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Developing fuels
DF 2 Getting energy from fuels
The role of oxygen
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Different fuels have
different enthalpy changes of
combustion
Enthalpy changes are negative
during combustion the
reactants lose energy to their
surroundings
Enthalpy of combustion varies
widely – why?
Fuels can’t release energy
until they have combined with
oxygen so we should think of
fuel – oxygen systems as the
energy sources
Enthalpy change of combustion
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2.
Depends on two things
Number of bonds to broken and made –
depends on the size of the molecule.
Larger molecules such as hexane have a
more negative ΔHcØ
ΔHcØ also depends on the type of bond
involved
Combustion of methanol and
methane
Combustion of methane
CH4 + 2O2  CO2
+ 2H20
H
l
H-C-H O=O  O=C=O
H-O-H
l
O=O
H-O-H
H
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Combustion of methanol
CH3OH + 1 ½ O2  CO2 + 2H20
H
l
H-C-O-H O=O
O=C=O H-O-H
l
½ [O=O]
H-O-H
H
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•Products are the same
•Key difference is
methanol has an O-H bond
already – one bond to the
oxygen is already made –
it is already partly
oxidised where methane is
not.
Combustion of methanol and
methane
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Energy released during combustion comes from making bonds
to oxygen. Methanol has one bond made already so it releases
less energy when burnt.
General rule, more oxygen a fuel has in its molecule, the less
energy it will give out.
Oxygenated fuels (alcohols and ethers) are less energy rich
than hydrocarbons.
Not poor fuels – some ways better, less pollution, higher
octane number (more later)
Important news for slimmers
Eating too much of an energy food, the excess
energy gets stored by the body as fat
The more energy rich, the more fattening
Glucose C6H12O6
Glycerol trioleate C57H104O6
(main constituent of olive oil)
Per carbon atom, glucose is much more
oxygenated than the olive oil, the olive oil is
much more oxygenated.
Fats are twice as energy rich than
carbohydrates
Alcohols are neither fat or carbohydrates.
Ethanol the alcohol present in drinks is used
as an alternative to petrol in some countries.
It releases energy in the body when
metabolised – beer bellies!!
Alcohol can be fattening
each of these drinks
provides 300kJ of
energy about 1 ½ slices
of bread
Carrying fuels around
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For a practical uses - fuel enthalpy change of combustion
may not be the most important thing to consider.
Energy density is often the decisive factor – how much
energy you get per kilogram of fuel
We can work this out from energy change of combustion
using relative molecular mass