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5.0Part2:Calorimetry

Tostudyenergychanges,weneedanisolatedsystem(noenergycanmoveinorout).Wecallthistypeofsystemacalorimeter.

Theprocessofpreciselymeasuringenergychangesinsidethissystemiscalledcalorimetry.

Oftenforsimplelaboratoryexperiments,asimple,lowcostcalorimeteriscreatedandused.

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Energyistransferredbetweenthechemicalsystemandthewater(knownasthesurroundings).

Thechangeinenergy(H)ofthechemicalsystemisequaltothequantityofheattransferredtothesurroundings,q.(i.e.,whatoneloses,theothergainsandviceversa.)

H=q(chemicalsystem)(calorimeter)

Theenergychangeofthechemicalsystem,H,dependsontwoquantities:

themassofthechemical(Ifyouhavemoremethanetoburn,moreenergywillbetransferred.)

Theenergyconstantforthechangeoccuring,whichiscalledspecificheat,h.ThisquantityhasunitsofJ/g.(Assumingyouburn1gramofpropane,itmaygiveoffmoreheatthanonegramofmethane.Therefore,propanewouldhaveahigherspecificheat.)

H=mh

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Ourequationsthusfar:

SinceH=qandweknowfromyesterdaythatq=mcT,thenH=mcT.

Example1Inacalorimetryexperiment,astudentdissolvessomepotassiumchloride,KF,in100mlofwaterandthetemperaturechangesfrom22oCto18.5oC.IfthespecificheatofsolutionforKFis5.72kJ/g,howmuchKFwasdissolvedinthesolution?

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BombCalorimetersandHeatCapacity

Manychemicalreactionsofinteresttochemistsdonottakeplaceinaqueoussolutions,likecombustion,forexample.Oilcompaniesarealwaysresearchingbettergasolines,andoneimportantcharacteristicofgasolineistheheatreleasedduringcombustion.Acalorimetermadeofstyrofoamcantbeusedtostudytheenergychangesofcombustionreactions.Soweneedsomethingelse.

Westudythesetypesofreactionsinabombcalorimeter.

Readpage578ofyourtextbookandusetheblankspaceprovidedtomakenotesontherelevantinformationyoudiscover.(Youshouldbeworkingtowardsdevelopingtheformulaq=CT.)

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Example2Anoxygenbombcalorimeterhasaheatcapacityof6.49kJ/oC.Thecompletecombustionof1.12gofacetyleneproducesatemperaturechangefrom18.60oCto27.15oC.Calculatethespecificheatofcombustion,hc,foracetylene,C2H2(g).

Homework:pg579#11,12pg581#2,3CalorimetryPracticeQuestions

Therefore, The specific heat of combustion for acetylene is -49.5kJ/g.

Surroundings Chemical System

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Solutions12.2AExtraPracticeQs.pdf

1. A student reacted a sodium hydroxide solution containing 12.0 g of solute with excess, dilute sulfuric acid. Both solu-tions were initially at 19.67C. The highest temperature reached by a 300.0 mL mixture was 34.06C. Calculate the spe-cific heat of neutralization for sodium hydroxide.

E qNaOH (calorimeter)

mhn mct

12.0 g hn 300.0 g g4.

18C

J (34.06 19.67)C

hn 1.50 kJ/g

The specific heat of neutralization for sodium hydroxide is an exothermic 1.50 kJ/g.

2. Acetylene has a specific heat of combustion of 49.5 kJ/C. Find the temperature increase expected for 1.00 kg of water inthe combustion of 1.45 g of acetylene.

E qC2H2 (calorimeter)

mhn mct

1.45 g 49.

g5 kJ 1.00 kg

g4.

18C

J t

hn 1.50 kJ/g

3. Combustion of 3.50 g of ethanol in a bomb calorimeter with a heat capacity of 15.2 kJ/C causes a temperature increasefrom 19.88C to 26.18C. Using this evidence, determine the specific heat of combustion for ethanol.

E qethanol (calorimeter)

mhc ct

3.50 g hc 15.

2C

kJ (26.18 19.88)C

hc 27.4 kJ/g

The specific heat of combustion for ethanol is an exothermic 27.4 kJ/g.

4. A reference gives the specific heat of combustion of methane as 50.0 kJ/C. What minimum mass of methane must beburned to warm 4.00 L of water from 22.4C to 87.6C, assuming no heat losses.

E qCH4 (calorimeter)

mhn mct

m 50.

g0 kJ 4.00 kg

g4.

18C

J (87.6 22.4)C

m 21.8 g

12.2B

Solutions for 12.2A Extra Practice Questions: Calorimetry

5-22 Unit 5 Lab and Study Masters Copyright 2002 Nelson Thomson Learning

SMART Notebook

Attachments

Solutions12.2AExtraPracticeQs.pdf

HeatTransfer.mov

1. A student reacted a sodium hydroxide solution containing 12.0 g of solute with excess, dilute sulfuric acid. Both solu-tions were initially at 19.67C. The highest temperature reached by a 300.0 mL mixture was 34.06C. Calculate the spe-cific heat of neutralization for sodium hydroxide.

E qNaOH (calorimeter)

mhn mct

12.0 g hn 300.0 g g4.

18C

J (34.06 19.67)C

hn 1.50 kJ/g

The specific heat of neutralization for sodium hydroxide is an exothermic 1.50 kJ/g.

2. Acetylene has a specific heat of combustion of 49.5 kJ/C. Find the temperature increase expected for 1.00 kg of water inthe combustion of 1.45 g of acetylene.

E qC2H2 (calorimeter)

mhn mct

1.45 g 49.

g5 kJ 1.00 kg

g4.

18C

J t

hn 1.50 kJ/g

3. Combustion of 3.50 g of ethanol in a bomb calorimeter with a heat capacity of 15.2 kJ/C causes a temperature increasefrom 19.88C to 26.18C. Using this evidence, determine the specific heat of combustion for ethanol.

E qethanol (calorimeter)

mhc ct

3.50 g hc 15.

2C

kJ (26.18 19.88)C

hc 27.4 kJ/g

The specific heat of combustion for ethanol is an exothermic 27.4 kJ/g.

4. A reference gives the specific heat of combustion of methane as 50.0 kJ/C. What minimum mass of methane must beburned to warm 4.00 L of water from 22.4C to 87.6C, assuming no heat losses.

E qCH4 (calorimeter)

mhn mct

m 50.

g0 kJ 4.00 kg

g4.

18C

J (87.6 22.4)C

m 21.8 g

12.2B

Solutions for 12.2A Extra Practice Questions: Calorimetry

5-22 Unit 5 Lab and Study Masters Copyright 2002 Nelson Thomson Learning

SMART Notebook

SMART Notebook

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