Yr9 Energy from fuels and incomplete combustion
Combustion is the rapid oxidation of a substance accompanied by a
flame. One example is the combustion of magnesium.
Q1. Write the word equation for the reaction between magnesium and
oxygen.
Magnesium + oxygen  magnesium oxide
A hydrocarbon is a compound made up only of carbon and hydrogen.
One example is methane, CH4. When hydrocarbons undergo complete
combustion they produce carbon dioxide and water. As long as the fuel
is volatile (forms gas easily) and there is a good supply of oxygen,
complete combustion will occur.
The following reaction represents the complete combustion of methane:
CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(g) + energy
Q2. Write the word equation for the complete combustion of ethane,
C2H6.
Extension- write the chemical equation for this reaction.
Ethane + oxygen  carbon dioxide + water
2C2H6(g) + 7O2(g)  4CO2(g) + 6H2O(l)
Many fuels such as petrol and natural gas are hydrocarbons.
Hydrocarbons with small molecules make better fuels than
hydrocarbons with large molecules. This is because small hydrocarbon
molecules:
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are more volatile - turn into a gas more easily
are less viscous - they flow more easily
are more easily ignited
burn with a cleaner, less sooty flame
Complete and incomplete combustion
Incomplete combustion is what occurs when there is not a good supply
of oxygen OR if the fuel molecules are too big to vaporise easily. If there
is not enough oxygen available for all the carbon to turn into carbon
dioxide (complete combustion), then some or all of the carbon turns to
carbon monoxide or soot. This happens with any hydrocarbon - we shall
take methane as an example. During incomplete combustion methane
gas burns with a yellow flame. (unlike the clear blue flame seen in
complete combustion). Carbon particles (sooty marks) may also be
seen.
methane + oxygen
4CH4(g) + 5O2(g)
carbon monoxide + soot + water.
2CO(g) + 2C(s) + 8H2O(l)
Carbon monoxide is a very poisonous gas. It cannot be seen or smelt.
Faulty gas fires or boilers may produce carbon monoxide and poison the
air in a room without anyone knowing. Carbon monoxide acts as a
poison by combining with haemoglobin in the blood. Haemoglobin
normally reacts with oxygen from the air and transports the oxygen to
the parts of the body which need it. Carbon monoxide is much more
reactive with haemoglobin than oxygen is. It combines to form a stable
compound with haemoglobin, preventing the transport of oxygen
around the body. The person dies by suffocating from the inside
(nasty!).
Soot is a health hazard too. The fine particles can get trapped in lungs
and cause respiratory problems.
The main sign of incomplete combustion is a “dirty” flame. The smoke is
the soot being produced. The flame burns yellow because all the tiny
soot particles glow red hot.
Q3. Which is more likely to undergo incomplete combustion, methane or
octane, C8H18? Explain your answer.
Octane because it is larger than methane and doesn’t vaporise as easily
or mix as well with air/oxygen.
FOSSIL FUELS
Fossil fuels are non-renewable resources formed over millions of years
from the remains of living things. They include coal, oil (petroleum) and
natural gas. Since the Industrial Revolution (a time of rapid
technological advancement in the 18th and 19th centuries), we have used
fossil fuels in ever-increasing amounts, primarily to run vehicles and for
electricity generation. The combustion of fossil fuels has resulted in an
increase in the carbon dioxide concentration in the atmosphere. This
has caused global warming due to the enhanced greenhouse effect.
Combustion of fossil fuels also causes photochemical smog and acid
rain.
Coal is formed from plant matter in ancient swamps.
Crude oil and natural gas are formed from the remains of marine
organisms, mainly plankton.
The organic matter is buried in sediment and then compressed and
heated over millions of years to form fossil fuels.
Q4. Watch the brainpop videos on fossil fuels and the greenhouse effect
and add your notes in the spaces below and on the back of this page.
Fossil fuels form over millions of years from the remains of living things.
They include coal, crude oil (petroleum) and natural gas.
The greenhouse effect is the trapping of heat (infrared radiation) in the
atmosphere by greenhouse gases such as carbon dioxide. It is essential
to life on Earth. The enhanced greenhouse effect is the extra warming of
the Earth due to greenhouse gases put into the atmosphere by human
activity. This is a bad thing because it causes global warming.