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Phase Changes, Diagrams

phase changes - changes of the state of a substance

  • vaporization - liquid to gas
  • condensation - gas to liquid
  • melting - solid to liquid
  • freezing - liquid to solid
  • deposition - gas to solid
  • sublimation - solid to gas
  • loss of movement >> loss of energy >> exothermic
  • more movement >> gain of energy >> endothermic
  • heat of fusion - energy needed to change solid to liquid
    • Hfus
  • heat of vaporization - energy needed to change liquid to gas
    • usually larger than heat of fusion since molecules must get rid of all intermolecular attractions when in transition from liquid to gas
    • Hvap
  • heating curve - temperature of system versus amount of heat added
    • temperature stops at the points where substance changes states
    • supercooling - temporarily cooling a liquid below its freezing point w/o it turning into a solid
  • critical temperature - highest temperature at which a liquid phase can form
  • distinct pressure - pressure needed to turn gas into liquid at the critical temperature

Find the enthalpy change as 10 g of a liquid at 70° C goes to a gas at 100° C  

  • Given:
    • liquid boils at 90° C
    • specific heat of liquid = 1.0 J/g-K
    • specific heat of gas = 0.3 J/g-K
    • enthalpy of vaporization = 8.5 J/g
  • enthalpy of heating liquid from 70° C to 90° C = (10)(1.0)(20) = 200 J
  • Hvap = (10)(8.5) = 85 J
  • enthalpy of heating gas from 90° C to 100° C = (10)(0.3)(10) = 30 J
  • total enthalpy change = 200 + 85 + 30 = 415 J

vapor pressure - pressure exerted by vapor during dynamic equilibrium

  • weaker attractive forces >> more molecules able to escape into gas phase >> higher vapor pressure
  • dynamic equilibrium - when evaporation/condensation occur at same rate
    • no net change in system
    • doesn’t occur in an open container
  • volatility - describes liquids that evaporate readily
    • higher vapor pressure >> more easily evaporates
  • normal boiling point - boiling point of liquid at 1 atm
    • occurs when vapor pressure equals external pressure on liquid’s surface
    • higher pressure >> higher boiling point

phase diagrams - graphic representation of equilibrium between phases

  • uses temperature/pressure as axis
  • lines represent equilibrium between phases
  • triple point - where 3 curves intersect
    • all 3 phases are in equilibrium at this point

 

Subject: 
Chemistry
Subject X2: 
Chemistry

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