Liquids Vapor Pressure Vapor gas phase of substance

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 Fig b: closed system rate of evaporation = rate of condensation system appears static due to equilibrium

How does evaporation occur?

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 quantity liquid present • VP depends ONLY on temperature

IMF in Liquids WEAK FORCES • • high VP high rate evaporation low bp small Hv STRONG FORCES • • low VP low rate evaporation high bp large Hv

Phase Changes • • • melting vaporization sublimation freezing deposition condensation

Phase Changes are accompanied by energy changes • Endothermic Process: Energy is absorbed • Exothermic Process: Energy is released G L S

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

Boiling Point • temperature at which: VP liquid = external atmospheric P • normal boiling point: oint 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 co-exist at equilibrium • higher the mp, the stronger the IMF

Freezing Point • temp at which liquid can be converted to crystalline solid How does fp compare to mp? ex: H 2 O mp & fp are same: for H 2 O = 0˚C, 273 K

Strong or Weak attractive forces? • • high vapor pressure large Hv high boiling point low vapor pressure small Hv evaporates rapidly evaporates slowly • • weak strong weak strong

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

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