H2 Chem Practical (1)

EXPERIMENTALPROCEDURES Titration 1.SF,DP ○ Burette–2d.p. ■ Allnumbersshouldeitherendwith.00or.05–notewhenuarefakingyourdata ○ Pipette–1d.p. ○ Temperature–1dp(halfof0.2degcel) ○ Mr–1dp,nounits 2.Notes ○ Remembertowriteunitsforcalculations–writeatthestartofthepaper+signs(theyseemtostressnegative/positivesignalotforqns,includingchangeintemp/gradientqns,somakesuretoinclsignsaswell) ○ Don’tstressifstuckatmolecalculations,readquestion,readprovidedinfo,understandaimofexperiment ○ Ifuusesolidtomakeastandardsolution,rememberthatinurcalculations,useurmeasuredmassofsolidandnotthemassuweresupposedtoadd. ○ Calculating%uncertainty: ■ Absoluteuncertainty=(½smallestdivision)x(numberofreadings) ■ Burettevolumerequires2readings,so±0.05x2.Masschange/lossalsorequires2readings.Pipetteonlyrequires1readingso0.1x1. ■ Inyourworkingandfinalanswer,remembertoadd±. ■ Egqn:whichpieceofapparatuscausesthegreatestpercentageerrorinyourexperiment?→askingforutocalculateandcompare%uncertainties. ● Enthalpy=mc(changeinT)/amountofLR:whencalculating%ucofchangeinT,needtox2tothe½ofsmallestdivision. ○ %error(difference):

2 ■ noneed± ■ 3sf ○ Ifqngivesumassofacompoundthathaswaterofcrystallisation,theMrthatudividebymustincludemassofwaterofcrystallisation! ○ Ifaskedtocalculatenumberofmolesofelectrons,leaveaswholenumber,cuzoxidationstatesonlyexistaswholenumbers. ○ Memoriseindicatorsandtheircolours(foruniversalindicator,checkthecolourofthepaperagainstthechartofpHvaluesgivenandstatethepHvalue)–methylorangeforacidic||thymolblueandphenolphthaleinforalkaline ○ Ifurnotsurewhatgastotestfor,lookattheeqnatthestartofthepractical→givesyoutheproductstotestfor ○ Whenqnsays‘calculate___change’,makesureyourfinalanswerhasa+/-signtoindicatewhetherthevalueincreasedordecreased. ○ Ifquestionsasksaboutwhythissolutionisacidic,explainintermsofcharge/sizeratio,highcharge/size,letsitpolariseandweakensO-HbondinH2O,causingittoundergohydrolysistogive[complexion],makingitanacidicsolution ○ Forplanningqns,rmbtodrawyourtable ○ Ifaskedtheroleofxyzinaparticularreaction,it’smostlyeitheracatalystoranacid/baseoraR.A/O.A.. ○ Tworeadingsfromaninstrumentmeanstheoveralluncertaintyis2xtheuncertaintyoftheinstrument ○ Ifyoupouroutasolidfromabottleandtheqnasksyoutoweighthebottleafterpouring,inyourtable,youmustplotas“massofemptybottle+RESIDUE”–thisisbecausethereisboundtoberesidueafterpouringasolidout. ○ Youquenchamixturebyeitherdilutingthesolutioninalargeamountofsolvent(likewater)orremovingthecatalystsothatreactionstopshappening. ○ Tabulationofdataandgraphingisahugechunkofmarksforprac,somakesureyoudon’tloseanysillymarksthere.

3 ○ Forinitialratesmethod,youhavetouserateagainstconcgraphtodeterminetheorder(ifthelineisastraightlinepassingthroughtheorigin,thisisfirstorder).Forcontinuousmethod,youwoulduseaconcagainsttimegraph(ifcurvehasaconstanthalflifeoft1/2,thenitisfirstorder). ○ BeSUPERCLEARaboutyourapparatusinyourplanningqn-eg.250cm^3conicalflask,100cm^3measuringcylinder,etc.Don’tlosesillymarksoverthis:( Theyaskedthelasteqnbefore,somaybeit’sgudtoknowthese(mostlythelastonehehe) Indicator Whentheequivalencepointis… AcidicmediumTurningpoint Alkalinemedium Phenolphthalein Alkaline Colourless – Pink Thymolphthalein Colourless – Blue Bromothymolblue- Minion!- Blueinbase Neutral Yellow Green Blue MethylOrange Acidic Red Orange Yellow Screenedmethylorange Purple Grey Green

4 Rememberacidic→alkaline:ROYGBIV Ifaskedforcolorchange,it’salwayscolorchangefromacidic/alkalinemediumtoturningpointcolor,neveracidictoalkalineoralkalinetoacidic. - KMnO4(self-indicating).“Titrateuntilthefirstpermanentpalepinkisobserved.”- Forplanning,juststicktomethylorangeandphenolphthalein.Othersarejustincaseqnsask. ○ Initialburettereading/cm 3 Finalburettereading/cm 3 Titrevolume/cm 3 Addyourticksforconsistentreadings(+-0.10cm 3 )→ 3.Howtowashequipment ○ Burette–tapwater,DIwater,titrant ○ Pipette–tapwater,DIwater,analyte ○ Conicalflask,volumetric/graduatedflask–tapwaterandDIwateronly ○ ImptbutIkeepnotdoingthis:RinseinnersideofconicalflaskwithDIwaternearendpointsothattotalvolumemeasuredbyburettereactswithanalyteinconicalflask ■ Thisisbecausesomeofthetitrantthatflowsintotheconicalflaskmaystickontothewallsoftheconicalflaskinsteadofparticipatinginreaction ■ DIwateraddedwillnotaffecttitrevalue

5 4.TitreLevelQuestions ○ Higherthanexpectedwhen ■ AddingtitranttoburetteimmediatelyafterwashingwithtapandDIwater ● Waterinburettedilutestitrant,lower[titrant]needslargervolumetobereactedwithanalyte,thusincreasingtitrevalue ■ Airgap/columninburettetip ● Solutionwillfillburettetipfirstbeforeitistransferred,hencevolumemeasuredwillbelargerthanactualvolumetransferred,thusincreasingtitrevalue ■ Blowinglastdropofsolutioninpipettetipintoconicalflask ● Noneedtodothisbecauselastdrophasalreadybeencalibratedfor,soadditionofthislastdropintotheconicalflaskwillonlyincreasetheamountofanalyte;greateramountoftitrantrequiredtoreactwithit,thusincreasingtitrevalue ■ Rinsingconicalflaskwithanalyte ● Greateramountofanalyteinvolvedthenthereshouldbe,thusincreasingtitrevalue ■ NotrinsingdownsidesofconicalflaskwithDIwaternearendpoint ● Ipersonallyhaven’tbeendoingthisgg,idkifyouguysdoitbutit’sgoodpracticesoplsdoitforprelimsandA’s!! ● Someofthetitrantonthesidesoftheconicalflaskmaynotreactwithanalytebutstillmeasuredbyburette,thusincreasingtitrevalue ■ Fillingburettetiptoe.g.25.00cm3,thentransferringallofitintograduatedflask ● [standardsolution]higherthanactualsincelargervolumeofsolutionaddedtograduatedflaskthanintended(asitincludesthevolumeinsidethetip–seebelow)

6 ● ● Thusincreasedamountofanalyte,increasedamountoftitrantrequiredtoreactwithit,increasingthetitrevalue ○ Lowerthanexpectedwhen ■ WashingpipettewithonlytapandDIwater,wipingtipwithtissueandthenimmediatelypipettinganalyte ● Waterinsidethepipetteresultsinsmallervolumeofanalytebeingdrawnup;smalleramountofanalyterequiresmalleramountoftitrant,thusdecreasingtitrevalue ■ Airbubbleinpipette ● Analytefilluppipetteto25.0cm3earlierthanitshould,smalleractualamountofanalyterequiresmalleramountoftitranttoreact,thusdecreasingtitrevalue ■ Rinsingconicalflaskwithtitrant ● Someofthetitrantalreadyintheconicalflaskmayhavealreadyreactedwiththeanalyte,solessvolumeoftitrantrequiredtobereleasedfromtheburetteforreaction,thusdecreasingtitrevalue ○ Inconsistentwhen ■ Usingstandardsolutionwithoutinvertingorshaking ● [standardsolution]inconsistentas[analyte]notuniform,couldbehigherorlowerindifferentpartsofthegraduatedflask,hencetitrevaluewillbeinconsistenttoo 5.Typesoftitration ○ Acid-base–nochangeinoxidationnumber,useindicator ○ Redox–changeinoxidationnumber,someareself-indicating(e.g.whenMnO4-⇒Mn2+,purple⇒permanentpalepink),somestillrequireindicator(e.g.iodometrictitrationrequiresstarchindicator)

7 ■ Iodometric ● Procedure+reactions ○ AddoxidisingagentXwithiodidecontainingsolutionintoconicalflask ○ AddH2SO4intotheconicalflaskaswell–providesacidicmediumforreactionbetweenOAandiodidetoproduceH2O,wherebytheHcomesfromH2SO4 ○ X+I-+H+→I2+anotherformofX+H2O ○ NowtitrateI2withS2O32-untilsolutionturnspaleyellow,thenadd10dropsofstarch,titratetillblue-blackturnscolourless ○ I2+2S2O32-→S4O62-+2I- ● Colourchangestonote ○ Rxn1:Colourless(iodide)⇒brown(iodine)⇒paleyellow(nowthisiswhenyouaddstarch,otherwiseit’shardtotellwhenpaleyellowturnscolourlesswhenreactionfinishes,sogottamakeitblue-blackforcolourchangetobeobvious) ○ Rxn2:Paleyellow+starch=blue-black(I2turnsstarchblue-black)⇒colourless(reactioncomplete!)(cuzI2hasbeencompletelyreactedawaybyS2O3 2- ) ● Whyisstarchonlyaddedwhentheend-pointisnear ○ Ifstarchwasaddedinlargeamountatthestartoftheexpt,I2formsinsolublestarch-iodinecomplex ○ I2takeslongertocompletelyreactwiththiosulfate ○ Dischargeofblue-blackcolorationisdelayed ○ Toobserveendpointcolorchange(blue-blacktocolourless),morethanexpectedvolumeofthiosulfatemaybeadded ○ Titrevolumeislargerthanexpected

8 ○ Backtitration&gascollectionoverwater ■ Backtitrationinvolvessolidsthatareinsolubleinwater/whenreactionbetweenanalyteandtitrantisveryslow

9 QualitativeAnalysis 1.Organic Precautions ● Don’tpouror