Chapter 4: Energy

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Chapter 4 Energy
Chem 100 Lecture 7 February
15, 2005
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• The formation of CO bonds is what make
  combustion reactions so exothermic.
• The greater the C content, the better the fuel.
• Fuels that contain O (like ethanol) are less
  efficient

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How does C get into fuels?

• Ultimately, through photosynthesis
• 6CO2 6H2O ? C6H12O6 6O2
• This is an endothermic reactionit requires 2800
  kJ for every mole of glucose produced
• Plants get the energy for this from light
• Glucose can be used as a fuel
• Cellulose (wood) is many glucose molecules bonded
  together

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• When plants die in the presence of O2
• C6H12O6 6O2 ? 6CO2 6H2O 2800 kJ/mole
• This is an exothermic process, but its very
  slow!
• This is what happens in a compost pile

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• Under certain conditions, dead plants are buried
  in anaerobic conditions (anaerobic means no O2!)
• Coal and oil are formed this way
• Because these are more concentrated sources of C,
  they are better fuels than wood
• Wood 10-14 kJ/g
• Coal 16-30 kJ/g (depends on type)
• Octane 47.8 kJ/g

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Coala mixture of compounds

• Can be approximated by
• C135H96O9NS
• What is the of C in coal?
• of C in wood?

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Grades of Coal

• Lower grades of coal contain a lower of C

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Advantages disadvantages of Coal Use

• Disadvantages
• Underground mining is dangerous
• Accidents
• Respiratory disease
• Strip mining is environmentally damaging
• Combustion doesnt produce just CO2 H2O
• Soot
• Sox, NOx

• Advantages
• Large supply (20-30x petroleum reserve)
• Large amount of energy/gram

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Ways to overcome strip mining?

• Environmental regulations on strip-mining
• Safety regulations on mining
• Use of higher quality coal to minimize SOx and
  NOx

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Petroleum

• Since the 1950s, this has been the major energy
  source in the U.S.
• Advantages over coal
• Liquid (makes transportation easier)
• A better energy source (48 kJ/g)
• Contains little S and other contaminants
• Disadvantages
• Crude oil must be refined (purified) before it
  can be used

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Crude oil

• Crude oil contains a mixture of many kinds of
  hydrocarbons
• Hydrocarbons can be classified by the of Cs
  they contain
• 1 C methane
• 2 Cs ethane
• 3 Cs propane
• Hydrocarbons can be separated because they boil
  at different temperatures
• The fewer the number of Cs, the lower the
  boiling point

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Liquid phase to gas phase

• It requires a lot of energy to go from the liquid
  phase to the gas phase

Liquid phase Molecules are very close together
(denser)
Gas phase Molecules are very far apart (less
dense)
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Separation is done by distillation

• Heat crude oil, vaporizing all the components.
• Heavier compounds quickly condense while lighter
  compounds take much longer to condense.

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What happens after separation of hydrocarbons by
distillation?

• Convert hydrocarbons with gt 8 Cs into C8H18 by
  cracking
• C16H34 ? C8H18 C8H16
• Convert hydrocarbons with lt 8 Cs into C8H18 by
  using catalysts
• 4 C8H18 ? C8H16 C8H18

3. Convert n-octane to iso-octane using catalyst
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Isomers

• n-octane normal octane
• (straight chain structure)
• Many other ways to arrange Cs and still have
  C8H18
• Examples

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Isomers

• There are a total of 18 different arrangements of
  C8H18
• Each of the different C8H18 compounds is an
  isomer different compounds with the same
  formula
• Isomers have similar (but not identical) chemical
  physical properties
• Example n-octane has a b.p. of 125oC
• iso-octane has a b.p. of 99oC

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Why is iso-octane the preferred form of octane?

• Car engines Gasoline air vapors mix in a
  cylinder, the vapors are compressed by a piston
  and ignited by a spark
• The expansion is what powers the car
• The expansion compression of all cylinders must
  be well-coordinated
• n-octane ignites much more readily than
  iso-octane
• It frequently ignites w/o the spark
• Premature firing (knocking)

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Octane rating

• Octane rating A measure of the tendency of
  fuels to cause knocking the higher the the
  better
• n-octane -19
• Iso-octane 100

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Status of leaded gas world-wide
http//www.eia.doe.gov/oiaf/ieo/pdf/environmental.
pdf
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Substitutes for Petroleum

• 1. Water gas
• Biomass
• Wood
• Fermentation produces alcohols
• C6H12O6 ? 2C2H5OH 2CO2
• Combustion of alcohols
• C2H5OH 3O2 ? 2CO2 3 H2O
• Heat of combustion 30 kJ/g vs 48 kJ/g for C8H18
• Garbage
• Collect methane produced in landfills
• Burning garbage ? heat ? electricity

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Price Comparison

• Cost of producing 1 gallon of ethanol from
  feedstock corn  1.10/gallon
• Wholesale price of gasoline is 0.90/gallon
• Data from http//www.energy.state.or.us/biomass/C
  ost.htm

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Biodiesel

• One obstacle Cost!
• Vegetable oil seed procurement, transport,
  storage and oil extraction accounts for at least
  75 percent of the cost of producing biodiesel.
• The cost varies depending on the feedstock used.
  For example, based on the market price for
  industrial rapeseed grown in Washington and
  Idaho, the estimated cost of producing biodiesel
  is 2.56 per gallon of rapeseed methyl ester.
• http//www.energy.state.or.us/biomass/Cost.htm
• Heat of combustion of rapeseed methyl ester is
  40.6 kJ/g

http//www.biodiesel.com/why_biodiesel.htm
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How is electricity generated?
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How is electricity generated?

• Fuel (garbage, coal) is combusted producing heat
  heat converts liquid water to vapor (high
  pressure steam)
• The steam drives a turbine which rotates a wire
  coil in a magnetic field? electricity
• Lower pressure steam passes through a heat
  exchanger where it condenses
• Liquid is then pumped back to the boiler
• Note Not all of the chemical energy will be
  converted to electricity

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Maximum Theoretical Efficiency

• Maximum theoretical efficiency for
• heat ? work conversion
• Efficiency Thighest Tlowest
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  Thighest
• Thighest Temperature to which water vapor is
• heated
• Tlowest Temperature to which condensed water is
  cooled
• Temperature must be in KELVINS
• T(Kelvins) T(oC) 273
• 0K -273oC is absolute zero
• the lowest possible temp

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Example Efficiency Problem

• Steam is heated to 550oC
• Water is condensed to 30oC
• What is efficiency of this system?

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Overall Efficiency

• Efficiencies are multiplicative
• Overall efficiency efficiency of (power plant)
  x (boiler) x (turbine) x (electrical generator) x
  (power transmission) x (home heat)

Home heat 98 efficient
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Comparison with gas furnace

• House could have also been heated directly with a
  gas furnace burning methane at 85 efficiency.

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Explanation for inefficiency

• Heat or Thermal Energy is characterized by the
  random motion of molecules
• Work is directed, organized motion of molecules
• Difficult to transform random motion of heat into
  organized motion

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2nd Law of Thermodynamics

• The 2nd law of thermodynamics states The amount
  of entropy of the universe is always increasing
• Entropy a measure of the amount of disorder
• Spontaneous process always involves an increase
  in entropy

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• Entropy provides a way for chemists to predict
  whether or not a reaction will occur
• C (graphite) ? C (diamond)
• entropy 5.7 kJ/mol
  entropy 2.4 kJ/mol
• Dentropy for this process is -3.3 kJ/mole
  (the disorder decreases)
• This process doesnt occur spontaneously!
• Implies that diamond to graphite transition
  should occur spontaneously (it does, but the
  kinetics are very slow)

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Can also estimate whether change in entropy will
be positive or negative

• C8H18 O2 ? 8CO2 9H2O
• 13.5 particles ? 17 particles
• Predict that this reaction will be spontaneous
  (positive entropy)

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2
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• Gases are more disordered than liquids
• Liquids are more disordered than solids

Increasing disorder Increasing entropy
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Conservation

• Limited supply of petroleum

• Easily recoverable oil will last 43 yrs
• Enough petroleum in heavy crude oil, bitumen, and
  oil shale to last 170 yrs at current rate of
  consumption
• This reserve will be more difficult and expensive
  to extract

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Energy Consumption Projections
http//www.eia.doe.gov/oiaf/ieo/download.html
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Smarter choices

• We can make more use of electricity by using more
  efficient appliances, etc.
• Example 75 watt light bulb vs. 18 watt compact
  fluorescent lamp
• In 10,000 hrs of use, can save 770 lbs coal!