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
  • H_fus
• 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
  • H_vap
• 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
• H_vap = (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