Download CHM111 Lab – Atomic Emission Spectroscopy – Grading Rubric

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CHM111Lab–AtomicEmissionSpectroscopy–GradingRubric
Criteria
Pointspossible
Pointsearned
LabPerformance
Printedlabhandoutandrubricwasbroughttolab
Followedprocedurecorrectlywithoutdependingtoomuch
oninstructororlabpartner
Workspaceandglasswarewascleanedup
3
3
1
1
2
4
2
2
2
20
LabReport
Observationsanddatarecordedwithproperunits
Correctidentificationofunknowns
Calculationsshownclearlyandcompletelywithunits.
Question1
Question2(calculationsshownindetailwithunits)
Question3(calculationsshownindetailwithunits)
Total
Subjecttoadditionalpenaltiesaspertheinstructor
AtomicEmissionSpectroscopy
Introduction
Spectroscopy is the study of the interaction of light with matter. This interaction can be in the form of the
absorptionortheemissionofelectromagneticradiation.Whenelementsorcompoundsareexposedtolargeamounts
ofenergyintheformofheat,lightorelectricity,theymayabsorbthisenergy.Whenenergyisabsorbedelectronscan
jumpfromtheirgroundstate,orlowestenergylevel,toanexcitedstate,orhigherenergylevel.Electronsinexcited
statesareunstableandwilleventuallyreleaseenergyagaintoreturntolowerenergystates.Thisreleaseofenergyis
whatweobserveinatomicemissionspectra.
Abasicprincipleofquantumtheorystatesthatelectronscanonlyhavecertainspecificenergylevels.Hence,
when electrons move from one energy level to another, a specific amount of energy (a quantum) is released or
absorbed.Theamountofenergyinanyformofradiationisdirectlyproportionaltoitsfrequency(E=hν),sotheenergy
emittedwhenanelectronmovestoalowerenergystatewillhaveadistinctfrequencyandwavelength.Takentogether,
all of the wavelengths of light emitted from a particular atom from these electron movements constitute that atom's
emissionspectrum.Eachelementorcompoundhasadistinctemissionspectrumthatcanbeusedtohelpidentifyit.
Atomicemissionspectracanbethoughtofasatomicfingerprints.
Whenahighelectricalpotentialisappliedtoatubeofhydrogengas,theatomswillabsorbsomeoftheenergy
and reemit it as light. The distinct wavelengths emitted appear as lines when viewed through a spectroscope.
Hydrogenemitslightintheinfrared,visibleandultravioletregions.Thelinesinthevisibleregion,whichcorrespondto
electronsdroppingfromhigherenergylevelston=2,areknownastheBalmerseries.Youwillobservethelinesofthe
Balmerseriesinthislab.HydrogenalsoemitswavelengthsintheUVregion,knownastheLymanseries,whenelectrons
drop to n=1, and in the infrared, known as the Paschen series, when electrons drop ton=3. Theenergylevelsofthe
hydrogenatomareschematicallyrepresentedinthediagrambelow.
CalculationsInvolvingEnergyLevelsofHydrogen
Theenergyoftheelectroninahydrogenatomisgivenbyequation(1)wherenisthequantumnumberofthe
energylevel(n=1,2,3,...)
(1)
Theenergydifferencebetweentwoenergylevelsinhydrogenisgivenbyequation(2)wherenfisthefinalenergylevel
oftheelectronandniistheinitialenergyleveloftheelectron.Thiscanbesimplifiedtoequation(3).
(2)
(3)
Whennf>ni,ΔEwillbepositive–energyisabsorbedwhenanelectrongoestoahigherenergylevel.Whennf<ni,ΔE
willbenegative–energyisreleasedwhenanelectrondropstoalowerenergylevel.Theenergydifferencebetween
levels(ΔE)isequaltotheenergyofthephotonabsorbedoremitted(Ephoton).Theenergyofaphotoniscalculatedwith
equation(4)wherePlanck’sconstant(h)=6.626x10-34Js,andthespeedoflight(c)=3.00x108m/s.
(4)
Oncewemeasurethewavelengthoflightinthehydrogenspectrum,wecanuseequation(4)todeterminetheenergyof
thephotons.Sinceweareobservinghydrogen’semissionspectrum,wemustusethenegativeEphotonvalueforΔEin
equation(3).
(5)
WeknowthatfortheBalmerSeries(thevisiblewavelengthsofemittedlightthatwillbeobservedintoday’slab)nf=2.
Wecanuseequation(3)tocalculatetheinitialenergylevel(ni)thattheelectrondroppedfrom.InpartBofthe
experiment,youwillmeasurethewavelengthsemittedbyhydrogenatomsandultimatelydeterminewhichenergylevel
transitionitcorrespondsto.
Everyelementhasadistinctspectrumwhichcanbeusedtoidentifyit,muchlikeafingerprint.Heliumwas
discoveredwhenscientistslookingatlightfromthesunnoticedanabsorptionspectrumpatternthatdidn’tcorrespond
toanyknownelement.InpartAofthethisexperiment,threelampscontaining“unknown"gaseousatomswillbe
analyzed.Youwilldeterminetheiridentitybycomparingyourobservedwavelengthvaluestoatableofknownvalues.
NotethattheBohrequation(3)onlyappliestohydrogen,sowewillnotcalculateenergylevelsofotherelements.
UsefulPhysicalConstantsandConversionFactors
Planck'sconstant
h=6.626×10−34Js 1angstrom=10─10meter
speedoflight c=3.00×108m/s
1nm=10─9meter
LaboratoryActivity
Equipment
handheldspectroscopes,spectroscopes,5000volttransformer,lampscontainingH,He,Hg,andNe.
SafetyHazards-DONOTTOUCHTHELAMPORTHEMETALCONNECTIONSWHILETHEAPPARATUSISON!
Instructors:Pleasesetupeachapparatusforallthelamps.Placeeachpowersupplyononeofthelabjacksandadjust
theheightofthelabjacksothattheslitonthespectroscopelinesupwiththecenterofthelamp.Insertthelamp
carefullyinthepowersupplybeingcarefulnottotouchthemiddleofthelamp.Onlyhandletheendsofthelamp.
Students:
PartA
1.Holdaplastichandheldspectroscopetothefluorescentlightsinthe
room,thentosunlightcomingthroughthewindow.Aligntheslitwiththe
brightestpartofthelightforthebestresults.Describewhatyouobserve.
2.Turnononeoftheunknownlampsforuptooneminute.Youmay
touchthesidesoflamptosteadyitasyoufliptheswitch,butdonot
touchthecenterofthelamportheelectricalconnections.Afterone
minute,turnthelaboffandallowittorestforatleastoneminutebefore
turningitonagainifnecessary.
3.Lookthroughthespectroscopesinfrontofthelampsforunknowns#1,
#2,and#3.Recordthewavelengthsofthemajorlinestotwosignificant
figures.Notetheunitonthespectroscope.Forsomeoftheunknowns,
youwillseeanalmostcontinuousspectrumratherthandiscretelines.
Recordtherangeandcenterwavelengthofthisbroadbandofcolor.
ComparethewavelengthstothoseinTable1anddeterminetheidentity
ofeachunknown.
PartB
3.UsethesametechniqueasinPartAtorecordthecolorandwavelengthsofthehydrogenspectrum.Youshouldbe
abletoseethreeorfourlines.UsethesewavelengthstocalculatetheΔEforthetransitionandthentheinitialenergy
leveloftheelectronforeachtransition(ni)Rememberthatnf=2forthevisibleregion(theBalmerseries).
AtomicEmissionSpectroscopy:DataSheetName_____________________________
PartA:
1.Recordobservations:
fluorescentlight
sunlight
Basedonwhatyouobserved,describeisthedifferencebetweenacontinuousspectrumandalinespectrum.
2.Recordcolorandwavelengthofmajorspectrallinesfortheunknowns.Includethecorrectunitswiththewavelength.
Unknown#1
Unknown#2
Unknown#3
Color
Wavelength
Color
Wavelength
Color
Wavelength
Usethefollowingtabletodentifyeachoftheunknownelements:
CHARACTERISTICSPECTRALLINES*
He(nm)
Hg(nm)
Ne(nm)
447
405
540
502
436
583
588
546
585
668
579
640
707
Unknown#1__________________Unknown#2__________________Unknown#3__________________
*Thisisanabbreviatedtable,butitshowssufficientwavelengthsforidentifyingtheunknownelements.Longertables
areintheHANDBOOKOFCHEMISTRYandPHYSICS
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AtomicEmissionSpectroscopy:PostLabName_____________________________
PartB:
1.Recordcolorandwavelengthsforthehydrogenspectrum,thenuseequations(4),(5)and(3)tocalculateΔEandni.
Color wavelength Ephoton
nf ni(makeΔEnegative,useseparatepageifneeded)
2
2
2
2
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AtomicEmissionSpectroscopy:PostLabName_____________________________
1.Rankthefollowingradiationsfromshortesttolongestwavelength.
Radiowaves
Infraredwaves Gammarays
microwaves Xrays
Shortestλ____________________________________________________________longestλ
2.WhichofthefollowingelectronswillemitlightofLONGERWAVELENGTH?
Anelectrondroppingfromn=3ton=2ORanelectrondroppingfromn=4ton=3?Calculatethewavelengthfor
eachtransitiontojustifyyouranswer.
3.Theionizationenergyistheenergyneededtoremoveanelectronfromanatomwhichcorrespondstoaraisingthe
electronfromn=1toanorbitthathasn=∞.Whatistheenergyneededtoremovetheelectronfromahydrogenatom?
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