Using Heats of Combustion and Altering Fuels

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Using Heats of CombustionandAltering Fuels
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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.

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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
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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
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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

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Assignment

• Heats of Combustion
• p.252-253 1-3

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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.

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Cracking

• 1913 chemists discovered that it was possible to
  convert a large hydrocarbon (kerosene) into a
  smaller one (gasoline) by heating it to 600-700C
• Cracking process of converting large
  hydrocarbon molecules into smaller ones through
  the application of thermal energy and a catalyst.

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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 (500C instead of 700C)

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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)

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Octane

• Branched-chain alkanes less likely to combust
  during compression (dont ping)
• Example
• Isooctane or 2,2,4-trimethylpentane

Can you see how the name relates to the structure?
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Octane Rating

• Determined by testing the fuels 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)

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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

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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)

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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

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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 (its a POLAR
  molecule!!)
• Policies to reduce or ban MTBE are under
  consideration

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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
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Assignment

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