Thermodynamics

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Thermodynamics Phase Change
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• Heat can only do one thing at a time either
  change the temperature OR change the state!

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• Changes of state seen in phase change diagram

temperature
heat added
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• Diagonal regions where thermal energy is added
  and particles increase in motion (temperature
  changes)
• Flat regions where changes in state occur. All
  added thermal energy goes to overcome the forces
  holding the particles together.

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• Heat of fusion The amount of energy required to
  melt one kilogram of a substance
• Heat of vaporization the amount of energy
  required to vaporize one kilogram of a substance

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Heat of Fusion Vaporization for Water

• Heat of fusion for water
• Hf 3.34x105 J/kg
• positive when melting, negative when freezing
• Heat of vaporization for water
• Hv 2.26x106 J/kg
• positive when evaporating, negative when
  condensing

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• Heat required to melt
• Q mHf
• use for melting freezing!
• Heat required to vaporize
• Q mHv
• use for evaporating condensing!

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Specific Heat Values

• Cice 2060 J/kg?C
• Cwater 4180 J/kg?C
• Csteam 2020 J/kg?C
• Use these values for temps in Celsius or Kelvin!

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Example 1

• You are asked to melt 0.100 kg of ice at its
  melting point and warm the resulting water to
  20.0 ºC. How much heat is needed?

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Example 1 Strategy

• Calculate heat needed to melt the ice.
• Q1 mHf
• Calculate heat needed to raise water temperature.
• Q2 mC?T
• Add all heats together to get the total heat
  needed.
• Q Q1 Q2

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Example 1 Solution

• Q1 33400 J
• Q2 8360 J
• Q 41,760 J

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Example 2

• A 40.0-g sample of chloroform is condensed from a
  vapor at 61.6 ºC. It liberates 9,870 J of heat.
  What is the heat of vaporization of chloroform?

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Example 2 Solution

• Q mHv
• -9870 (.040)(Hv)
• Hv -246,750 J/kg
• (negative because its cooling)

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Law of Conservation of Energy

• Energy cannot be destroyed or created only
  transferred.

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1st Law of Thermodynamics

• The total increase in the thermal energy of a
  system is the sum of the heat added to it and the
  work done on it.
• Heat engines are devices which convert thermal
  energy to mechanical energy continuously along
  with some waste heat.

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2nd Law of Thermodynamics

• Entropy of the universe is always maintained or
  increased. All things will become more and more
  disordered unless some action is taken.

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2nd Law of Thermodynamics

• Entropy is a measure of the disorder in a system.
• Entropy increases when heat is added and
  decreases when heat is removed.

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Thermal Engine efficiency

• Called Carnot efficiency, or the ideal efficiency
  of a heat engine
• (Thot-Tcold)/Thot Ideal efficiency
• Temperatures must be in Kelvin.
• Result is a percentage.