Thermodynamics - PowerPoint PPT Presentation

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Thermodynamics

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Thermodynamics & Phase Change Heat can only do one thing at a time: either change the temperature OR change the state! Changes of state seen in phase change ... – PowerPoint PPT presentation

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Title: Thermodynamics


1
Thermodynamics Phase Change
2
  • Heat can only do one thing at a time either
    change the temperature OR change the state!

3
  • Changes of state seen in phase change diagram

temperature
heat added
4
  • 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.

5
  • 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

6
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

7
  • Heat required to melt
  • Q mHf
  • use for melting freezing!
  • Heat required to vaporize
  • Q mHv
  • use for evaporating condensing!

8
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!

9
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?

10
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

11
Example 1 Solution
  • Q1 33400 J
  • Q2 8360 J
  • Q 41,760 J

12
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?

13
Example 2 Solution
  • Q mHv
  • -9870 (.040)(Hv)
  • Hv -246,750 J/kg
  • (negative because its cooling)

14
(No Transcript)
15
Law of Conservation of Energy
  • Energy cannot be destroyed or created only
    transferred.

16
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.

17
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.

18
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.

19
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.
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