Coffee Cup Calorimetry

Home , Calorimeter, Coffee cup

calcium turnings ammonium chloride weigh boat sensor, temperature, graduated cylinder, data collectiondevice balance EACH GROUP MATERIALS RESOURCES AND stainless steel 100 mL Copyright © 2013 National Math + Science Initiative, Dallas, Texas. All rights reserved. Visit us online at www.nms.org. at online us Visit reserved. rights Texas. All Dallas, Initiative, +Science Math National ©2013 Copyright water, distilled spoon, plastic 2 cups,8-ozStyrofoam sodium hydroxide, hydrochloric acid,0.5 with lids 0.5 M M ® , I exothermic chemicalreactions. process ofanendothermicdissolutionandtwo design calorimetryinvestigationsforthephysical school laboratory. With someguidance, studentswill ABOUT THIS LESSON THIS ABOUT Measuring Heat Transfer Calorimetry Cup Coffee Chemistry LEVEL Students will: OBJECTIVES Organize andanalyze datatodetermineenthalpy • Designcalorimetricinvestigationsusing • Review andevaluatesampleproceduresdata • required forquantifyingheatexchangeinahigh investigation toreviewconceptsandtechniques n thislesson,studentswillevaluateasample coffee cupcalorimeters changes fortheprocessescarriedoutintheir chemical systems parameters andassumptionsforphysical water inacoffee cup calorimeter analysis fortheheatexchangebetweenmetaland Chemistry

i TEACHER PAGES NEXT GENERATION SCIENCE STANDARDS PLANNING/CARRYING OUT ENERGY ANDMATTER INVESTIGATIONS PS3: ENERGY Copyright © 2013 National Math + Science Initiative, Dallas, Texas. All rights reserved. Visit us online at www.nms.org. at online us Visit reserved. rights Texas. All Dallas, Initiative, +Science Math National ©2013 Copyright SCALE, PROPORTION, INTERPRETING DATA ANALYZING AND AND QUANTITY CONSTRUCTING EXPLANATIONS DESIGNING SOLUTIONS company ofDow. Dow ChemicalCompany(“Dow”)oranaffiliated Styrofoam ACKNOWLEDGEMENTS embedded inthislesson: The followingtypesofformativeassessmentsare ASSESSMENTS Assessmentofstudent-developedproceduresand • Assessmentofstudentunderstandingthrough • and questioningduringinvestigation laboratory techniquethroughactivemonitoring sample investigation discussion ofPre-LabExercisesregardingthe ® Brandisaregisteredtrademarkof The Chemistry –Coffee CupCalorimetry ii TEACHER PAGES displays. interpret thescaleandoriginingraphsdata interpret unitsconsistentlyinformulas;chooseand guide thesolutionofmulti-stepproblems;chooseand Use unitsasawaytounderstandproblemsand (MATH) N-Q.1 using thesamereasoningasinsolvingequations. Rearrange formulastohighlightaquantityofinterest, (MATH) A-CED.4 relevant andsufficient evidence. substantive topicsortexts,usingvalidreasoningand Write arguments tosupportclaims inananalysisof W.1(LITERACY) relevant togrades9–10textsandtopics. are usedinaspecificscientificortechnicalcontext other domain-specificwordsandphrasesasthey Determine themeaningofsymbols,keyterms,and RST.9-10.4 (LITERACY) STATE STANDARDS CORE COMMON Copyright © 2013 National Math + Science Initiative, Dallas, Texas. All rights reserved. Visit us online at www.nms.org. at online us Visit reserved. rights Texas. All Dallas, Initiative, +Science Math National ©2013 Copyright CONNECTIONS TO AP* *Advanced Placement involved intheproduction ofthisproduct. College EntranceExaminationBoard. TheCollegeBoard was not B.2 B.1 A.2 B.4 B.3 work. can occurthrougheitherheatexchangeor systems remainsfixed.Energy transfer system. The combinedenergy ofthe two equal totheenergy thatgoesintotheother energy thatcomesoutofonesystemis each otherandareotherwiseisolated,the When twosystemsareincontactwith system doingworkontheothersystem. either throughheattransferorone Energy istransferredbetweensystems always fromahottocoldbody. spontaneous directionofthetransferis in thiscourseasheattransfer, andthe at theparticulatescaleisreferredto The processofkineticenergy transfer energy ofachemicalsystem. that isusedtomeasurethechangein Calorimetry isanexperimentaltechnique chemical reactions. heating/cooling, phasetransitions,and processes thatchangetheirenergy; Chemical systemsundergo threemain Chemistry –Coffee CupCalorimetry ® and AP CHEMISTRYC APA H E M I P S T R Y ® are registered trademarksofthe 5 iii TEACHER PAGES system, theamountofheatgained( and “Burn,Baby, Burn.”Fromtheperspectiveof in otherlessonslike“Mass,Heat,and Temperature” This equationusesthesameconventionestablished gained byanotherbecomes heat lostbyonecomponentofasystemandthe Manipulating theequation,relationshipbetween equation thedirectionalityofheatlost( positive value. When valuesaresubstitutedintothe 0 According tothelawofconservationenergy, = SUGGESTIONS TEACHING Relating heattransfertothelawofconservation Working withthermochemicalequationstocalculate Differentiating between Solving forvariablesusingtheequation, Understanding therelationshipbetweenmass,heat, energy heat orchemicalamounts processes and temperature Copyright © 2013 National Math + Science Initiative, Dallas, Texas. All rights reserved. Visit us online at www.nms.org. at online us Visit reserved. rights Texas. All Dallas, Initiative, +Science Math National ©2013 Copyright Prerequisite Knowledge andSkills q = mC − Calorimetry q q lost ∆ exothermic gained T =

q + gained q

lost (Eq. C) and (Eq. B) q gained endothermic q lost ) isalwaysa (Eq. A) ), alwaysa

is often prepared forothervariationsoftheheatvariable.It the physicsteacherstomakesurethatstudentsare For thepurposesofverticalalignment,checkwith equal. negative value,isresolvedandthemagnitudesbecome procedural points. Pre-Lab Exerciseshighlightessentialconceptual and of thetechniquesusedincoffee cupcalorimetry. and analysis—ispresentedtoscaffold understanding sample investigation—completewithprocedures,data, procedures forthreecalorimetryinvestigations. A In thelab,studentsareaskedtodesigntheirown and work,isbeyondthescopeofcourse. ∆ types ofcalorimeters. The distinctionbetween the approachtoproblemsolvingissameforboth in energy, calorimetry bomb calorimetry at constantpressure. Textbooks thatintroduce is equaltothechangeinenthalpy, barometric pressureintheroom. The heattransfer, sealed andthereforemaintainpressuresequaltothe Note thatcoffee cupcalorimetersarenotperfectly it withyourstudents. there isadifference betweendisciplines,makenoteof entirely, suchaswith are trainedthatcapitalizationchangesthevariable H , basedontechnicaldefinitionsforenthalpy, energy q inchemistrybut ∆ , equateheattransfertoanoverallchange E . Forthepurposesofafirst-yearcourse, Chemistry –Coffee CupCalorimetry , referredtoas t (time)and Q inphysics.Students constant-volume T ∆ (temperature).If H , forprocesses ∆ E and q , iv TEACHER PAGES

ideas shouldsurface: In discussingthesampleinvestigation,following TEACHING SUGGESTIONS (CONTINUED) The changeintemperatureisdefinedas • Equation A isusefulforcalculatingtheheat • When equatingtwoheatvalues,theunitsmustbe • Two substancesatdifferent temperatures • The coppermetalisatthermalequilibriumwith • Thespecificheatcapacityforwateris • Ifthetemperaturedecreasesforasampleof • Ifthetemperatureincreasesforasampleof • Heatistransferredbetweentwoobjectsat • Thehotplatewasrequiredinthesample • measurable temperaturechange. lost orgainedbyasampleofmatterwith the same. experiencing equalchangesintemperature. do notalwaysreachthermalequilibriumby the copper. temperature ofwaterastheinitial calorimeter, thereforewecanusetheboiling the boilingwaterbeforeitistransferredto matter orendothermicprocesses. 4.184 Jg matter, it sample ofmatterorexothermicprocess. matter, it “cold.” different temperaturesinthedirectionof“hot”to source. investigations donotrequire anexternalheat the metalisplacedinwater. The other physical orchemicalchangeoccurswhen between themetalandwaterbecauseno investigation tocreateatemperaturedifferential Copyright © 2013 National Math + Science Initiative, Dallas, Texas. All rights reserved. Visit us online at www.nms.org. at online us Visit reserved. rights Texas. All Dallas, Initiative, +Science Math National ©2013 Copyright − lost gained 1 ·°C thermalenergy toanothersampleof − 1 ∆ andthedensityis1g·mL thermalenergy fromanother T = T f

−

T i (Eq.D) − 1 . problems. the followingrecurringstepsinsolvingcalorimetry transfer. The examplecalculationsshownemphasize approach canbeappliedtotheanalysisofheat useful forstudentstoseethataconsistentconceptual very different inthedetailsofnumbersandunits,itis Although mostcalorimetrycalculationscanlook issues andaskthemtobeginanewtrial. with designorexperimentation,discusstheerrors/ procedures. Ifyounoticethatagroupisstruggling prior toinvestigation,encouragegroupscompare feasible tocheckoff theprocedureforeachgroup progress andproceduraldesigns.Ifitisnotlogistically As studentsdesigntheirinvestigations,monitor . Insimplecalorimetryproblems,heatgainedby 3. Identifythecomponentsofsystemthatare 2. AlwaysstartwithEquationC. This equation 1. stated. investigation, theunitsforvariableareclearly change inenthalpytoafinaltemperature.Inthis for canrangefromaspecificheatcapacityto it toacomponent. The variablebeingsolved to includetheappropriateunits,andattribute Establish theunknownvariable,makingsure heat absorbedbythesystem. calculations, thisvalueshouldbeaddedtothe absorbed bythecalorimeterisincludedin the calorimeterisoftenneglected.Ifheat including thespecificcomponentsinproblem. losing andgainingenergy. Rewrite EquationC, conceptual frameworkoftheproblem. seemingly unnecessary, groundsstudentsinthe is theentirebasisofheattransferand,though Chemistry –Coffee CupCalorimetry v TEACHER PAGES TEACHING SUGGESTIONS (CONTINUED) . Plugthevaluesintoequationestablishedin 5. . Solvefortheamountofheattransferred 4. do notchange. q Step 2,beingsuretomultiplythe reactant mustbeusedinthecalculation. or chemicalchange.Forallprocesses,alimiting are appliedinsituationslike discrete samplesofmatter. Processenthalpies shown inthedimensionalanalysis,unitsof by ( situations like they mustcomplete.Equation A isappliedin them aboutthenatureofheatcalculations The waystudentslabeltheircomponentscantell into thecalculation. joules forconsistency. The variable, the analysesshown,heatisalwayscalculatedin units canceluntilonlyanenergy unitisleft.In Dimensional analysiscanclearlyshowhow all componentsofthesystemindividually. reaction − 1). Solvefor , andforothertermsdescribingaphysical Copyright © 2013 National Math + Science Initiative, Dallas, Texas. All rights reserved. Visit us online at www.nms.org. at online us Visit reserved. rights Texas. All Dallas, Initiative, +Science Math National ©2013 Copyright q water x , . Despitetheunitcancelation q solution , q metal q dissolution , andother q lost , x component q , shouldfit neutralization , x

Notes: gives thedissolutionequationforsodiumhydroxide. equation directthemtoPre-LabExercise9,which aqueous ions.Ifstudentsarestrugglingwiththis a physicalchangewherethesoliddissociatesinto reactant. However, inthiscasetheprocessrepresents to representachemicalreactionwherewateris ammonium chloride.Somestudentsmaybetempted equation fortheendothermicdissolutionprocessof Part Irequiresthestudentstowriteabalanced SOLUTION HEATPART OF I: Thevariablebeingsolvedforisestablishedas • • The terms There isadifference betweenthephysical • Thesolutionexperiencesadecreasein • Besurethatstudentsaddthemassof • ∆ struggling withappropriatenotation. between whichstudentsmustdistinguish. “solution” andtheprocessof“dissolution” dissolution mustbegainingthatenergy. Deductive reasoningstatesthattheprocessof temperature, andmustbelosingenergy. solution, asspecifiedintheassumptionssection. when determiningtheheatchangefor ammonium chloridetothemassofwater chloride. H ° soln withunitsofkJ·mol Chemistry –Coffee CupCalorimetry q solution and q dissolution − 1 canhelpstudents ofammonium vi TEACHER PAGES Notes: clearly establishthelimitingreactant. of theequimolarsolutions,astheywillthenneedto Students mustbecarefuliftheyusedifferent volumes make surethetemperaturechangeisuniform. described inthesampleproceduresisrecommendedto The reactioncontinuestocompletionbutswirlingas the othersolutioncanbepouredintocalorimeter. the calorimeter. Soonafterthestartofdatacollection, procedures workbestwhenonesolutionisalreadyin reaction ofastrongacidwithbase. The Part IIdealswiththeexothermicneutralization HEAT NEUTRALIZATION PART OF II: TEACHING SUGGESTIONS (CONTINUED) The sodiumhydroxideisthelimitingreactant,so • Some studentsmaywanttousebothvolumes • The variablebeingsolvedforisestablishedas • • The terms The temperatureincreaseofthesolutionindicates • Besurestudentsusethe total massofthe • Thecoefficients inthebalancedequationprovide • q it isusedinthethermochemicalcalculationfor calculate ∆ struggling withappropriatenotation. energy. that theneutralizationprocessmustbelosing it isgainingenergy. Deductivereasoningstates assumptions section. solution intheircalculation, asspecifiedinthe reaction. the ratiobetweenmolesofreactantand heat change. solution, itisnotappropriateforestablishingthe excess reactantisnecessaryforthetotalmassof the limitingreactant. Although thevolumeof neutralization H ° neut Copyright © 2013 National Math + Science Initiative, Dallas, Texas. All rights reserved. Visit us online at www.nms.org. at online us Visit reserved. rights Texas. All Dallas, Initiative, +Science Math National ©2013 Copyright withunitsofkJ·mol . q q neutralization neutralization . Redirectthemtodetermine and q solution − 1 rxn canhelpstudents . Notes: the process. their calciumsample,asitisthelimitingreactantin to obtainasample.Studentsmustmassfor students gettingshockedwhenusingametalscoopula calcium samplesintoabeakerorweighboattoavoid up easilyinthebottleofcalciumturnings.Poursome reaction ofcalciumwithwater. Staticelectricitybuilds Part IIIinvolvestheexothermicsinglereplacement HEAT REACTION OF PART III: • The terms Thetemperatureincreaseofthesolutionindicates • Besurestudentsaddthemassofcalciumto • Thecoefficients inthebalancedequationprovide • Thevariablebeingsolvedforisestablishedas • that thereactionmustbelosingenergy. it isgainingenergy. Deductivereasoningstates mass ofwater, asspecifiedintheassumptions. reaction. the ratiobetweenmolesofreactantand ∆ struggling withappropriatenotation. H ° rxn withunitsofkJ·mol Chemistry –Coffee CupCalorimetry q reaction and q solution − canhelpstudents 1 rxn . vii TEACHER PAGES analysis questions. The Conclusion Questions2–5aredesignedaserror- CLAIM-EVIDENCE-REASONING (C-E-R) TEACHING SUGGESTIONS (CONTINUED) question. is appropriateforformulatingresponsestoeach (C-E-R) frameworkforcraftingscientificexplanations In thecontextofalaboratoryinvestigation • • The evidence componentisexpectedtobeless Note thatforthisparticularinvestigationthe inference. information. The evidenceisalwaysabsentof knowledge, orothercitablesourcesofobjective from backgroundreading,theprocedures,prior claim. Evidencestatementsmightalsobederived just theportionofdatathatrelatesto is asubsetofallthedatacollectedandrepresents the student’s claim.Inmostcases,theevidence observations, orothercollecteddatathatsupport Evidence either “toohigh,”low,” or“thesame.” (as comparedtoatheoreticalvalue)andwillstate the effect anerrorwillhaveonacalculatedvalue In theConclusionQuestions,claimregards behind thestatement. but doesnotexaminetheevidenceorreasoning claim stateswhatthestudentbelievestobetrue students stopifnotpromptedtogofurther. A in simple,straightforwardterms.Itiswheremost Claim Copyright © 2013 National Math + Science Initiative, Dallas, Texas. All rights reserved. Visit us online at www.nms.org. at online us Visit reserved. rights Texas. All Dallas, Initiative, +Science Math National ©2013 Copyright generallycitesnumbers,calculations, isthestudent’s answertothequestion Claim-Evidence-Reasoning

difference between calorimetry. Studentsmayneedclarificationonthe heat offusion,exploredin“HeatingCurves,”with a procedurethatrequiresice. This connectslatent In theGoingFurthersection,moststudentswillfind formed inPartIIIisaqueousandnotsolid. data carefully. As anexamplethe calciumhydroxide including aqueous,sostudentsshouldselecttheir data existsfordifferent phasesofpuresubstances, products ineachinvestigation. Thermodynamic up standardheatsofformationforallreactantsand Conclusion Question6requiresstudentstolook • The calculations. logic throughpropagationoferrorintheir substantial thanthereasoning,asstudentsmust justify theclaimandselectionofevidence. background information,facts,andtheoriesthat The reasoningstatementweavestogether applicable scientificprinciplesandknowledge. counts asevidencefortheclaimandinclude statement shouldjustifywhythedatapresented evidence isconnectedtotheclaim. The reasoning reasoning, istheprecisionwithwhich data provided. The heartoftheresponse, and adequateevidenceasasubsetofallthe reasoning requirestheselectionofappropriate difficult forstudentstoarticulate.Proficient Reasoning Chemistry –Coffee CupCalorimetry specific heat statementistypicallythemost and heat capacity . viii

TEACHER PAGES Chemistry – Coffee Cup Calorimetry

PRE-LAB EXERCISES

1. Sketch a diagram for the setup described in the Sample Procedure. See Figure A.

temperature probe

data collection device 100 mL water

hot metal

Figure A. Calorimeter setup ANSWER KEY

2. Describe the major safety concerns for the 3. Consider the transition from Equation 2 to Sample Procedure. Equation 3 of the Sample Analysis. Explain how • Avoid touching the hot plate with hands or the experimenter can determine which system cords. loses heat and which system gains heat based on the data. • Place the metal sample carefully into the boiling water to avoid splashing hot water The temperature changes for the copper and and to avoid cracking the glass beaker. water indicate whether they lost or gained heat. • Remember that hot and cold glass look the The copper metal has an initial temperature same. equal to the boiling water, 99.8°C. The initial temperature of water was recorded to be 22.3°C. The final temperature of the system was recorded as 24.1°C. As the temperature of the copper went down, heat must have been lost. Therefore the water gains that heat, which increased its temperature.

PRE-LAB EXERCISES (CONTINUED)

4. State what the variable x represents in Equation 4 7. Justify why the units from Equation 4 and of the Sample Analysis, and report its units. Equation 5 of the Sample Analysis must be the The x represents the value for the specific heat same. capacity of the metal (refer to Equation 4). The Equation 4 represents the amount of energy units are J/g·°C. lost by the copper and Equation 5 represents 5. In Equation 4 of the Sample Analysis, why is the amount of energy gained by the water. The the temperature of the boiling water used as the energy lost equals the energy gained, as shown in initial temperature of the copper? Equation 6. If the units are not the same, then the equation does not work and variables cannot be The metal was allowed to sit in the boiling water calculated correctly. for a period of time. Metals absorb heat quickly, so the copper would have reached thermal 8. Considering the actual data collected, state the equilibrium with the water and have been at the assumption made in Equation 5 of the Sample same temperature initially. Analysis. 6. Explain why the final temperature of the sample The assumption is that the density of water is is closer to 22.3°C rather than 99.8°C. Why are 1 g/mL, which allows us to use the volume of the final temperatures of water and metal the water as its mass. same? 9. The heat calculation, q, differs from the sample The final temperature for the copper and water is analysis when the system losing or gaining 24.1°C when they reach thermal equilibrium. The heat is a process for which a heat capacity and mass of the water in the sample is greater than the temperature change are not applicable. Calculate mass of the copper used, so the heat lost by the the heat transferred by the dissolution of 3.0 g of metal is spread out over more matter. NaOH(s) in water.

Also, the specific heat calculated for the metal NaOH(s) → Na+(aq) + OH−(aq) ANSWER KEY is much smaller than the given specific heat for the water. The metal changes temperature more ΔH° = −44.5 kJ·mol−1 readily than the water when heat is lost or gained. soln

1mol NaOH 1molrxnæö- 44.5kJ 3.0g NaOH´´´=-ç ÷ 3.34kJ 40.01g NaOH 1mol NaOHèøç 1molrxn÷

DATA AND OBSERVATIONS

PART I: HEAT OF SOLUTION

BALANCED EQUATION FOR THE DISSOLUTION DATA TABLE

+ − Table A. Heat of Solution NH4Cl(s) → NH4 (aq) + Cl (aq) Mass of ammonium chloride 1.30 g

SUMMARY OF PROCEDURES Volume of water 65.5 mL T of water in calorimeter 22.1°C • Measure out ammonium chloride. Record the i

mass in the data table. Tf of water in calorimeter 21.0°C • Obtain two stacked coffee cups and a fitted lid. Transfer a known volume of water into the cups, replace the lid, and record the initial temperature with the temperature probe. • Set up the data collection device to record the temperature every second for 180 seconds. • Add the solid ammonium chloride, stir, and continue recording the temperature until the data collection device stops.

ANALYSIS ANSWER KEY

-=qqlost gained

-=qqsolution dissolution 4.184 J q =+(65.5 1.30)g ´ ´-=-() 21.0 22.1 C 307.4 J solution gC⋅

1molNH4 Cl x kJ 1000 J qxdissolution =´1.30 g ´ ´ = 24.30 J 53.5 g NH44 Cl 1 mol NH Cl 1 kJ --()307.4 = 24.30x

307.4 kJ x ==13 24.30 mol NH4 Cl

DATA AND OBSERVATIONS (CONTINUED)

PART II: HEAT OF NEUTRALIZATION

BALANCED EQUATION FOR THE NEUTRALIZATION DATA TABLE

Table B. Heat of Neutralization HCl(aq) + NaOH(aq) → NaCl(aq) + H2O(l) Volume of 0.5 M HCl 30.0 mL or Volume of 0.5 M NaOH 28.4 mL

+ − Ti of solution 22.1°C H(aq) + OH (aq) → H2O(l)

Tf of solution 25.1°C

SUMMARY OF PROCEDURES • Rinse and dry the cups used in Part I. • Transfer a known volume of hydrochloric acid into the cups, replace the lid, and record the initial temperature with the temperature probe. • Set up the data collection device to record the temperature every second for 180 seconds. • Add a known volume of sodium hydroxide, stir, and continue recording the temperature until the data collection device stops. ANSWER KEY

ANALYSIS

-=qqlost gained

-=qqneutralization solution 1 L 0.5 mol NaOH 1 molrxnx kJ 1000 J q =´28.4 mL ´ ´ ´ ´= 14.2x J neutralization 1000 mL 1 L 1 mol NaOH 1 molrxn 1 kJ 4.184 J q =+(30.0 28.4)g ´ ´-=() 25.1 22.1 C 733 J solution gC⋅ -=()14.2x 733

733 kJ x ==-52 -14.2 mol rxn

DATA AND OBSERVATIONS (CONTINUED)

PART III: HEAT OF REACTION

BALANCED EQUATION FOR THE REACTION DATA TABLE

Table C. Heat of Reaction Ca(s) + 2H2O(l) → Ca(OH)2(aq) + 2H2(g) Mass of calcium 0.35 g or Volume of water 120.5 mL

2+ − Ti of water in calorimeter 22.1°C Ca(s) + 2H2O(l) → Ca (aq) + 2 OH (aq) + 2H2(g)

Tf of water in calorimeter 29.3°C

SUMMARY OF PROCEDURES • Rinse and dry the cups used in Part I. • Measure out a small amount of calcium metal. Record the mass in the data table. • Transfer a known volume of water into the cups, replace the lid, and record the initial temperature with the temperature probe. • Set up the data collection device to record the temperature every second for 180 seconds. • Add the solid calcium metal, stir, and continue recording the temperature until the data collection device stops. ANSWER KEY

ANALYSIS

-=qqlost gained

-=qqreaction solution 1 molCa 1 molrxnx kJ 1000 J qx=´´´´=0.35 g 8.73 J reaction 40.08 g Ca 1 molCa 1 molrxn 1 kJ 4.184 J q =+´(120.5 0.35)g ´-=() 29.3 22.1 °C 3641 J solution g°C⋅ -=()8.73x 3641

3641 kJ x ==-420 -8.73 mol rxn

CONCLUSION QUESTIONS

1. For each of the three investigations, determine 3. In Part I, would your calculated value for the heat whether each process is endothermic or of solution be reported as too large, too small, or exothermic. State your reasoning. the same as the theoretical value if some of the Part I: Endothermic. The temperature change was solid sample remained in the weigh boat instead negative, indicating that the solution was cooler; of being transferred into the calorimeter? Explain the enthalpy change was positive. your reasoning. Part II: Exothermic. The temperature change was The calculated value for heat of solution would positive, indicating that the solution was warmer; be reported as too low if some of the solid the enthalpy change was negative. remained in the weigh boat. Part III: Exothermic. The temperature change was The recorded mass of solid would be larger than positive, indicating that the solution was warmer; the actual amount that dissociated. This means the enthalpy change was negative. that the unit of moles, found in the denominator, would be larger than the actual value. When 2. In the sample experiment to determine the divided by a larger number, the amount of energy specific heat capacity of a metal, would the in the numerator yields a smaller value for heat of calculated value be reported as too large, too solution. small, or no different than the theoretical value if the experimenter took too long to transfer the 4. In these experiments, it is assumed that heat is metal to the calorimeter? Explain your reasoning. neither transferred to nor from the calorimeter. This is not actually true. The heat transferred to The specific heat capacity would be reported as the calorimeter or by the calorimeter is a part of too low. The metal is at a higher temperature than the system. How would you decide whether the the air, so it will lose energy during the transfer. term q belongs as a component of q or The initial temperature of the metal would be calorimeter lost qgained in the analysis of the experiment? Explain. recorded as the boiling temperature of the water ANSWER KEY The term q can be a component of either but it is actually less. This would cause the calorimeter q or q . The temperature change for the calculated difference between the final and initial lost gained temperature to be too large. calorimeter will be the same as for the water or solution inside due to the thermal equilibrium. Temperature is in the denominator of the specific For exothermic processes happening in the heat ratio. Dividing by a larger change in calorimeter, the calorimeter will gain heat. For temperature will result in a specific heat capacity endothermic processes in the calorimeter, the that is smaller than it should actually be. calorimeter will lose heat.

CONCLUSION QUESTIONS (CONTINUED)

5. Calcium metal reacts readily with air. Consider 6. Research the standard heats of formation for the reaction shown in Equation 7: all reactants and products in each of your calorimetry investigations. Ca + ½ O → CaO (Eq. 7) 2 a. Determine the theoretical values for each of Assume the calcium turning used in the the three unknowns you solved for using experiment had a coating of calcium oxide. Equation 8: a. Would the recorded mass of calcium be DHH = D  - D H  greater, smaller, or the same as the actual value rxnååff (products) (reactants) of the calcium turning? Explain. If the calcium metal was actually calcium Part I:

oxide, the reported mass of calcium would be ∆H°rxn = [1(−132) + 1(−167)] – [1(−314)] recorded greater than the actual value. The = +15 kJ/mol rxn molar mass of calcium is less than the molar Part II: mass of the calcium oxide that results from a ∆H°rxn = [1(−286)] – [1(0) + 1(−230)] reaction with oxygen in the air. This causes the = −56 kJ/mol rxn mass of the sample to increase slightly. Part III: b. Would the calculated heat of reaction be ∆H° = [1(−543) + 1(−230) + 1(0)] reported as too large, too small, or the same? rxn – [1(0) + 1(−286)] = −487 kJ/mol rxn Explain. b. Calculate the percent error for the calculated The calculated heat of reaction would be values in each investigation based on the reported as too low. The mass of calcium is theoretical values from (a). reported as too great, so the number of moles would be as well. The moles of calcium are 13- 15 ANSWER KEY Part I: % error=´= 100 13.3% related to moles of reaction, which is in the 15 denominator for the heat of reaction. If the ---52 ( 56) denominator is artificially large, the calculation Part II: % error=´= 100 7.1% -56 will be less than the actual value. ---420 ( 487) Part III: % error=´= 100 13.8% -487 GOING FURTHER

Research a laboratory procedure to determine the heat capacity of the coffee cup calorimeter. If resources permit, determine the heat capacity in units of J·°C−1. Student procedures will vary depending on the source but will likely center around the mixing of hot and cold water.

Copyright © 2013 National Math + Science Initiative, Dallas, Texas. All rights reserved. Visit us online at www.nms.org. xv Chemistry – Coffee Cup CalorimetryChemistry Coffee Cup Calorimetry Measuring Heat Transfer

hemical reactions and physical processes are always accompanied, if not driven by, a change in energy. Heat, the energy transferred due to a Cdifference in temperature, is measured in calorimetry experiments. The analysis of a calorimetry experiment changes depending on the nature of the MATERIALS calorimeter and processes involved but is always based on the law of conservation balance of energy. data collection device As the unit for heat, q, is attributed with a positive or negative sign to indicate the graduated cylinder, direction of the transfer, we can say 100 mL sensor, temperature, −qlost = qgained (Eq. 1) stainless steel weigh boat In your investigations, you will be using a coffee cup calorimeter similar to the ammonium chloride one shown in Figure 1. Read through the sample procedures, data, and analysis for a calorimetry experiment for a physical process of heat transfer. Answer the calcium turnings questions in the Pre-Lab Exercises that follow. hydrochloric acid, 0.5 M sodium hydroxide, 0.5 M 2 cups, 8-oz Styrofoam®, with lids spoon, plastic thermometer water, distilled

insulated cover

Figure 1. Coffee cup calorimeter

styrofoam cups

DETERMINING THE SPECIFIC HEAT OF METAL

SAMPLE PROCEDURE 1. Fill a 400-mL beaker halfway with water. Bring the water to boil on a hot plate and record the boiling temperature using a temperature sensor. 2. Obtain two stacked coffee cups and a fitted lid. Add approximately 100 mL of water and record the exact volume of water used. Place the lid on the coffee cups and insert the temperature probe through the lid into the water. 3. Obtain a metal block. Record the identity and mass of the sample. Using crucible tongs, transfer the metal to the boiling water. Be sure that the metal is completely submerged in the water. 4. Set up a data collection device to record the temperature every second for 180 seconds. 5. Place the temperature probe in the water and begin data collection. Open the lid, allowing the temperature probe to briefly come out of the water. With extreme care, use crucible tongs to transfer the metal from the boiling water to the water in the calorimeter. Replace the lid and the temperature probe. 6. Keeping the base of the coffee cups on the table, swirl the water by moving the cup in small circular motions. Continue data collection until time has elapsed.

DETERMINING THE SPECIFIC HEAT OF METAL (CONTINUED)

Table 1. Sample Data

Identity of the Metal Copper Mass of metal 25.00 g Volume of water 98.5 mL T of boiling water 99.8°C

Ti of water in calorimeter 22.3°C

Tf of water in calorimeter 24.1°C

SAMPLE ANALYSIS

−qlost = qgained (Eq. 2)

−qCu = qwater (Eq. 3)

x J qx=´´-=-25.0 g (24.1 99.8) C 1892.5 J (Eq. 4) Cu gC⋅

4.184 J q =98.5 g ´ ´-= (24.1 22.3) C 741.8 J (Eq. 5) water gC⋅ −(−1892.5x) = 741.8 (Eq. 6)

741.8 J x ==0.39 1892.5 g⋅ C

PROCEDURE

PART I: HEAT OF SOLUTION

Design an experiment to determine the heat of solution, ΔH°soln, for the dissolution of ammonium chloride in water. Solve for units of kJ·mol−1 of ammonium chloride. The rate of dissolution is increased with agitation, so be sure to adequately swirl the solution.

PARAMETERS • Use between 1.0 g and 2.0 g of solid ammonium chloride. • Use between 50.0 mL and 80.0 mL of distilled water.

ASSUMPTIONS • The total mass of solution includes the mass of ammonium chloride. • The specific heat of the solution is the same as that for water.

PART II: HEAT OF NEUTRALIZATION

Design an experiment to determine the heat of neutralization, ΔH°neut, for the −1 reaction of sodium hydroxide and hydrochloric acid. Solve for units of kJ·mol rxn for the balanced equation using the lowest whole-number coefficients.

PARAMETERS • The solution concentrations are all 0.5 M. • The total volume of the solution must not exceed 60.0 mL.

ASSUMPTIONS • Volumes are additive. • Both solutions have the same starting temperature. • The density and specific heat of the solutions are the same as those for water.

PROCEDURE (CONTINUED)

PART III: HEAT OF REACTION

Design an experiment to determine the heat of reaction, ΔH°rxn, for the reaction of −1 calcium metal with water. Solve for units of kJ·mol rxn for the balanced equation using the lowest whole-number coefficients.

PARAMETERS • The amount of distilled water in the calorimeter can range from 80.0 mL to 150.0 mL. • Use only one (1) calcium turning.

ASSUMPTIONS • The total mass of the solution includes the mass of added calcium. • The specific heat of the solution is the same as that for water.

PRE-LAB EXERCISES

1. Sketch a diagram for the setup described in the Sample Procedure.

2. Describe the major safety concerns for the Sample Procedure.

3. Consider the transition from Equation 2 to Equation 3 of the Sample Analysis. Explain how the experimenter can determine which system loses heat and which system gains heat based on the data.

PRE-LAB EXERCISES (CONTINUED)

4. State what the variable x represents in Equation 4 of the Sample Analysis, and report its units.

5. In Equation 4 of the Sample Analysis, why is the temperature of the boiling water used as the initial temperature of the copper?

6. Explain why the final temperature of the sample is closer to 22.3°C rather than 99.8°C. Why are the final temperatures of water and metal the same?

7. Justify why the units from Equation 4 and Equation 5 of the Sample Analysis must be the same.

PRE-LAB EXERCISES (CONTINUED)

8. Considering the actual data collected, state the assumption made in Equation 5 of the Sample Analysis.

9. The heat calculation, q, differs from the sample analysis when the system losing or gaining heat is a process for which a heat capacity and temperature change are not applicable. Calculate the heat transferred by the dissolution of 3.0 g of NaOH(s) in water.

NaOH(s) → Na+(aq) + OH−(aq)

−1 ΔH°soln = −44.5 kJ·mol

DATA AND OBSERVATIONS

PART I: HEAT OF SOLUTION

BALANCED EQUATION FOR THE DISSOLUTION

SUMMARY OF PROCEDURES

DATA TABLE

ANALYSIS

DATA AND OBSERVATIONS (CONTINUED)

PART II: HEAT OF NEUTRALIZATION

BALANCED EQUATION FOR THE NEUTRALIZATION

SUMMARY OF PROCEDURES

DATA TABLE

ANALYSIS

DATA AND OBSERVATIONS (CONTINUED)

PART III: HEAT OF REACTION

BALANCED EQUATION FOR THE REACTION

SUMMARY OF PROCEDURES

DATA TABLE

ANALYSIS

CONCLUSION QUESTIONS

1. For each of the three investigations, determine whether each process is endothermic or exothermic. State your reasoning.

2. In the sample experiment to determine the specific heat capacity of a metal, would the calculated value be reported as too large, too small, or no different than the theoretical value if the experimenter took too long to transfer the metal to the calorimeter? Explain your reasoning.

3. In Part I, would your calculated value for the heat of solution be reported as too large, too small, or the same as the theoretical value if some of the solid sample remained in the weigh boat instead of being transferred into the calorimeter? Explain your reasoning.

CONCLUSION QUESTIONS (CONTINUED)

4. In these experiments, it is assumed that heat is neither transferred to nor from the calorimeter. This is not actually true. The heat transferred to the calorimeter or by the calorimeter is a part of the system. How would you

decide whether the term qcalorimeter belongs as a component of qlost or qgained in the analysis of the experiment? Explain.

5. Calcium metal reacts readily with air. Consider the reaction shown in Equation 7:

Ca + ½ O2 → CaO (Eq. 7)

Assume the calcium turning used in the experiment had a coating of calcium oxide. a. Would the recorded mass of calcium be greater, smaller, or the same as the actual value of the calcium turning? Explain.

b. Would the calculated heat of reaction be reported as too large, too small, or the same? Explain.

CONCLUSION QUESTIONS (CONTINUED)

6. Research the standard heats of formation for all reactants and products in each of your calorimetry investigations. a. Determine the theoretical values for each of the three unknowns you solved for using Equation 8:

DHHrxn =åå Dff  (products) - D H  (reactants) (Eq. 8)

b. Calculate the percent error for the calculated values in each investigation based on the theoretical values from (a).

GOING FURTHER

Research a laboratory procedure to determine the heat capacity of the coffee cup calorimeter. If resources permit, determine the heat capacity in units of J·°C−1.