Developing fuels

1
Developing fuels

• DF 2 Getting energy from fuels

2
The role of oxygen

• 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 cant release energy until they have
  combined with oxygen so we should think of fuel
  oxygen systems as the energy sources

3
Enthalpy change of combustion

• 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 ?HcØ
• ?HcØ also depends on the type of bond involved

4
Combustion of methanol and methane

• Combustion of methane
• CH4 2O2 ? CO2 2H20
• H
• l
• H-C-H OO ? OCO H-O-H
• l OO H-O-H
• H
• Combustion of methanol
• CH3OH 1 ½ O2 ? CO2 2H20
• H
• l
• H-C-O-H OO OCO H-O-H
• l ½ OO H-O-H
• H

• 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.

5
Combustion of methanol and methane

• 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)

6
Important news for slimmers
Alcohol can be fattening each of these drinks
provides 300kJ of energy about 1 ½ slices of bread
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 Alcoho
ls 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!!
7
Carrying fuels around

• 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