Download CHM112 Lab – Heat of Neutralization – Grading Rubric

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CHM112Lab–HeatofNeutralization–GradingRubric
Criteria
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Pointsearned
LabPerformance
Printedlabhandoutandrubricwasbroughttolab
Safetyandproperwastedisposalproceduresobserved
Followedprocedurecorrectlywithoutdependingtoomuch
oninstructororlabpartner
Workspaceandglasswarewascleanedup
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3
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11
20
LabReport
CalculationsQ1-Q11(1pointeach)
Total
Subjecttoadditionalpenaltiesatthediscretionoftheinstructor.
EnthalpyofNeutralization
Introduction
Inthecourseofmostphysicalprocessesandchemicalreactionsthereisachangeinenergy.Inchemistrywhat
isnormallymeasuredisΔH(enthalpychange),thechangeinheatatconstantpressureandignoringanyworkdoneby
thereactingsystem.Ifthereactionisexothermic,heatisgivenoffandΔHhasanegativevalue.Whenheatisabsorbed
fromthesurroundingsduringthereaction,thereactionisendothermicandΔHhasapositivevalue.Thechangein
enthalpyisameasureofthedifferenceinenergybetweenthebondsintheproductsandthebondsinthereactants.
Theabsoluteenergyofacompoundcannotbemeasureddirectly,butthechangeinenthalpythatoccursduringa
chemicalreactioncanbemeasured.
Thereactionintoday’sexperimentisacid/baseneutralizationwhichisanexothermicprocess.
HCl(aq)+NaOH(aq)→ H2O(l)+NaCl(aq)+heat
Theheatreleasedbythereactionwillbeabsorbedbythesurroundings(aqueoussolution).CoffeeCup
Calorimetrywillbeemployedtodeterminetheamountofheatlostbythereactionandgainedbythesaltwater
solution.Acalorimeterissimplyacontainerusedtomeasuretheheatchange.CoffeeCupCalorimetryjustmeansthat
wewillbemeasuringheatatconstantpressure,ΔH.Theheatlostbythereactionwillactuallybetransferredtoboth
thesaltwateranditssurroundings(thecalorimeter.)Theheatcapacityofthecalorimeter(inthiscase,twoStyrofoam
cups)usuallywouldbecalculatedfirst;however,wehavefoundthattheheatcapacityofthecupsissosmallthatitcan
beneglected.
AstheFirstLawofThermodynamicsapplies,
Heat(q)lostbythereaction+heat(q)gainedbythesolution=0
orqrxn=–qsoln
Tofindtheheatlostbythereactionweuse:
q=mxCsxΔT
where: m=massofthesolution(Thedensityofthesolutionis1.04g/mL)
Cs=specificheatcapacityofthesolution(3.87J/g°C)
ΔT=changeintemperatureofthesolution(Tfinal–Tinitial)
Equipment
2Styrofoamcups
(2)Thermometers
50or100mLgraduatedcylinder
Stirringrod
150mLbeaker
Chemicals
2.00MHCl
2.00MNaOH
Procedure
1. RinseanddrytheStyrofoamcups(calorimeter).Placeonecupinsidetheother.Measureout25.0mLof2.00M
NaOHandpouritintothecalorimeter.Inacleandry150mLbeaker,take25.0mLof2.00MHCl.
2. Determinethetemperatureofboththeacidandthebasetothenearest0.1°C.Averagethetemperatures.Record
thisaverageastheinitialtemperatureTinitial.
3. Pourtheacidintothebasequicklyandcarefullywithgentlestirring.Startmonitoringthetemperatureassoonas
thetwoaremixed.Continuetostirandmonitorthetemperature.Recordthemaximumtemperaturethatthe
solutionreaches.ThisisTfinal
4. RepeatSteps1-3(above)forasecondtrial.
Disposal
Allreactantsandproductsmaybedisposedofintothesink.
HeatofNeutralization:DataSheet
Name________________________
Trial1
Trial2
Volumeof2.00MHCl
Volumeof2.00MNaOH
Volumeofsolution
Tinitial
Tfinal
ΔT(Tfinal—Tinitial)
SpecificheatcapacityofNaClsolution
3.87J/g°C
3.87J/g°C
Calculations(Showallyourworkcompletelyandclearly.Includeproperunits)
1. Calculatethemassofthesolutionforeachtrial(giventhedensityofthesolutionis1.04g/ml)
2. Calculatetheheatabsorbedbythesolutionineachtrial.(qsoln)
qsoln(Trial1)____________
qsoln(Trial2)_____________ Averageqsoln_________
3.Calculateqrxn=____________________________
4.Calculatethenumberofmolesofwaterformed(ThisisthesameasmolesofHCl):
molesofwater=molesofHCl=__________________
5.CalculateΔHneutralization(ΔH°)permoleofwater(kJ/mole)formed.Makesurethesigniscorrect.
ΔH°=_________________
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HeatofNeutralization:Calculations
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6.Writethechemicalreactionfortheautoionizationofwater(equilibriumconstantKw).
7.LookupthenumericalvalueforKwatTinitial(TaketheaverageofTinitialfrombothtrials)
8.Howisthereactioninquestion1relatedtothenetionicreactionfortheneutralizationshownbelow?
H3O+(aq)+OH—(aq)DH2O(l)+H2O(l)
9.UsethevalueofKwandtherelationshipbetweenthereactionsinquestions1and3tofindthenumericalvaluefor
theKfortheneutralizationreactioninquestion3.
10.GiventheKcalculatedfortheneutralizationreaction,calculateΔGofortheneutralizationreaction.(Useaverage
initialTfromthetwotrialsforthereactionTemperature.)
11.FromΔGoabove,andtheΔH°thatyouhavecalculated,calculatetheΔS°forthereaction.(UseaverageinitialT
fromthetwotrialsforthereactionTemperature.)
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