1. Using Heats of Combustion and Altering Fuels

2. Combustion
With plenty of oxygen and compete combustion , burning of a hydrocarbon is…
Hydrocarbon + Oxygen gas  Carbon dioxide + Water + Thermal energy
A highly exothermic reaction.

3. Thermal Energy Burning ethane:
2 C2H6 + 7 O2  4 CO2 + 6 H2O + _?_ kJ thermal energy
Use table 3.6 p.250 to find out the molar heat of combustion for ethane…
…1560 kJ/mol (burning 1 mol of ethane releases that much energy)
How many moles of ethane are consumed in the above reaction?
How much thermal energy is released?
Ethane, U of Idaho

4. Thermal energy Can also be expressed as kJ/g (instead of kJ/mol)
Useful in finding out how much energy is released when a certain mass of fuel is burned
Ethane fire in Saskatchewan. Ethane part of natural gas, stored in underground salt caves. Chem.queensu.ca

5. Sample problem
How much thermal energy would be produced by burning 12.0 g octane, C8H18?
How much thermal energy released by burning 1.0 g of octane (table 3.6)?
12.0 x 47.8 kJ = 574 kJ

6. Assignment
Heats of Combustion
p #1-3

7. Altering Fuels Gasoline is only about 18% of crude oil
Could it be possible to alter the structure of other components of crude oil so that more of a barrel of oil could be gasoline?
Yes.

8. Cracking
1913 chemists discovered that it was possible to convert a large hydrocarbon (kerosene) into a smaller one (gasoline) by heating it to °C
Cracking = process of converting large hydrocarbon molecules into smaller ones through the application of thermal energy and a catalyst.

9. Cracking Today more than 1/3 of crude oil undergoes cracking
Process is improved with catalyst
Catalyst increases the speed of a reaction
Catalytic cracking is more efficient because it occurs at lower temp (500°C instead of 700°C)

10. Problems with Pure Gasoline
Gasoline composed of straight-chain alkanes (C6H14, C7H16, C8H18)
Gasoline-air mixture compressed before ignited by spark plugs (in most engines)
Compression can sometimes cause gas to ignite before spark, this is called “pinging” or “knocking” (piston bangs backwards against crankshaft at wrong time)

11. Octane
Branched-chain alkanes less likely to combust during compression (don’t “ping”)
Example:
Isooctane or 2,2,4-trimethylpentane
Can you see how the name relates to the structure?

12. Octane Rating
Determined by testing the fuel’s burning efficiency under two conditions:
A free running engine, and
Engine under load (towing or passing)
Results of these two tests are averaged to obtain octane rating
The higher the octane rating, the better its antiknock characteristics (examples: 87, 89, 92)

13. Leaded Gasoline
1920s to 1970s, tetraethyl lead (C2H4)4Pb was added to increase octane rating
It increased efficiency and added 3 points to octane rating
However, lead particulates entered atmosphere and were found to be very harmful to the environment and human health
Outlawed in 1970s

14. Cleaner Burning Fuels
Alternative to lead that is octane-boosting are additives called oxygenated fuels
These molecules contain oxygen as well as carbon and hydrogen
Deliver less energy per gallon, but reduce exhaust-gas pollutants
Also, often encourage more complete combustion producing lower emissions of air pollutions such as carbon monoxide (CO)

15. Example Oxygenated Fuels
Methanol
Also called methyl alcohol, CH3OH
Added to gas at distribution locations
Can be made from natural gas, coal, corn or wood
Ethanol
Blend of 10% ethyl alcohol, CH3CH2OH and 90% gasoline (called gasohol) can also be used in all modern engines

16. MTBE Methyl tertiary-butyl ether (MTBE) Octane rating 116
Introduced late 1970s
Most common fuel additive in 1990s
Late 1990s found that it was seeping from underground storage into groundwater and drinking water
Has unpleasant taste and smell, difficult to remove through filtration (it’s a POLAR molecule!!)
Policies to reduce or ban MTBE are under consideration

17. Isomerization
= straight chain hydrocarbons are converted to branched-chain hydrocarbons
Process requires heating hydrocarbon vapor with a catalyst
Both cracking and isomerizing are more expensive because fuel is needed to create this type of gasoline
Isomerization refinery run by Shell in Martinez, CA

18. Assignment Fuel for transportation p.257 #1-6
(Relates to alternative fuel project)