Revisi,ng)Vapour)Pressure)•  Why$do$liquid$molecules$escape$into$the$gas$phase$even$though$the$temperature$is$not$equal$to$the$boiling$point?$

•  At#any#temperature,#the#energy#of#individual#molecules#following#a#distribution#(known#as#Boltzmann’s+Distribution).#Not#all#molecules#have#the#same#energy.#

•  At#higher+temperatures,#the#average#energy#is#higher,#but#that#does#not#mean#ALL#molecules#have#higher#energy#than#the#ones#at#a#lower#temperature#

#of studentsw/ grade

i¥¥a.

⑨

i ;in

Revisi,ng)Vapour)Pressure)•  #Boltzmann’s#Distribution#Curve#

Width = domain of KE

Height = # of molecules w/ KE

Area = Total # of molecules

( KJIMOI )

Revisi&ng)Vapour)Pressure)•  !Boltzmann’s!Distribution!Curve!

Why!is!the!higher!temperature!curve!shaped!that!way…?!1.  At!higher!T,!the!max!KE!is!greater!(wider!curve),!

and!the!average!KE!is!greater!(peak!to!the!right)!2.  Yet!the!#!of!molecules!(area!under!the!curve)!

remains!the!same!as!lower!T.!3.  This!mean!the!height!of!the!curve!must!be!lower.!_#

ofmolecules wi as itrowkemp . DE.yywhyvpkas.tn}• ←High Temp . @ night, there are moremolecules w/ enough KE¥oA*******:÷÷÷÷÷:÷÷÷

Min- KE needed for molecules to Vaporize .

= " Threshold "

Revisi,ng)Vapour)Pressure)•  Notice#that#at#a#higher+temperature,#the#number#of#molecules#with#enough#energy#to#vapourize#(escape#as#gas)#is#greater#than#the#number#of#molecules#able#to#do#so#at#a#lower#temperature.#

Revisi,ng)Vapour)Pressure)•  This#conTirms#what#we#noted#earlier#in#the#Bonding#unit:##! #At#a#higher#temperature,#more#molecules#have#enough#energy#to#escape#into#the#gas#state#

! #More#molecules#in#the#gas#state#means#greater#pressure#(PV#=#nRT)#

! Higher+temperatures+result+in+higher+vapour+pressures+for+all+substances#

Vapour)Pressure)and)Boiling)Point)•  Let#us#look#at#some#data#for#H2O#########

•  What#do#we#notice#about#the#vapour#pressure#and#temperature#in#the#last#row?#

Temperature+(oC)+ Vapour+Pressure+(kPa)+

0# 0.61#20# 2.34#40# 7.37#60# 19.9#80# 47.3#100# 101.3#b. p. = @ = Eatin = Patm .

Vapour)Pressure)and)Boiling)Point)•  The#boiling+point+of#a#substance#is#the#temperature#at#which##

++++vapour+pressure+=+atmospheric+pressure+•  At#higher#altitudes,#when#atmospheric#pressure#is#lower#(such#as#on#mountains),#the#boiling#point#of#substances#decrease#! #On#the#peak#of#Mt.#Everest,#water#boils#at#70oC…#this#just#means#you#cannot#get#liquid#water#about#70oC#since#it#will#boil#away#quickly.#Can’t#make#a#good#cup#of#coffee#on#Everest#"#Patm#~#250#torr. ##

Vapour)Pressure)and)Boiling)Point)-

a:#b.p .

CETOH)

v e78.50C atseauvel

70°C