Developing fuels DF 2 Getting energy from fuels

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 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 Depends on two things 1. Number of bonds to broken and made – depends on the size of the molecule. Larger molecules such as hexane have a more negative ΔH c Ø 2. ΔH c Ø also depends on the type of bond involved

Combustion of methanol and methane Combustion of methane CH 4 + 2O 2  CO 2 + 2H 2 0 H l H-C-H O=O  O=C=O H-O-H l O=O H-O-H H Combustion of methanol CH 3 OH + 1 ½ O 2  CO 2 + 2H 2 0 H l H-C-O-H O=O O=C=O H-O-H l ½ [O=O] 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.

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)

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 C 6 H 12 O 6 Glycerol trioleate C 57 H 104 O 6 (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!!

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