2(g)

What processes remove carbon dioxide?

What are the other types of greenhouse gases?Methane (CH4) and nitrous oxide (N2O)

The Greenhouse EffectThe natural greenhouse effect is crucial for maintaining the proper temperature needed to sustain life on Earth.

Without these greenhouse gases, Earth’s surface temperature would be –40 °C and it would be permanently covered with ice.

Greenhouse gases, such as carbon dioxide, are building up rapidly in the atmosphere.

Why?Human activities like the burning of fossil fuels and large scale cutting down of forests

Global Warming

• Therefore, carbon dioxide is being added to the atmosphere at a higher rate than photosynthesis can remove the excess gas.

• The effect of carbon dioxide build-up is an increase in the Earth’s average temperature. global warming

Global Warming

• a decrease in crop yields world-wide because the areas that are currently covered by vegetation may become deserts,

• the melting of large quantities of ice in the North Pole and South Pole. This will cause the levels of oceans to rise and flood low-lying countries such as the Netherlands.

• the rapid evaporation of water from the Earth’s surface. When this happens, carbon dioxide dissolved in the oceans will be driven out into the atmosphere. This adds further to the greenhouse effect.

Consequences of Global Warming

Click on the video on global warming.