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Understanding Phase Changes: Heat and Latent Energy Fundamentals

Explore the fascinating world of phase changes and latent heat in this comprehensive guide. Learn about temperature variations, equilibrium pressures, volume shifts, and heat exchanges during state transitions. Identify critical and triple points, calculate latent heat, and understand phase diagrams. Discover how enthalpy and latent heat correlate, and master the concepts of wet bulb temperature and dew point. Enhance your understanding of gas, liquid, and solid transformations, including sublimation, evaporation, condensation, fusion, and more. Dive into the intricate relationships between energy and molecular structures across different phases.

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Understanding Phase Changes: Heat and Latent Energy Fundamentals

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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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