Chem 1012_Diagonal A_Prof Marques_Chapter 5 _ 2010

8/8/2019 Chem 1012_Diagonal A_Prof Marques_Chapter 5 _ 2010

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Chapter5ThermochemistryChapter5ThermochemistryThermochemistry: relationship betweenchemical reactions and energy changes


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The nature of energy

Energy = ability to do work (i.e., move a mass through somedistance; w = F d) or to produce heat

Two important types of energy:

-potential: energy due toposition in a field such as agravitational field

-kinetic: energy due tomotion

2k1E= mv2Kinetic energy


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Ep stored energy with the potential to do workDue to the position of a mass in a gravitationalfieldpE= mghDue to the position of a charge in an electricfield

elec pot E = K qQreg: e-in the electric field of thenucleus of an atomThis is the energy that getsconverted during chemicalreactionsk = 8.99 109 J m C-2.Ep stored energy with the potential to do workDue to the position of a mass in a gravitationalfieldpE= mgh

Due to the position of a charge in an electricfieldelec pot E = K qQreg: e-in the electric field of thenucleus of an atomThis is the energy that getsconverted during chemicalreactionsk = 8.99 109 J m C-2.Chemical reaction:

Rearrangement of electrons causes energy to flow in

or out of a chemical system

Thermal energy:

Energy a substance possess due to its temperatureabove the absolute zero of temperature. Associatedwith the kinetic energy of atoms and molecules.


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SI Unit for energy is the joule, J:212=kEmv1 2 -1 2 2-2

Ek = mv =kg(m s ) =kg m s

2

J = kg m2s-2

=pE mgh-2 2-2

Ep = mgh =kg ms m = kg ms

=elec potK qQErKqQ J m C-2 C C

E = ==J

elec pot

r m

Example

How much energy (J) is there is one kilowatt-hour, theunit in which power companies like Eskom bill theircustomers?


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Systems and Surroundings Systems and SurroundingsThermodynamic system: that part of the universewe are interested in.Surroundings: the rest of the universe.

For example: C(s) + CO2(g) .2CO(g)

System: C, CO2, CO; everything else: surroundings

Closed systems:

exchange energy but not matter with surroundings

Open systems:

exchange energy and matter with surroundings

Isolated systems:

exchange neither


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Example

Identify each of the following systems as open, closedor isolated:

Engine coolant in the cooling system of a car

Mercury in a thermometerA living plantCoffee in a thermos flask

Transferring Energy: Work and Heat


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Transferring Energy: Work and HeatSir Benjamin Thompson, Count RumfordDemonstrated the equivalence of mechanical work and heatTransferring Energy: Work and HeatSir Benjamin Thompson, Count RumfordDemonstrated the equivalence of mechanical work and heat

Workofteneitherelectricalormechanical.Mechanicalworkinvolvingexpandinggasesiscalledpressure-volumeworkorP-VworkTransferring Energy: Work and Heatpistonreaction chamberSYSTEM:C2H5OH(g) + 3O2(g) .2CO2(g) + 2H2O(g) +energyheat flows from ahot body to coldsurroundingsmechanical work

doneENERGY:ability to do WORK and/or toTRANSFER HEAT


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TheFirstLawofThermodynamicsEnergy cannot be created or destroyed, butmerely converted from one form to another


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Ifapieceofmetalat

85Cisaddedtowaterat25C,thefinaltemperatureofthe

systemis30C.Whichofthefollowingistrue?

1.Heat

lostbythemetal>heatgainedbywater2.Heatgainedbywater>heatlostbythemetal3.Heatlostbymetal

>heatlost


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bythewater4.Heatlostby

themetal=heatgainedbywater5.Moreinformationisrequired.

Internal Energy, U (textbook uses E)

Sum of all Ek and Ep of a system

Motion of moleculesMotion of electronsPosition of electrons in electrostatic field of the nucleusInteractions between molecules, etc.


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Virtually impossible to determine or calculate UDuring a chemical reaction can only determine changes in U.U = Ufinal -UinitialThe quantity .U can be measuredVirtually impossible to determine or calculate UDuring a chemical reaction can only determine changes in U

.U = Ufinal -UinitialThe quantity .U can be measuredTwo ways for energy to be transferred:Heat (q) Work (w)SystemSystemq qw w+ work done on the system work done by the system.U = q + w


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So... So...Example

The battery does 555 kJ of workevery hour driving the pump. Thesystem loses 124 kJ of energy per

hour to the surroundings.

Xsystem What is .Usystem per hour?


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Exothermic and Endothermic Processes

Endothermic: absorbs heat from the surroundings.Exothermic: transfers heat to the surroundings.

Note:

An exothermic reaction (q0) feels coldExample

Length of arrow = relative magnitude of energy transferred

Which of these processes is exothermic?For which (if any) is .U < 0? For which (if any) is there a net gain of internal energy?


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Many chemical systems expand or contract when theyreact. These systems are doing expansion(PV) work.HeatCaCO3expanding CO2Many chemical systems expand or contract when they

react. These systems are doing expansion(PV) work.HeatCaCO3expanding CO2


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Expansion work done by a gas

.h

15w = -P.VHaveoppositesignsbecausewhengasexpands(.V>0),systemdoeswork(w


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Example

A 10.0 W heater heats gas in a cylinder for 1 min. Thecylinders volume expands from 1.00 l to 1.50 l. Calculate.U for the gas if Patm = 0.965 atm.

Measuring .U.U = q + w= q -P.VIf the reaction is carried out in a sealed rigid container, .V = 0.U = qVThe change in internal energy of a system is equal to theheat transferred to or from the system at constant volume


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Most reactions carried our under constant pressureDefinitionDefine a new state function called the ENTHALPY ofthe system, H, such thatH = U + PVAs for U, it is virtually impossible to measure or to

calculate H, only .H. Under constant P conditions.H = .U + P.VMost reactions carried our under constant pressureDefinitionDefine a new state function called the ENTHALPY ofthe system, H, such thatH = U + PVAs for U, it is virtually impossible to measure or tocalculate H, only .H. Under constant P conditions.H = .U + P.VMeasuring .H.H = .U + P.V

= q + w + P.V= q P.V + P.V= qP.H = qPThechangeinenthalpyofasystemhasnoeasilyinterpretedmeaningexceptatconstantPwhereitisequaltotheenergyflowasheatForaprocesscarriedutatconstantP,whereonlyPVworkisallowed:At constant P, exothermic means .H < 0 andendothermic means .H > 0


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Example

Which will have the higher enthalpy (same T unlessotherwise indicated)?

1 mol CO2(g) or 1 mol CO2(s)

2 mol of H atoms or 1 mol of H2 molecules1 mol H2(g) and 0.5 mol O2(g) or 1 mol H2O?1 mol N2(g) at 100 oC or 1 mol N2(g) at 300 oC?Measuring heat flows: Calorimetryreactionchamberwater baththermometerT increases for an exothermic reaction and decreases foran endothermic one


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Need to relate q to .THeat capacity, Cheat suppliedC =change in temperatureqC =T.JJKJ KJ K-1K-1

Need to relate q to .THeat capacity, Cheat suppliedC =change in temperatureqC =T.JJKJ KJ K-1K-1Whichmetalwillundergothegreatest

temperaturechangeifanequalamountofheatisaddedtoeach?

1.Fe,Cm=0.45J/gK2.Al,Cm=0.90J/gK3.Cu,Cm=0.38J/gK4.Pb,Cm

=0.13J/g


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K5.Sn,Cm=0.22J/g

K


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Recall that size of 1 degree Kelvin = size of 1 degree centigrade

.T (K) = .T (oC)

reactionchamber

water baththermometer.T will depend on the quantity ofwater; so more common to use theintensive property:

SPECIFIC HEAT CAPACITY Cs; or

MOLAR HEAT CAPCITY Cm

Cs

J K-1 g-1J K-1 g-1CmJ K-1 mol-1J K-1 mol-1q = m Cs/m .TJJg ormolsg ormols J K-1 g-1orJ K-1 mol-1J K-1 g-1orJ K-1 mol-1KK


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ExampleThe molar heat capacity of water is 75.4 J K-1 mol-1. How longmust a 1.000 kW heater run to raise the temperature of 500.0 gof water from 20.0 oC to 80.0 oC?StrategyMass mols energy time

ExampleThe molar heat capacity of water is 75.4 J K-1 mol-1. How longmust a 1.000 kW heater run to raise the temperature of 500.0 gof water from 20.0 oC to 80.0 oC?StrategyMass mols energy timeReaction is open to theatmosphereqP = .Hand we measure reactionenthalpy


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Reaction occurs underconstant volume conditionsqV = .Uand we measure change ininternal energyp. 186 10th Ed

Reaction occurs underconstant volume conditionsqV = .Uand we measure change ininternal energyp. 186 10th EdExample

When 100 ml of an aqueous solution of certain reactantsare mixed, it is known that their reaction releases 1.78 kJ ofenergy.

When this reaction occurred in an open calorimeter, thetemperature rose by 3.65 oC.

Next 50 ml HCl was mixed with 50 ml NaOH in the samecalorimeter; in this case the temperature rise was 1.26 oC.

What is the heat output of the neutralisation reaction?


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Example

A 2.200 g sample of quinone (C6H4O2) is burned in a bombcalorimeter with a heat capacity of 7.854 kJ K-1.

The temperature of the calorimeter increases from 23.44 to

30.57 oC.What is the molar heat of combustion of quinone?Enthalpies of Reactionfinal initialproducts reactants.H = H - H= H - HFor a reaction:Enthalpy is an extensive property (magnitude of .H is directlyproportional to amount):CH4(g) + 2O2(g) .CO2(g) + 2H2O(g) .H = -802 kJ mol-1

2CH4(g) + 4O2(g) .2CO2(g) + 4H2O(g) .H = -1604 kJ mol-1kJ mol-1 for the reaction as written


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For the reverse reaction the sign of .H changes:

CO2(g) + 2H2O(g) .CH4(g) + 2O2(g) .H = +802 kJ mol-1

Values of .H depend on conditions such as pressure and

physical state of reactants and products.

Therefore define the standard enthalpy of reaction .Ho asenthalpy change that occurs when the reaction is under

standard thermodynamic conditions

Pressure: 1 atmosphereTemp: 25 oC

If a reaction is carried out in a number of steps, .H for the

overall reaction is the sum of .H for each individual step.

This is known as Hess Law. It is merely a consequence of theFirst Law of Thermodynamics.

.H1 = .H2 + .H3


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What is the value of the unknown .H in the diagram?

1. +329.5 kJ2. -329.5 kJ3. +285.8 kJ4. -241.8 kJ

5. +241.8 kJMost useful application of Hess Law:

Use extensive data available for heats of combustionsto determine .H for other reactions

Example

Given:C3H8(g) + 5O2(g) .3CO2(g) + 4H2O(l) .Ho = -2220 kJ mol-1C(s) + O2(g) .

CO2(g) .Ho = -394 kJ mol-1H2(g) + O2(g) .H2O(l) .Ho = -286 kJ mol-1

Calculate .Ho for:3C(s) + 4H2(g) .C3H8(g)


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Standard Enthalpies of Formation Standard Enthalpies of FormationDefinition

If 1 mol of compound is formed from its constituentelements in their standard states, then the enthalpychange for the reaction is called the standard

enthalpy of formation, .Hof .

Corollary

Standard enthalpy of formation of the most stableform of an element is zero.

p. 192 in 10th Ed. See also Appendix C


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Whichreactionrepresentsthe .HfreactionforNaNO3?Whichreactionrepresentsthe .HfreactionforNaNO3?Usefulness?We use Hess Law to calculate enthalpies of areaction from enthalpies of formationFor any reaction:

( ) ( ).=-=.-.SSSo o orxn final initialo of fH H Hn H prod n H reactstoichiometriccoefficientsstoichiometriccoefficients


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Example

Calculate .Ho for the combustion of 1 mol of acetylene C2H2.

Lets see how we have used Hess Law to get this result


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Integrated Exercise

Combustion of a hydrocarbon produces 21.83 g ofCO2, 4.47 g of H2O and 311 kJ of heat.

(i)

What mass of hydrocarbon wascombusted?(ii) What is its empirical formula?(iii) Calculate .Hof per empirical formula unit forthe hydrocarbonEnd of Chapter 5:Thermochemistry


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Chem 1012_Diagonal A_Prof Marques_Chapter 5 _ 2010