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Thermodynamics & Phase Change

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 diagram. temperature. heat added.

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Thermodynamics & Phase Change

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  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 = mCΔT • 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 • How much energy will be used to convert a 3.5 kg block of ice at 240 K into steam at 390K?

  13. Example 2 • First, look at your phase change diagram and locate where you are starting in the process and where you are ending.

  14. ⁰C = K - 273.15 Q=mCΔt Δt=tf-ti mCΔta = -mCΔtb Water Phase Change Diagram Water Vapor Steam Q=m(2020)Δt Q=m Hv Hf = 3.34 x 105 Hv = 2.26 x 106 100⁰ C 373K Temperature Liquid Water Q=m(4180)Δt 0⁰ C 273K Q=m Hf Solid Water/Ice Q=m(2060)ΔT Heat

  15. Ex. 2 Cont’d • Then write down all the formulas you will need Q=3.5(2060)(33) Q=3.5(3.34x10^5) Q=3.5(4180)(100) Q=3.5(2.26x10^6) Q=3.5(2020)(17)

  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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