quiz 1

• dH is (-) in exothermic reactions.

  The reverse reaction has a (+) dH because it would be endothermic.
• dSsys is (+) when

  • phase change [s->l->g] from left to right
  • (+)dn from left to right
  • molecule size and complexity increases
  • mixing/dissovling
  • system is at high T
• Phase change:

  • making IMFS = - dH (remove to form) --> more restriction = - dS
  • breaking IMFs = + dH (add to break) --> less restriction = + dS
• (-)dG = (-)dH - (+)TdS : spontaneous always

  [combustion --> H2 balloon explosion]

  (-)dG = (-)dH - (-)TdS : spontaneous low T

  [freezing water --> 0ºC to go l ->s]

  (-)dG = (+)dH - (+)TdS : spontaneous high T

  [boiling water --> 100ºC to go l->g]

  (+)dG = (+)dH - (-)TdS : spontaneous never

  [cant add heat to change water to ice]
• Tphase = dH/dS

  • dH and dS always have same sign
  • melting = endothermic (+ dH/+ dS)
  • freezing = exothermic (- dH/- dS)
  • dH and dS used to find phase transition temp
• Vapor Pressure:

  • temp dependent
  • volume independent
  • surface phenomenon
  • measured at equilibrium (dG=0) in a closed system
  • **strong IMF = weak VP (inverse relationship)**
• phase diagrams:

  • water phase diagram: negative slope that indicates HBonds make solid form less dense than liq form
  • solid line: point of equilibrium = phase transition
  • low T/high P = solid
  • high T/low P = gas
  • triple point: all 3 phases exist together
  • supercritical region: high P and T ; cant turn VP -> liq
  • heating curve: flat = equilibrium / steep = T increase