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Liquids & Vapor Pressure

Liquids & Vapor Pressure. Vapor. Gas phase of substance that is normally liquid at room temperature Some evaporation occurs at all temperatures The easier a substance evaporates, the weaker the IMF. Fig a: open system Evaporation continues until all liquid is gone

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Liquids & Vapor Pressure

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  1. Liquids & Vapor Pressure

  2. Vapor • Gas phase of substance that is normally liquid at room temperature • Some evaporation occurs at all temperatures • The easier a substance evaporates, the weaker the IMF

  3. Fig a:open system Evaporation continues until all liquid is gone Fig b:closed system Rate of evaporation = rate of condensation System appears static due to equilibrium

  4. How does evaporation occur?

  5. molecules in liquid phase have range of KE; some have enough KE to“escape”from liquid phase

  6. Vapor Pressure • Pressure exerted by vapor over its liquid • VP is affected by temperature: • higher the T, higher the VP

  7. all liquids: VP ↑as T ↑

  8. Vapor Pressure • VP does NOT depend on how much liquid is present • VP depends ONLY on temperature

  9. STRONG FORCES Low VP Low rate evaporation High bp Large Hv IMF in Liquids WEAK FORCES • High VP • High rate evaporation • Low bp • Small Hv

  10. Phase Changes • Melting • Boiling • Vaporization • occurs throughout liquid; T constant) • Evaporation (occurs only at surface; T ↓) • Sublimation • Freezing • Deposition • Condensation

  11. Phase Changes are accompanied by energy changes • Endothermic Process: Energy is absorbed • Exothermic Process: Energy is released

  12. freezing , condensation, deposition • Which phase changes absorb energy (endothermic)? • Which phase changes release energy (exothermic)? melting, boiling (vaporization), sublimation

  13. Boiling Point • Temperature at which: VP liquid = external atmospheric P • Normal Boiling Point: temp at which VP liquid = 1 atm • Substances with high bp’s have strong IMF’s

  14. Melting Point • = temp at which liquid & solid phases of substance coexist at equilibrium • higher the mp, the stronger the IMF

  15. They’re the same: for H2O = 0˚C, 273K Freezing Point • Temp at which liquid is converted to crystalline solid • How does fp compare to mp?

  16. Weak Strong Strong Weak Strong Weak Weak Strong Strong or Weak attractive forces? • High vapor pressure • Large Hv • High boiling point • Low boiling point • Low vapor pressure • Small Hv • Evaporates rapidly • Evaporates slowly

  17. > < Boiling and Pressure • If increase external pressure (ex: camping in Death Valley), boiling point is ____ than 100oC • If decrease external pressure (ex: eating Raman noodles at top of Mt. Whitney), the boiling point is ____ than 100oC

  18. EVAPORATION occurs at surface of liquid (or solid) occurs at all T liquid cools off as evaporates spontaneous all the time Boiling vs. Evaporation BOILING • occurs throughout liquid • occurs at definite T • occurs at constant T • only spontaneous when VP = external pressure

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