Energy

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Energy Chemistry

• ENERGY is the capacity to do work.
• HEAT is the form of energy that flows between 2
  objects because of their difference in
  temperature.
• Other forms of energy
• light
• electrical
• kinetic and potential

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Potential Energy on the Atomic Scale

• Positive and negative particles (ions) attract
  one another.
• Two atoms can bond
• As the particles attract they have a lower
  potential energy

NaCl composed of Na and Cl- ions.
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Thermodynamics

• Thermodynamics is the science of heat (energy)
  transfer.

Heat energy is associated with molecular motions.
Heat transfers until thermal equilibrium is
established.
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Directionality of Heat Transfer

• Heat always transfer from hotter object to cooler
  one.
• EXOthermic heat transfers from SYSTEM to
  SURROUNDINGS.

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Directionality of Heat Transfer

• Heat always transfer from hotter object to cooler
  one.
• ENDOthermic heat transfers from SURROUNDINGS to
  the SYSTEM.

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UNITS OF ENERGY

• 1 calorie heat required to raise temp. of 1.00
  g of H2O by 1.0 oC.
• 1000 cal 1 kilocalorie 1 kcal
• 1 kcal 1 Calorie (a food calorie)
• But we use the unit called the JOULE
• 1 cal 4.184 joules

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HEAT CAPACITY

• The heat required to raise an objects T by 1 C.

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Specific Heat Capacity

• How much energy is transferred due to T
  difference?
• The heat (q) lost or gained is related to
• a) sample mass
• b) change in T and
• c) specific heat capacity

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Specific Heat Capacity

• Substance Spec. Heat (J/gK)
• H2O 4.184
• Ethylene glycol 2.39
• Al 0.897
• glass 0.84

Aluminum
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Exercise Specific Heat Capacity

• If 25.0 g of Al cool from 310 oC to 37 oC, how
  many joules of heat energy are lost by the Al?

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Answer Specific Heat Capacity

• If 25.0 g of Al cool from 310 oC to 37 oC, how
  many joules of heat energy are lost by the Al?

where ?T Tfinal - Tinitial q
___________________ q _______ J
Notice that the negative sign on q signals heat
lost by or transferred OUT of Al.
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Measuring Heats of Reaction
CALORIMETRY

• Constant Volume Bomb Calorimeter
• Burn combustible sample.
• Measure heat evolved in a reaction.
• Derive ?E for reaction.

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Calorimetry
Total heat evolved qtotal qwater qbomb
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Measuring Heats of ReactionCALORIMETRY

• Calculate heat of combustion of octane. C8H18
  25/2 O2 --gt 8 CO2 9 H2O
• Burn 1.00 g of octane
• Temp rises from 25.00 to 33.20 oC
• Calorimeter contains 1200 g water
• Heat capacity of bomb 837 J/K

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Heat TransferNo Change in State

• q transferred (sp. ht.)(mass)(?T)

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Heat Transfer with Change of State

• Changes of state involve energy (at constant T)
• Ice 333 J/g (heat of fusion) -----gt Liquid
  water
• q (heat of fusion)(mass)

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Heat Transfer and Changes of State
Liquid ---gt Vapor

• Requires energy (heat of vaporization). T
  remains constant.

energy
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Heating/Cooling Curve for Water
Evaporate water
Heat water
Melt ice
Note that T is constant as ice melts
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Heat Changes of State

• Calculate the total amount of heat required to
  convert 500.0 g of water at -50.00 oC to steam at
  200.0 oC. The heat of fusion of water is 333 J/g
  and the heat of vaporization is 2256 J/g. The
  heat capacity of ice is 2.06 J/g.K and the
  specific heat capacity of steam is 1.92 J/g.K.