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Phase Changes and Latent Heat

Phase Changes and Latent Heat. Gas. Where’s the heat?. Solid. Liquid. Reading. Hess Phase Diagram pp 49 – 51 Dew Point, Wet Bulb Temperature and Wet Bulb Potential Temperature pp 60 – 63 Bohren & Albrecht pp 218-223 Wallace & Hobbs p. 84. Objectives.

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Phase Changes and Latent Heat

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  1. Phase Changesand Latent Heat Gas Where’s the heat? Solid Liquid

  2. Reading • Hess • Phase Diagram • pp 49 – 51 • Dew Point, Wet Bulb Temperature and Wet Bulb Potential Temperature • pp 60 – 63 • Bohren & Albrecht • pp 218-223 • Wallace & Hobbs • p. 84

  3. Objectives • Be able to describe the changes in temperature, equilibrium pressure, volume and heat during various phase changes

  4. Objectives • Be able to recall from memory the definition of critical point • Be able to recall from memory the definition of triple point

  5. Objectives • Be able recall from memory the values of temperature and pressure for the triple point of water • Be able to recall from memory the values of temperature and pressure at the critical point of water

  6. Objectives • Be able to show isobaric, isochoric and isothermal changes on phase diagrams • Be able to determine changes of boiling and melting temperatures with changes in atmospheric pressure

  7. Objectives • Be able to recall from memory the definition of latent heat • Be able to determine whether latent heat is released or absorbed during a phase change • Be able to provide the name given to each type of phase change

  8. Objectives • Be able to describe how enthalpy and latent heat are related • Be able to perform calculations to determine the amount of latent heat released during a phase change • Be able to perform calculations to determine the change in latent heat with temperature

  9. Objective • Be able to recall from memory the definition of wet bulb temperature • Be able to compare the differences between wet bulb temperature and dew point temperature

  10. Phase Changes • Phase change results in a transformation of the molecular structure Gas Liquid Solid

  11. T Phase Change • Temperature of substance does not change during transformation

  12. Phase Change • Equilibrium (or saturation) pressure does not change during phase change

  13. Phase Change • Can Occur at Various Temperatures and Equilibrium Pressures Water Pressure (e) T5 Ice Vapor Water & Vapor T4 T3 T2 Ice & Vapor T1 Volume (V)

  14. Phase Change • Volume changes significantly during phase change Condensation

  15. Phase Change • Entropy also changes Increasing Entropy Liquid Gas Solid

  16. Phase Change • Phase Change (P-V Diagram) • Vapor to Ice • Water to Ice • Triple Line • The thermodynamic state at which three phases of a substance exist in equilibrium. Ice & Water Water Pressure (e) Water & Vapor Vapor Ice Triple Line 0oC Ice & Vapor T Volume (V)

  17. Phase Change • Phase Change (P-V Diagram) • Triple Line • T = 273.16K • es = 6.107 mb Ice & Water Water Pressure (e) Water & Vapor Vapor Ice Triple Line 0oC Ice & Vapor T Volume (V)

  18. Ice & Water Critical Point Water Pressure (e) Water & Vapor Vapor Ice 0oC Ice & Vapor T Volume (V) Phase Change • Phase Change (P-V Diagram) • Vapor to Water • Critical Point (Pc) • The thermodynamic state in which liquid and gas phases of a substance coexist in equilibrium at the highest possible temperature.

  19. Ice & Water Critical Point Water Pressure (e) Water & Vapor Vapor Ice 0oC Ice & Vapor T Volume (V) Phase Change • Phase Change (P-V Diagram) • Vapor to Water • Critical Point (Pc) • No liquid phase can exist at temperatures higher than the critical temperature • Tc = 647 K • Pc = 222,000 mb

  20. Phase Change • Phase Change (P-T Diagram) esw Liquid Pressure Solid Gas esi Temperature

  21. Phase Change • Isothermal Compression esw Liquid Pressure Solid Gas esi Temperature

  22. Phase Change • Isobaric Cooling esw Liquid Pressure Solid Gas esi Temperature

  23. Phase Change • Changes in Atmospheric Pressure • Change in Freezing Point -.007oC atm-1 Liquid Solid Pressure Gas Temperature

  24. Phase Change • Changes in Atmospheric Pressure • Change in Boiling Point Liquid Solid Pressure Gas Temperature

  25. Phase Change • Critical Point Critical Point Liquid Pressure Solid Gas esi Temperature

  26. Phase Change • Triple Point Critical Point Liquid Pressure Solid Triple Point 6.11 mb Gas esi 0.01oC Temperature

  27. Ice & Water (hidden) Critical Point Water Water & Vapor Vapor Vapor Pressure Ice Triple State Ice & water Temperature Specific Volume Three Dimensional Phase Diagram

  28. Three Dimensional Phase Diagram

  29. Phase Change • Liquid Water Molecule • Hydrogen Bonds • Shearing Energy too great

  30. Phase Change • Ice • Volume Increases

  31. Gas Solid Liquid Phase Change • Heat is absorbed or released during the phase changes

  32. Phase Change • Heat Absorbed Sublimation Gas Evaporation Solid Liquid Melting

  33. Phase Change • Heat Released Deposition Gas Condensation Solid Liquid Freezing

  34. Phase Change • Latent Heat • The heat required to change the molecular configuration of a substance Gas Liquid Solid

  35. Phase Change • Latent Heat Sublimation (ls) Gas Vaporization (lv) Solid Liquid Fusion (lf)

  36. Phase Change • Latent Heat • Increase in internal energy results from the change in molecular configuration Gas Liquid Solid

  37. Latent Heat • First Law of Thermodynamics • Internal Energy changes • Temperature is constant! • Pressure is constant • Volume changes • Work is done

  38. Latent Heat • Rearrange • For a phase change from liquid to vapor av = specific volume of vapor aw = specific volume of liquid

  39. Latent Heat • Substitute • Into • Define the change in Internal Energy uv = internal energy of vapor uw = internal energy of liquid

  40. Latent Heat • Latent Heat (lv) = Change in Heat (dq) • Rearrange

  41. Latent Heat • Enthalpy is defined as • Substitute or • Latent Heat is a change in Enthalpy!

  42. Latent Heat • Latent Heat of Transformation (l) • ratio of the heat absorbed (Q) to the mass undergoing a phase change

  43. Latent Heat • The amount of heat absorbed (or released) during a phase change is

  44. Latent Heat • Representative Values at 0oC • Latent Heat of Fusion (lf) • 3.34x105 J kg-1 • Latent Heat of Vaporization (lv) • 2.500x106 J kg-1

  45. esw Liquid Pressure Solid Triple Point 6.11 mb Gas esi 0.01oC Temperature Latent Heat • Latent Heat of Sublimation (ls) at 0oC ls = lf + lv ls = 2.834x106 J kg-1

  46. Latent Heat • Varies with temperature Water dQ Pressure (e) dQ Ice Vapor dQ 0oC dQ T Volume (V)

  47. Variation of Latent Heat • Let’s examine the latent heat of vaporization • It’s easier to show the variation using entropy, but we’ll follow Hess

  48. Variation of Latent Heat • First Law of Thermodynamics • Substitute

  49. Variation of Latent Heat • Expand • And since aw << av

  50. Variation of Latent Heat • The Ideal Gas Law (or Equation of State) • Substitute

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