8.5 Environmental Chemistry
8.5.1 Fossil fuels
Fossil fuels are formed by the action of heat and pressure on the remains of
plants and animals (on land or on the sea bed) over millions of years. The remains
were buried in sediments which excluded the air (kept out oxygen) and stopped
them decaying. More sediment buried the remains deeper and deeper until
pressure and heat eventually turned them into coal, oil and natural gas.
Complete combustion of fuels
Hydrocarbons are mainly used as fuels. The burning of a substance in air is
called combustion. In chemistry, combustion (or burning) means reacting with
oxygen. The main products of the combustion of hydrocarbons are carbon
dioxide, water and heat. The reaction is exothermic (gives out heat energy). The
following experiment can be used to confirm the products of complete
combustion of a hydrocarbon.
During the combustion of a candle (which contains hydrocarbons) the
thermometer increases in temperature showing the reaction is exothermic,
Water (which turns blue cobalt chloride paper pink, and white anhydrous copper
sulphate blue) condenses in the U-tube, while the limewater turns cloudy
indicating the presence of carbon dioxide.
In general for complete combustion:
hydrocarbon + oxygen
→
carbon dioxide + water
TOPIC 8.5 ENVIRONMENTAL CHEMISTRY 1
We are currently adding over 60 million
tonnes of carbon dioxide EVERY DAY
to the atmosphere. Carbon dioxide is
not poisonous, and is naturally present
in the atmosphere, however, millions of
tonnes of carbon dioxide going
constantly into the atmosphere is
causing global warming, resulting in
climate change all over the planet.
Carbon dioxide is called a
greenhouse gas. It is causing
the global temperature of the
Earth to rise. This rise in
temperature
affects
the
climate all over the planet in
ways which are hard to predict.
Some areas may become wetter
or dryer, other areas may
become hotter. The main
(predictable) directions of wind
and ocean currents on the
planet may slow down, speed up,
or change direction. This could have catastrophic consequences for life on
planet Earth including rising sea levels because of ice melting at the poles and
more extreme weather because of more convection in the hotter wetter
atmosphere.
Incomplete combustion
Incomplete combustion occurs when the supply of air or oxygen is poor. Water
is still produced, but carbon monoxide and carbon are produced instead of
carbon dioxide.
In general for incomplete combustion:
hydrocarbon + oxygen
→
carbon monoxide + carbon + water
TOPIC 8.5 ENVIRONMENTAL CHEMISTRY 2
Carbon monoxide is a poisonous gas, which is one reason why complete
combustion is preferred to incomplete combustion. Gas fires and boilers must
be serviced regularly to ensure they do not produce carbon monoxide. The
carbon is released as soot. These particles (sometimes called particulates)
contain mainly carbon atoms and come from unburnt hydrocarbons. The particles
are a pollutant and health hazard which cause breathing problems for people
who suffer from asthma.
In addition these particles have two effects on the temperature of the
environment. High up in the atmosphere they absorb some of the energy from
the Sun and prevent it from reaching the Earth's surface. This is called global
dimming and has a cooling effect on the climate. Scientists have evidence that
particles in the atmosphere have this effect. Big volcanoes erupting in the past
have put a lot of particles high up into the atmosphere and have had a global
cooling effect.
The particles also cause an increase in the rate at which water vapour
in the atmosphere condenses to form clouds. Clouds reflect some of the Sun's
energy back into space. An increase in the rate of cloud formation also has a
cooling effect. The cooling effect from particles in the atmosphere is, however,
small compared to the effect of global warming from burning fossil fuels.
Petrol and diesel engines also produce poisonous carbon monoxide and release
unburnt hydrocarbons. All of this pollution is produced at ground level, mainly in
cities where road vehicles gather. The obvious solution is to fit road vehicles
with electric motors rather than petrol or diesel engines. One day this will
happen.
In the meantime, exhausts are fitted with catalytic converters in an attempt to
minimise road vehicle pollution.
TOPIC 8.5 ENVIRONMENTAL CHEMISTRY 3
8.5.2 Air Pollution
Combustion of fossil fuels (coal, oil and natural gas) releases the energy which
has been stored up in the plant and animal remains for millions of years. Fossil
fuels are a finite resource (there are only a certain amount of them). Fossil
fuels are a non-renewable energy source. Once used, they are gone. We would
have to wait for millions of years for nature to replace our coal, oil and natural
gas reserves
Fossil fuels are burnt on a huge scale to generate electricity. Coal (and to a
lesser degree Oil and Natural Gas) contain sulphur impurities. When they are
burned in a power station the sulphur oxidises (reacts with oxygen) to form
sulphur dioxide gas.
sulphur + oxygen
sulphur dioxide
Sulphur dioxide gas is acidic, poisonous, and smells like burnt matches. Sulphur
dioxide can be removed from the burnt waste gases (called flue gases) in the
power station chimney by wet scrubbing. If sulphur dioxide gets into the
atmosphere it reacts with water and oxygen in the air to form a dilute solution
of sulphuric acid. This sulphuric acid is the main pollutant in acid rain. Natural
rain is slightly acidic due to dissolved carbon dioxide. Natural rain has a pH of
about 6, acid rain has a pH of 4 or even less.
Acid rain can kill trees and aquatic life. It runs into
rivers and gathers in lakes. Eventually, lakes become
too acidic, and plants and fish begin to die. Acid rain
reacts with limestone and damages limestone buildings.
Look at the statue below before and after acid rain
damaged it.
TOPIC 8.5 ENVIRONMENTAL CHEMISTRY 4
Powdered limestone or slaked lime can be added to soils or lakes to make them
less acidic. It would be better if we could avoid or reduce pollutant gas
emissions in the first place.
Another important pollutant in acid rain is nitric acid.
High temperature combustion (in petrol and diesel engines used in cars and
lorries)
produces toxic gases in addition to carbon dioxide. At around 1000oC nitrogen in
the air reacts with oxygen producing oxides of nitrogen.
nitrogen + oxygen
nitrogen monoxide
As well as being poisonous, nitrogen monoxide is a greenhouse gas.
It reacts with oxygen in the air to form nitrogen dioxide (NO2). NO and NO2
together are called NOx. It also reacts with water and oxygen in the air to form
dilute nitric acid. This acid dissolves in clouds and contributes to acid rain.
8.5.3 Renewable fuels
Renewable alternatives to fossil fuels to generate electricity include
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Bio-Fuel Plantations (bio-ethanol and bio-diesel)
Geothermal Power
Fuel Cells
Hydroelectric Power
Solar Cells
Wind Turbines
Tidal Power.
These energy sources are called renewable because they don't run out. They can
often produce cheaper electricity than burning fossil fuels, and they are nonpolluting. Not all of these energy sources are suited to all parts of the planet,
but in the absence of non-polluting energy yet to be discovered, they are the
power sources of the future.
TOPIC 8.5 ENVIRONMENTAL CHEMISTRY 5
TOPIC SUMMARY
Fossil fuels are formed from buried plant and animal remains which have been
subjected to intense heat and _________ for millions of years.
Hydrocarbons are compounds consisting only of the elements __________ and
_________. When they are burned in air ( a process called ___________ ) the main
products are __________ _________ and water; much ________ is also given off.
If the supply of oxygen is limited, ____________ combustion occurs, the products of
which are __________ _________ ( a ____________ gas ) and carbon can also be
produced.
These carbon particles can cause many problems: they are a pollutant which may cause
breathing problems for individuals suffering from _________; higher in the
atmosphere, they are able to _________ and deflect energy from the sun ( a
phenomenon known as global _____________ ) which has the effect of cooling the
earth. The particles also speed up the rate of cloud ______________ which also has a
cooling effect upon the planet’s surface.
Carbon dioxide is a ______________ gas which contributes to the overall increase in
the earth’s average temperature ( a phenomenon known as __________ warming ).
Fossil fuels ( coal, oil and __________ ______ ) provide us with much of our energy
requirements, but they are _____________________ - we are using them faster
than they can be created naturally.
When coal is burned, sulphur impurities are oxidised to form __________
___________. This gas reacts with water and oxygen in the atmosphere to form a
dilute solution of ______________ acid, the main component of acid rain.
TOPIC 8.5 ENVIRONMENTAL CHEMISTRY 6
High temperature combustion in vehicle engines produces oxides of ___________ ,
which are another major contributor to acid rain.
Alternatives to fossil fuels, such as ______________, __________ ________ and
_________ ____________ provide a cleaner, renewable resource which will become
ever more crucial as fossil fuels run out.
incomplete
biofuels
carbon
asthma
heat
greenhouse
nitrogen
natural gas
carbon monoxide
combustion
absorb
dimming
hydrogen
sulphur dioxide
global
sulphuric
non-renewable
solar cells
pressure
formation
wind turbines
carbon dioxide
toxic
TOPIC 8.5 ENVIRONMENTAL CHEMISTRY 7