Experiment109November2017

IntermolecularForcesTodaywewillaskthatage-oldques3on:Whydomoleculesdothat?

Andhereisoneofourcontestants…

Objec&ve:Toseehowintermolecularforcesaffectratesofevapora&on.

Overview:1.  Intermolecularforces2.  Connec&ngrateofevapora&onwith

intermolecularforces3.  Procedure:UsingLoggerPro4.  Procedure:Whattodotoday5.  Yourlabreport

Moleculeswithweakintermolecularforcestendtoevaporate

faster…...asweshallsee.

Todaywewillbegatheringqualita3ve,empiricalinforma3onabout

intermolecularforces.

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Thisispentane,C5H12.Itcanbedrawnthreedifferentways.

1.Intermolecularforces

Thisisthe“space-filling”modelandisthemost

accurateofthethemall–butItshardtoseeeveryatom.

Hereistheball-and-s3ckmodel.Itshowseachatom

andthebondangles. AndthisoneIcallthe“road-kill”model.

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Pentane,C5H12hasonlyhydrogenandcarbonatoms

andisamemberofthehydrocarbonfamily.

Carbon’selectronega3vityis2.5andhydrogen’sis2.1.Theyaresocloseinvalue

thatC-Hbondsarerela3velynon-polar.Thepentanemoleculeis

func3onallynon-polar.

ThatmeansthepentanehasonlyLondondispersion

intermolecularforces.

Ooo!Ooo!Londonforcesarepropor3onaltoMM.Thismaybegood-to-know!

1.Intermolecularforces

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Theothertypeofmoleculewewillbestudyingtodayisanalcohol.Hereispentaneand1-

pentanol–theylookrathersimilarexceptforanimportance

difference!

Thehydrocarbonendof1-pentanolisnon-polar,buttheendwiththe–OHgroupisacompletely

differentstory!

1-pentanol

pentane

O-Hasinoxygen-hydrogen!

Electronpairs!Ohboy!Hydrogen

bonding!

1.Intermolecularforces

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2.Connec&ngrateofevapora&onwithintermolecularforces

1-pentanolLondondispersionforces

andH-bonding

pentaneLondondispersionforces

1-Pentanolshouldhavelargerintermolecular

forcesduetoH-bonding,meaningthemoleculesaremore

aVractedtoeachotherthaninpentane.

Evapora3onrequiresthebreakingofall

intermolecularforces.Itwouldtakemoreenergytobreaktheintermolecularforcesin1-pentanol.

Whenevapora3ontakesplace,theheat

comesfromthesurroundings.

IfeellikeIcanreadhermind!

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1-pentanolLondondispersionforces

andH-bonding

pentaneLondondispersionforces

So1-pentanolisslowertoevaporateandthesurroundingsdon’tgetquiteas

cold.

Atthetemperatureofthelab,fewermoleculesof1-pentanolwould

haveenoughenergytoevaporate.

Sothewimpiertheintermolecular

forces,thecooleritgetsduetoevapora3on

Iknowitseemsbackwards…

2.Connec&ngrateofevapora&onwithintermolecularforces

Soooo,largeΔTmeansfastevapora3on …andhigh

vaporpressuremeansweakintermolecular

forces!…andfastevapora3on

meansalotofmoleculeshavesufficientkine3c

energytoescapetheliquidwhichmeanstheliquidhasahighvapor

pressure

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2.Connec&ngrateofevapora&onwithintermolecularforces

Soooo,let’scompletethislastcolumn

together…

Herearethemoleculeswearestudyingtoday.

MoleculeFormulaMM(g/mol)IMF?

water H2O 18 LDF+H-bonding

ethanol C2H5OH 46

hexane C6H14 86

pentane C5H12 72

pentanol C5H11OH 88

methanol CH3OH 32

9Go!

2.Connec&ngrateofevapora&onwithintermolecularforces

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3.Procedure:UsingLoggerPro

We’lluseLoggerProforthefirst3me.Makesureitis

downloadedontoyourlaptop.

Seehowtheproberestsovertheedgeofthetable?

DownloadlinkatourChm204website.

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Thisisatypicalcoolingcurve.Evapora3onisendothermicsothetemperaturedrops.

Seewherethetemperaturestartstodrop?Thatis

whentheprobewasremovedfromthe

liquidandevapora3onstarted.

Hereatabout200s,evapora3onis

completeandyoucanstop.

Temperatureisameasureofenergy…sowhenenergygoesintomakingliquidevaporate,thesurroundingscooldown

3.Procedure:UsingLoggerPro

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Thisistheini3altemperature–anywherealongtheflatlinebeforetheprobeisremovedfromtheliquid

ΔT=Tfinal-Tini3alAndI’mstanding

onthefinaltemperature–whenthingsflaVenoff.

3.Procedure:UsingLoggerPro

4.Procedure:Whattodotoday

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I.  Wearyoursafetyglassestoday.

II.  Wefollowthemanualcarefully–excepteachpairofuswillonlydothreeandwaterisnotlistedinmanual.

III.  Youandyourpartnerwilldothreesolventsandtheotherpeopleatyoursta3onwilldotheotherthree.Thetwosetsare:

GroupA.Water,ethanol,hexane

GroupB.Pentane,pentanol,methanol

IV.  Recordobserva3onsanddetailsascarefullyaspossible.

S&ckpeopleinspiredbyxkcdcartoonsbyRandallMunroe

(www.xkcd.com)

IfyouareatSta3on1A,2A,etc.,youdoGroupAsolvents.Sta3ons1B,2BandsoondoGroupB

solvents.

5.Yourlabreport

S&ckpeopleinspiredbyxkcdcartoonsbyRandallMunroe

(www.xkcd.com) 14

①  First,thecoverpagewithTAini&als.②  Next,thetrimmedcopypagesfromyourlabnotebook

stapledtogether.③   On-lineresultsdueattheendofclasstoday..Late

submissionsarenotgraded–seethesyllabus.④  Turninlabreporttodayorbeforethestartofclass

tomorrow.Latelabsmaynotbegraded–seethesyllabus.

LoggerProwillbeusedforthelasttwolabs.

Also,dressforamess–bothweeks!