BuildingaSpectrometertoExploreInfraredRadiation
andGreenhouseGases
Anexplorationofclimatescience:thethermaleffectsofmolecularabsorption
ofinfraredradiation
Goals:
1. Toconductanexperimentcomparingthethermaleffectofinfraredradiation
(light)onnitrogenandcarbondioxidegases.
2. Toexplorea“MoleculesandLight”simulationtogaininsightintowhathappens
whendifferenttypesofmoleculesareexposedtoinfraredlight.
3. ToexploretheAmericanChemicalSocietyClimateScienceToolkit:aresourcefor
understandingandcommunicatingclimatescience.
4. Tocommunicateyourobservationsandfindingsbyconstructingaposterand
presentingyourresultstothelabclass.
Introduction
The American Chemical Society (ACS) has created The ACS Climate Science Toolkit
(http://www.acs.org/content/acs/en/climatescience.html) to help you understand and
communicatethefundamentalscientificprinciplesinvolvedinclimatescience:
“Global climate change, whether a result of natural variability or of human
activity, is a vital issue for life on Earth and involves many processes and
conceptsrelatedtochemistry.Engagingwiththisissueindeliberativediscourse
with colleagues and others requires understanding the fundamental science
thatdeterminesEarth’sclimate.Thisfundamentalscienceisthecorecontentof
theACSClimateScienceToolkit.”
In this lab, you will use a technique called infrared spectroscopy, which involves the
absorptionofinfraredradiationbyamoleculetopromoteitfromalowervibrationalenergy
leveltoahighervibrationalenergylevel.Youwillusethistechniquetoexplorehowinfrared
radiationinteractswithnitrogenandcarbondioxidegases.
1
Whatisinfraredlight?
Infrared (IR) light is a type of
electromagnetic radiation. It is
associatedwithlessenergythanthe
wavelengths of electro-magnetic
radiation that we call visible light.
IR light is invisible to the human
eye,butwecanfeelit.TheFigure
here shows a portion of the
electromagnetic spectrum (EMS)
illustrating
the
relationships
between wavelength, frequency
andenergy.
When you’re standing near a woodstove, the warmth you
sense is the result of your skin absorbing infrared light
emittedfromthestove.Youmayalsobefamiliarwithaway
of “seeing” infrared light: thermal imaging. Thermographic
camerasproduceimagesbydetectinginfraredlightemitted
by surfaces. The Figure here shows a hot plate turned on.
Whiterregionsoftheimagecorrespondwithmoreinfrared
light being emitted from these areas and hotter surface
temperatures.
Whatisinfraredabsorption?
WhenlightintheEMSisabsorbed,lightquantamatchingtheenergyspacingrequirements
areacceptedtoincreasetheenergyoftheatomormolecule.ForUV-vis,thisinvolvesthe
promotionofanelectronfromalowerenergytoahigherenergyorbital.Forinfrared,it
involvesvibratingbonds.Whenabsorptionoflightoccurs,itincreasesthevibrationalstate,
whichasshowninthecartoonbelow,canresultinthebondlengthincreasing.Wheninfrared
light is absorbed, various changes in the vibrational states can occur, including bond
lengthening,and,ifthreeormoreatomsareinvolved,bending.
hn
ž
In today’s experiment you will use an infrared thermometer to observe how plastic bags
filledwithnitrogen(N2)gasandcarbondioxide(CO2)gasinteractwithinfraredlight.You
willdothisbybuildinganinfraredspectrometer,whichhasanIRsource,sample(bagfilled
withgas),anddetectorallinastraightlineasshownbelow:
Infraredheatsourcežsample(bagfilledwithgas)žinfrareddetector
2
Infraredspectrometers
TheIRspectrometeryouwillbuildandusetodayisasimplifiedversionofaninstrument
thatchemistsuseinresearchlabscalledaFourierTransformInfrared(FTIR)Spectrometer.
An FTIR spectrometer is used to record the amount of infrared radiation transmitted
throughasampleasafunctionoftheenergyoftheinfraredradiation(seeFigurebelow).
There are several online databases that provide infrared spectra of many different
molecules; see for example the National Institutes of Standards (NIST) database at
http://webbook.nist.gov/chemistry/.
Infraredspectrumofroomair.Yaxis:transmittancevs.Xaxis:wavenumber(cm-1)
Whatdoesaninfraredthermometer(sensor)measure?
IRsensorsallowyoutomakeinferencesabouttheamountofinfraredlight
emittedfromthesurfaceofanobject.Forexample,seetheimagesofthehot
plates shown below. The IR sensor contains an electronic element that
converts the rate of incoming infrared photons to an electric current. The
magnitude of the current is measured and converted into a temperature
reading. The higher the amount of infrared light entering the sensor, the
higherthetemperaturereadingonthesensorwillbe.Pleasenotethattheaccuracyofthe
sensor may not be very good. However, we can experimentally adjust for this by taking
relativemeasurementsofthetemperature.Wecancomparethechanges
intemperaturewegetwheneachbagfilledwithadifferentgasisplaced
inthesampleholder.
23()'/)"+
The distance between the sensor and the target object affects the
diameterofthebottomoftheconethatthesensorcan“see.”(Seepicture
attheright).Thelargerthedistancethesensorisfromtheobject,the
largerthediameterofthebottomofthecone.TheIRsensorhasalaser
tohelpyouseewhereitisaimedbutitisnotinvolvedinthemeasurement.
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./-+&+'0(%.1!#
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6'&)*+'0
!"#$%&#'(&)(&()"*+,'(
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3
A.
B.
C.
Imagesofhotplate.A.Photographbeforeturningonhotplate.B.Thermalimagejustafter
turningonhotplate.C.Thermalimageafewminuteslater(Seekthermalcamera).
Hazards
Ø Donottouchthehotsurfaceofthehotplate.
Ø Placingthehotplateonitssidemaycreateaburnor
fire hazard. Precautions need to be taken to ensure
thatthesurfaceitisplacedonisnonflammableandan
appropriatenoticeispostedtowarnstudentsofthis
potentialhazard.
Ø DoNOTtestmethaneoranyotherflammablegases.
Ø Theinfraredthermometerusesalaserforalignment
purposes.Pleasedonotpointthelaseratsomeoneor
lookintothebeamofthelaser.
Noteagain:thelaserlightisusefulinaligningthepathofourspectrometerbutisnot
importanttothemeasurementofIRradiation.
Materials:
Chemicals:
Supplies&Equipment:
Nitrogengas(N2)
Carbondioxide(CO2)
Polyethylenebagsandtwist-ties.
Ringstands&clamps
Hotplate
IRthermometer
Plastictesttuberack(sampleholder)
Posterboardandmarkers
Optional:
Deionizedwater
Sodiumchloride(NaCl)
4
Pre-LabAssignment:
1. Writeashortsummaryoftheexperimentsyouwillperformtoday.
2. Prepareasafetytablethatliststhechemicals,anyhazards,andtheprecautionsyouwill
takewhenhandlingthechemicalsandequipment.
Overview
The investigations you will engage in today are open-ended in the sense that you will be
askedtocomeupwithyourowninterpretationofwhatyourobservationssuggest.Forallof
today’sactivities,youwillworkinagroupofthreetofourstudents(twopairsofpartners).
Discussanyquestionsmarkedwitha“Q”asagroup,asyouencounterthem.
Afterperformingtheexperiments,yourtaskwillbetocometogethertoexplainthephysical
phenomenayouhaveobservedandthenuseyourexplanationtomakeapredictionabouta
largerscalescenario.Eachgroupwillconstructaposterandthenpresentyourfindingsto
othergroupsina5-minpresentationattheendofthelaboratoryperiod.Yourlabscorewill
bebasedontheprelabassignment(10pts),theposter(70pts),andthepresentation(20
pts).Inaddition,thereisextracreditavailableforthislab(upto20pts).Allextracreditmust
tobesubmittedwithintwodaysofpresentingtheposter.
Part1
Part2
Part3
Part4
5
Part1:ThePhETsimulation“MoleculesandLight”
Experiment with “Molecules and Light”. PhET
simulations are interactive animations featuring a
varietyoftopicsinchemistry,physics,andmaththat
aredifficulttovisualize.Thesimulationsaredesigned
to provide insight into difficult topics, however, just
liketheanalogiesyouworkedwithinearlierlabs,keep
in mind that they are imperfect representations of
complex physical phenomena. The “Molecules and
Light”simulationallowsyoutoexposedifferentkinds
ofmoleculestodifferentkindsoflightand“observe” ScreenshotforthePhETsimulation:Molecules
whathappens.ToaccessthemoleculesandlightPhET andLight,usedwithpermissionfromPhET
InteractiveSimulations,UniversityofColorado
simulationusethislink:
Boulder,http://phet.colorado.edu
http://phet.colorado.edu/en/simulation/moleculesand-light
Choosethe“runnow”optionandexperimentwiththesimulationtoanswerthefollowing
questions:
Q:WhatdoesthePhETsimulationsuggestabouthowcarbondioxideandnitrogen
reactdifferentlytoinfraredlight?
Q:Whichoftheothergasesinthesimulationbehavethesamewayascarbon
dioxidewhenexposedtoinfraredlight?Whichgasesactlikenitrogen?
6
Part2:IRabsorptionofvarioussubstances:usingahomebuilt
spectrometer.
BuildinganIRspectrometer
Yourlabinstructorcanhelpyourgroupassemblethespectrometerbelow.Animportantfeatureis
thattheremustbeaclearpathfortheIRlighttogothroughtheholesinthesampleholdertothe
infrareddetector.ThehotplatestabilizershouldnotblocktheIRlighttothedetector.
InfraredSource(hotplate)
Sample
PathofIRlight
Sampleholder
InfraredSensor
Ringstand
&Clamp
HotplateStabilizer
Placingthehotplateonitssidemaycreateaburnorfirehazard.Precautionsneedtobetakento
ensurethatthesurfaceitisplacedonisnonflammable.Keepplasticbagsawayfromthehotplate
surface.Makesurethehotplatestabilizerdoesnotblockthebeampathandthatthereisaclearpath
fromtheinfraredsourcethroughthesampleholderandtothesensor.
Yourfinishedsetupshouldlooklikethis.
7
MakingMeasurements
1. ObtainbagsofN2andCO2fromthetankswithyourinstructor’sassistance.Donotoverfill
bags,sothatyoucanpressthemintotheshapeofthesampleholder(asshowninthepictures
below).
A.
B.
PlasticBagFigure.Partiallyfilledplasticbag:(A.)inthesampleholder,and(B.)inthesampleholder-
butpressinggentlydowntomoldthebagintotheshapeofthesampleholder.
2. Tomakemeasurements,onestudentholdsdowntheIRthermometertriggerfor10-15
secondstostabilizethedetectorbeforemeasurementscanbemade.Anotherstudentcan
placethesampleinthesamplecompartment,gentlyholdingdownthebag,whileanother
studentcanrecordthetemperatureofablankmeasurement(nothinginthecompartment)
orasampleinthecompartment.
Theexperiments:
Designasetofexperimentstoexplorethefollowingquestion:
Scientific Question #1: Comparing the absorption of infrared light in the presence and
absence of each different sample of gas, are there any significant differences that can be
observedexperimentally?
UsingtheDesigningExperimentsworksheettosummarizethisprocess,yourgroupshould
describealabproceduretoexplorethescientificquestion#1.Acopyofthisworksheetwill
befixedtotheposterthatyourgroupwillconstructinpartfour.Beforecollectingdata,you
needtoexplainyourproceduretoyourlabTA.
NotethattheplasticbagabsorbssomeIRradiation,butsincewearecomparingthe
absorptionofIRlightinthepresenceandabsenceofeachsample,theinfluenceofthebag
issubtractedoutofthemeasurements.
ExtraCredit-ExtraChallenge:IRspectroscopy(10pts)
Yourgroupcanextenditsdatasetbyinvestigatingothersubstances.Thepresenceofsubstancesuchas
waterorsaltcaninfluenceIRabsorptionintheatmosphere.Yourgroupcandesignexperimentsto
extendyouroriginalstudybyincludingtheinfluenceofwaterorsaltinthepresenceofnitrogenandor
carbondioxide.Watermaybeintroducedintoabag,andthebagmovedaroundtopromotethe
formationofwatervapor.Saltcanbecrushed,toincreaseitssurfacearea,andthenintroducedintoa
bag.Yourgroupcanalsoproposetheinvestigationofothersubstances.Inordertoreceiveextracredit,
youshoulddescribetheresultsonyourposteraswellasinthesummarysubmitted.Caution:DoNOT
testmethaneoranyotherflammablegases.
8
DesigningExperimentsWorksheet
Pleaseusethissheettosummarizeyourlabgroup’sexperimentandfindings.Beforegoinginto
lab,haveyourlabinstructorcheckandinitialit.
Names:
Experiment#:
Signatures:
Section:
Pleasedescribeyour
proposedexperiment.
(attachextrapagesifneeded)
(Checkinwithyourlab
instructorbefore
performingexperiments)
Instructor’sinitials:________
Describethedatayou
collected:
(attachextrapagesifneeded)
Whatclaimscanyou
make?
9
Part3.Thinkingaboutclimatescienceandresults.
A.TheACSClimateScienceToolkit:TheACSClimateScience
Toolkit is a resource designed to provide insight into the
chemistryoftheEarth’sclimate.Inparticular,itdiscusseshow
certain molecules interact differently with infrared light. We’d
likeyourgrouptolookatthiswebsitetoanswerthequestions
below.Theanswersshouldbewritteninyourlabnotebook.
YoucanaccesstheACSClimateScienceToolkithere:
http://www.acs.org/content/acs/en/climatescience.html
ClimateScienceToolkitLogo
usedwithpermissionfromthe
AmericanChemicalSociety.
Q:WhatdoestheACSClimateToolkitpagetellyouaboutwhysomemoleculesinteractwith
infraredlightdifferentlythanothermolecules?
Q:Whatarethepropertiesofagreenhousegas?
Q:Whateffectsdogreenhousegaseshaveontheatmosphere?
B. Inquiry into your experimental results: Each group should discuss your results and
deviseamolecularexplanationforwhatisgoingonwiththegasesandthehotplate.Didyou
observe any differences between gases? The PhET simulation and the ACS Climate Science
toolkitareresourcestohelpyouthinkaboutyourexplanation.Thequestionsbelowaremeant
tohelpguideyourthinkingasagroup.Youdonotneedtoindividuallywriteanswerstoeach
oneinyourlaboratorynotebook.We’dratherhaveyoutalkingandinteractingwitheachother
thanwritingbyyourself!Asagroupyouwillbepresentingyourcollectiveideasinaposter
formattotherestoftheclassattheendoflab.
1. Whatwereyourresults?
2. Howcanthedatabepresentedtohelpunderstandtheresults?
3. Weredifferencesseenthatarestatisticallysignificant?
4. Howdidyourgroup’sprocedureshelporhindergooddatafrombeingcollected?
5. Whatmolecularinterpretationcanbeinvokedtohelpexplainthedata?
10
Part4.Presentingdatatothepublic:posterpresentation
UsetheevidenceyoucollectedfromyourIRsensorexperiment,thePhETsimulation,andthe
ACSclimatesciencetoolkittoanswerscientificquestion#2belowasagroup.Prepareaposter
(poster board and markers are provided) that summarizes your group’s answer and the
reasoningbehindthatanswertosharewiththeclassina5-minutepresentation.
Scientific Question #2: How do you think increasing the concentration of CO2 in the
atmospherewouldinfluencetheamountofinfraredlightabsorbedandtheenergypresent
intheatmosphere?
Duringthepresentationsyouareencouragedtoaskothergroupsquestions!
C.Inordertoreceivecreditfortheposterpresentation,everyoneineachgroup
isrequiredtosubmittheDesigningExperimentsWorksheettoyourTAbefore
yourpresentation.
Yourlabscorewillbebasedontheprelabassignment(10pts),theposter(70pts),and
thepresentation(20pts).Inaddition,thereisextracreditavailableforthislab(upto20pts).
Allextracreditmustbesubmittedwithintwodaysofpresentingtheposter.
Note:therearetwotypesofextracreditpossibilitiesforthisexperiment:
a. ExtraCredit-ExtraChallenge(seepage8):upto10pts
b. Postlabquestion(seebelow):upto10points.
Tosubmitextracredit,youshoulduploadawordorpdffiletoICNexactlyasyouwoulda
labreport.FortheExtraCredit-ExtraChallenge,thefileshouldcontainasummaryof
whatadditionalexperimentsyoucarriedout(seepg8).Forthepostlab extra credit,thefile
shouldincludeyouranswerstothepostlabquestion(shownbelow).
Post lab question (extra credit): Considering the home-built spectrometer below that you worked
with in lab, do you think there is an analogy to be made between how the home built IR
spectrometer works (IR source => Gas Sample => Detector) and the interactions of radiation in
our atmosphere with greenhouse gases? Please explain. In your explanation, you should describe
the similarities and differences between a spectrometer and atmospheric interactions.
InfraredSource(hotplate)
InfraredSensor
Sample
11
Rubric for the Poster
Title and Team
Member Names
Introduction:
Contains title, Team member names, and brief statement about the poster
(5 pts)
ACS Climate
Science Toolkit (5
pts)
ACS Climate Science Toolkit
Explain the information you found on the ACS Climate Science Toolkit
PhET Simulation (5
pts)
PhET Simulation
Designing
Experiments
Worksheet (10 pts)
Designing Experiments Worksheet
Results:
Designing Experiments Worksheet (attach to poster)
Scientific data that
supports the claim.
(10 pts total)
Results
Summarize your experimental data in a table or figure.
Reasoning:
Reasoning:
Present an analysis of your results§
Scientific
explanations that use
evidence and
appropriate chemistry
concepts to construct
claims.
(20 pts total)
Explain the procedure used in the experiments you designed.
§
You may be able to use the PhET simulation in explaining your data.
Claim(s):
Summary of claims:
Statement(s), derived
from evidence, using
scientific reasoning.
What claims can you construct?
(15 pts total)
Explain the “Molecules and Light” simulation
Present your evaluation of the scientific question #2: "How do you think increasing
the concentration of CO2 in the atmosphere would influence the amount of infrared
light absorbed and the energy present in the atmosphere?"
12