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Understanding Liquids, Vapor Pressure, and Phase Changes in Chemistry

This overview explores the properties and behaviors of liquids and their transition to vapor. It explains evaporation, the factors affecting vapor pressure, and the significance of intermolecular forces (IMFs). Key concepts include how temperature influences vapor pressure and boiling points, as well as the differences between boiling and evaporation. Additionally, we discuss endothermic and exothermic processes during phase changes, including melting, freezing, and condensation. The effects of external pressure on boiling points are also examined, highlighting the complex interplay of thermal dynamics in liquids.

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Understanding Liquids, Vapor Pressure, and Phase Changes in Chemistry

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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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