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

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

How does evaporation occur?

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

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

all liquids: VP ↑as T ↑

Vapor Pressure VP does NOT depend on how much liquid is present VP depends only on temperature

IMF in Liquids WEAK FORCES High VP High rate evaporation Low bp Small H v STRONG FORCES Low VP Low rate evaporation High bp Large H v

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

Phase Changes are accompanied by energy changes Endothermic Process: Energy is absorbedEndothermic Process: Energy is absorbed Exothermic Process: Energy is releasedExothermic Process: Energy is released

Which phase changes absorb energy (endothermic)? Which phase changes release energy (exothermic)? Melting, Boiling (vaporization) FreezingCondensation Freezing, Condensation

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

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

Freezing Point Temp at which liquid is converted to crystalline solid How does fp compare to mp? They’re the same: for H 2 O = 0˚C

Strong or Weak attractive forces? High vapor pressure Large H v High boiling point Low boiling point Low vapor pressure Small H v Evaporates rapidly Evaporates slowly Weak Strong Weak Strong Weak Strong

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

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