11 dh neutralization - creighton...

Experiment1112November2019

EnthalpyofNeutralization

Oh!Thethingswewilldo.Oh!Thethingswewilllearn!And

allinjustthreehours.

H3O+(aq)+OH-(aq)à2H2O(l)ΔHrxn=?

qsurr=csol’nxmsol’nxΔTsol’n

qsurr=-qrxn

Objective:Tousecalorimetrytomeasuretheheatreleasedbyachemicalreaction.

Overview:1.  ThereactionandΔHrxn2.  Measuringheat,q,bycalorimetry3.  Dataanalysisandcalculations4.  Procedure:Whatwedotoday5.  Yourlabreport

Theconceptofmeasuringheattransferredcanbeextendedtoallsortsof

situations.

Todaywewillbemeasuringhowmuchheatisreleasedwhenastrongacidandastrongbasearereacted.

2

Hereisareactionsimilartotheoneyouaredoingtoday.Thisistheoverallreaction.Ifwebreakthestrongacidandsolublesaltsintoions,wegettheionicequation,shownbymyfeet.

1.  ThereactionandΔHrxn

HClO4(aq)+LiOH(aq)àLiClO4(aq)+H2O(l)ΔHrxn=?

H3O+(aq)+ClO4-(aq)+Li+(aq)+OH-(aq)à

Li+(aq)+ClO4-(aq)+2H2O(l)ΔHrxn=?

SeehowhemadeH+intoH3O+?Hehadtoadda

watertotherightsidetokeep

thingsbalanced.

3H3O+(aq)+OH-(aq)à2H2O(l)ΔHrxn=?

Cancelstuff!Beruthless.Thenetionicequationis

simply…

Hereisadramatizationofwhatthereactantsaredoing.Whatyouseedepictedhereistheionicreaction.–themiddlereactiononthepreviousslide.Notethatwaterisalsoa

product,butwatermoleculesarenotshown.

H3O+H3O+

ClO4-

ClO4-

ClO4-

+Li+

Li+

Li+Li+

OH-

OH-

OH-

OH-

Li+ClO4-

ClO4-

ClO4-

ClO4-Li+

Li+Li+

Andherearetheoverallandnetionicequations 4

H3O+(aq)+OH-(aq)à2H2O(l)ΔHrxn=?HClO4(aq)+LiOH(aq)àLiClO4(aq)+H2O(l)

Weuseabout60mLofbothacidandandbase.Recordtheprecisevolumeoftheacidyouusedto+/-0.1mL.Dothesameforthevolumeofbase.


H3O+


5

Yourreactionwillbeoneofthesefour–thetwoacidsare~2.0MHCl(aq)and~2.0

MHNO3.Thetwobasesare~2.0MNaOH(aq)and~2.0MKOH(aq).

2.HCl(aq)+KOH(aq)àKCl(aq)+H2O(l)ΔHrxn=?3.HNO3(aq)+NaOH(aq)àNaNO3(aq)+H2O(l)ΔHrxn=?4.HNO3(aq)+KOH(aq)àKNO3(aq)+H2O(l)ΔHrxn=?

1.HCl(aq)+NaOH(aq)àNaCl(aq)+H2O(l)ΔHrxn=?

Fourcombinations.

Notmuchroomformehereandthedrawingmakesmelooklikeaspider!Anyway,inyourlabnotebook,writetheoverallequationforyourcombination(oneofthe4givenabove),theionicandnetionic

equationsforyourcombination.Compareyouranswerwiththeotherpairsaroundyourstation.Whatdothenetionicshaveincommon?

6

Ok,zoomoutforasecond…Haveyoufiguredoutthatallthesereactionsgivetheverysamenetionicequation?Theotherionsfromtheacidandthebasearespectatorions.

Theonlythingthatproducesheatistheacid-basereactionshownabove.Thespectatorionsare…justalongfortheride.

Tocatchtheheat,wearegoingtouseacalorimeter.Calorimeterscanbeveryexpensiveandextremelyprecise.Buttheydon’thavetobe.

Theonewewilluseisrightbehindmehere.



7

Ok,payattention.Thiswillbeonthequiz.Heatisreferredtobythesymbolq.Andqisanextensiveproperty.

2.Measuringheat,q,bycalorimetry

Thatmeansqdependsontheamountofstuffthatproducedtheheat.Forexample,ifweused0.2molesofacidand0.2molbase,wewouldmaketwiceasmuchheatthanifweused

0.1molofeach.


Hereistheequationwewilluse,wherecisthespecificheatinunitsofJg-1deg-1.

qsurr=csol’nxmsol’nxΔTsol’nAndmisformass.FYIInfofor

Introduction

8

Thereactionproducestheheat.Thesolutionholdstheheatandthe

calorimeterminimizesheatlossfromdissipation.

Thisisthecalorimeter!The

thingystickingoutofitisthetemperatureprobeanditisusedwithLoggerPro.Weuseditlastweekwiththeintermolecular

forceslab.


Seeqsurr?The“surrounding”isthesolutioninthecup–

errrr–calorimeter.

Youcancallqsurr“qcal”or“qsol’n”–it’sallthesamething.


9

We’lluseLoggerProagain.PlugintheprobeandlaunchLoggerProonyourlaptop.

Seehowsheisswirlingthesolution

duringdatacollection?

ThisnicestraightlinethatI’mseeingmeanstherewasexcellentswirling.


10

Thisishowcoolinglooks.Wewantto

knowthetemperaturebeforetheheatlossstarted.ThatwillbeTfinal.Thetemperaturewearelookingforisin

theredcircle.

TherearetwowaystofigureoutTfinal.Let’sfindoutaboutthem…

Youwillneedtoknowtheexacttimewhenyouaddedthechemicals

togetherandthereactiontookplace.

HerearethefirstdatapairsfromLoggerPro.You

canseethatTinitial=20.4oC.Seewhenthe

chemicalswereaddedtogether.

Tinitial

Tfinal

3.Dataanalysisandcalculations

Theprecisetimethechemicalswereaddedwas77s,asmeasuredwiththestopwatch

featureonaphone.ThelabmanualdoesnotmentionthisinStep5..

11

Displaytheequationofthegreenline.

BothwaysofdeterminingTfinalrequireustocreatealinethatgoesthroughthelinearcoolingcurveportionofthegraph.

Thisisshownabovewithagreenline.YourTAorDr.Mattsonwillshowyouhowtodothis.Wejustimaginethebluelinemarkingthetimewhenthechemicalswereaddedtogether.

Makesureyourecordthistimeinyourlabnotebook.

Tfinal=???Thisisn’thowwefigured

outTfinallastweekwhenwedidintermolecularforces.Thiswayisappropriateforsituationsinwhichthe

reactionisinstantaneousuponmixingandfollowedbyheatloss.Lastweek,wewerewatchingaprocessthattookseveralminutes.


Today,however,wewillusetheequationofthelinetodetermineTfinal.TheTAsorDr.Mattsonwillhelpyoudisplaytheequationofthelineshowningreen.Itis:

Tfinal=??? temperature=mt+bm(slope)=-0.003759deg/sb(y-intercept)=33.57oC

Thisbluelinerepresents77s,theexacttimethesolutionsweremixed.

OnewaytodetermineTfinalistoplaceyourcurserrightatthe

intersectionofthegreenlineandtheexacttimethechemicalsweremixed.ThetemperaturedisplayedisTfinal.

HereisTfinalfromusingtheformulaandpluggingin77sfortime:

Tfinal=-0.003759x77+33.57oC

=-0.289443deg+33.57oC=-0.29deg+33.57oC(sigfig)=33.28oC(sigfig)

12Infofor

calculations


WeknowTfinalandTinitial.SowecanfigureoutΔT.Thesubtractiongivessigfigsto

thetenthsplaceonly.

13

ΔT=Tfinal–Tinitial=33.28oC–20.4oC=12.88deg=12.9deg(sf) qsurr=csol’nxmsol’nxΔTsol’n

FromΔT,wecancalculateqsurr.Ofcourseweneedtoknowthespecificheatandthemassofthesolution…

Thespecificheatofthesolutionwemadeis3.71Jg-1deg-1.Itsayssointhelabmanual.Werecorded

theexactvolumesofacidandbase–wewillusethatnext.

Let’ssuppose,forexample,weused

60.8mLHClacidand59.7mLKOHbase.Whathappensnext?Infofor

calculations


14

DensitiesofOurAcidsandBases

Weneedthemassofthesolutionnext.The

densitiesofthetwoacidsandtwobasesaregivenin

thetable.

WhatwasIsaying?Oh,yeah.Let’ssupposeweused

60.8mLHCl(aq)and59.7mLKOH(aq).

2.0MHCl1.033gmL-1

Infoforcalculations

2.0MHNO31.065gmL-12.0MNaOH1.079gmL-12.0MKOH1.093gmL-1

Sooo…60.8mLoftheHCl(aq)and59.7oftheKOHwouldhaveamass

of…

60.8mLx1.033gmL-1+59.7mLx1.093gmL-1,

or128.1g


qsurr=csol’nxmsol’nxΔTsol’nNowwe’recooking!Wehaveeverythingweneed–specificheat,massandΔT!Let’sbecarefulwith

theunits.

=3.71J128.1g12.88deggdeg

=6121.497595J=6120Jwith3sigfigs

Infoforcalculations

Remember,ΔThasonlythreesignificantfigures,12.9deg(Slide13),butweusethe

numberobtainedforallcalculationsandthenattheend,adjustforsignificant

figures. 15Progress.


Thevalueforqwillincreaseordecreasewithamountofstuffused.Thisiscalled

anextensiveproperty.

16

Ok,let’spulluphereforasecondandthink

whatthismeans.Ourcoffeecupcalorimetercollected6120Jofheat.

Ahhh,thedistinctsmellofaquiz

question.


Wouldwehavecollectedmoreheatifwehadused50mLofacidand70mLbase

insteadofabout60ofeach?Thinklimitingreagent...

Weused~2.0Macidand~2.0Mbase.Wouldwehavecollectedmoreheatifwehadused30mLof3.0Macidand60mLof1.5Mbase?Whatelsewouldhave

changed?(assumethespecificheatisnotchangedmuch)


So,thesolutioninthecalorimetercollected6120Jofheat.Thisheatcamefromourreaction–whichwehaven’t

seensinceSlide7!Thecalorimetergainedheat

becausethereactionreleasedheat.Themathematical

relationshipisprettysimple:


qsurr=-qreaction

Wearegettingclosehere.Wecanalmostcalculate ΔH,then

cleanupandgohome.

Rememberinourexamplehowweused60.8mLacidand59.7mLbase?Wewillbegiventheexactmolaritiesoftheacidandbase.

Supposetheacidwas2.040Mandthebasewas1.962M.Usingn=MV,wecancalculate

themolesofacidandmolesofbase:

nacid=2.040molL-1x0.0608L=0.1240molH3O+

nbase=1.962molL-1x0.0597L=0.1171molOH-

17Infofor

calculations



The-6120Jfromthereactionisassociatedwiththelimitingreagent–inourexample,the

base.FromthatwecancalculateΔH!

18

ΔHneutralization=qrxn/nLR

=-6120J1x10-3kJ

0.1171molOH-1J=-52.27581208kJ/mol

SeehowtheequationfeaturesoneOH-?SoΔHhastobeforone

moleofOH-.

Theliteraturevalueis-55.8kJ/mol.Ooops.

So…isn’tthatalotofsignificant

figures?


4.Procedure:Whattodotoday

19

Wefollowtheprocedureinthemanual,exceptforStep5,whichshouldmentionthatyouneedtorecordthetimewhentheacidwasadded.

SeeSlides10-12forhowwedetermineTf.

Ifyourgraphlooksgood(meaningyouswirledwithoutsplashingstuffaround),youaredoneafterjustonetrialwiththeexperimentalpart.Notsogood?Thereistimetotryagain!

Includeallthecalculationsshowninthispresentation,

alongwithyourcalculationofpercenterrorfromthe

literaturevalue..

Inyourconclusion,compareyourΔHvaluewiththatofanotherpairof

students.

5.Yourlabreport

StickpeopleinspiredbyxkcdcartoonsbyRandallMunroe

(www.xkcd.com) 20

①  First,thecoverpagewithTAinitials.②  Next,thetrimmedcopypagesfromyourlabnotebook

stapledtogether.③   On-lineresultsdueattheendofclasstoday.Late

submissionsarenotgraded–seethesyllabus.④  YourLoggerProchartattached.⑤  Turninlabreporttodayorbeforethestartofclass

tomorrow.Latelabsmaynotbegraded–seethesyllabus.

Dressforamess–thisweekandforExperiment12!

ChemLabwiththeStickPeopleandBirdwascreatedandproducedbyDr.BruceMattson,CreightonChemistry.Enjoyitandshareitifyouwish.

11 dh neutralization - creighton...

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