ITT Chng Ch 06 Thermochemistry

7/29/2019 ITT Chng Ch 06 Thermochemistry

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Thermochemistry


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Energyis the capacity to do work

Radiant energy comes from the sun and is

earths primary energy source Thermal energyis the energy associated with

the random motion of atoms and molecules

Chemical energyis the energy stored within the

bonds of chemical substances

Nuclear energyis the energy stored within the

collection of neutrons and protons in the atom

Potential energy is the energy available by virtueof an objects position

6.1


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Heatis the transfer of thermal energybetween two bodies that

are at different temperatures.

Energy hanges in hemical !eactions

Temperatureis a measure of the thermal energy.

"##$##

greater thermal energy 6.%

Temperature & Thermal Energy


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Thermochemistryis the study of heat change in chemical

reactions.

The systemis the specific part of the universe that is of

interest in the study.

open

mass ' energyExchange:

closed

energy

isolated

nothing 6.%


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Exothermic processis any process that gives off heat (

transfers thermal energy from the system to the surroundings.

Endothermic processis any process in which heat has to be

supplied to the system from the surroundings.

%)%*g+, -%*g+ %)%- *l+, energy

)%- *g+ )%- *l+, energy

energy , %)g- *s+ %)g *l+, -%*g+

6.%

energy , )%- *s+ )%- *l+


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Eothermic Endothermic

6.%


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Thermodynamicsis the scientific study of the

interconversion of heat and other kinds of energy.

State functionsare properties that are determined by the stateof the system/ regardless of how that condition was achieved.

0otential energy of hiker 1and hiker %

is the same even though they took

different paths.

energy/ pressure/ volume/ temperature

6.

E& Efinal2 Einitial

P& Pfinal2 Pinitial

V& Vfinal2 Vinitial

T& Tfinal2 Tinitial


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First law of thermodynamics( energy

can be converted from one form to another/

but cannot be created or destroyed.Esystem, Esurroundings& #

or

Esystem& -Esurroundings

)3, 4-% -%, $)%-

Eothermic chemical reaction5

6.

hemical energy lostby combustion & Energy gainedby the surroundingssystem surroundings


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nother form of the first lawfor Esystem

6.

E& q, w

E is the change in internal energy of a system

qis the heat echange between the system and the surroundings

wis the work done on *or by+ the system

w& 2P

V when a gas epands against a constant eternal pressure


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7ork 8one -n the 9ystem

6.

w& F x d

w& 2P V

P x V = x d3= F x d = wF

d2

: ; #

-PV < 0

wsys< #

7ork is

not a

statefunction5

w& wfinal2 winitial

initial final


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sample of nitrogen gas epands in volume from 1.6 = to

4.$ = at constant temperature. 7hat is the work done in

joules if the gas epands *a+ against a vacuum and *b+against a constant pressure of .> atm?

w& 2P V

*a+

V& 4.$ = ( 1.6 = & .3 = P& # atmW& 2# atm .3 = & # =atm & # joules

*b+ V& 4.$ = ( 1.6 = & .3 = P& .> atm

w& 2.> atm .3 = & 21$.1 =atm

w& 21$.1 =atm 1#1. @1=atm

& 21$# @

6.


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Chemistry in Action: Making Snow

E& q, w

q& #

w< #/ E < #

E& T

T < #/ 9A-75


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Enthalpy and the Birst =aw of Thermodynamics

6.$

E& q, w

E& ! 2 PV

!& E , PV

q& !and w& 2PVt constant pressureC


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Enthalpy (His used to Duantify the heat flow into or out of a

system in a process that occurs at constant pressure.

!& !*products+ ( !*reactants+

!& heat given off or absorbed during a reaction at constant pressure

!products < !reactants!< #

!products ; !reactants!; # 6.$


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Thermochemical EDuations

)%- *s+ )%- *l+ ! & 6.#1 k@

s !negative or positive?

9ystem absorbs heat

Endothermic

!; #

6.#1 k@ are absorbed for every 1 mole of ice that

melts at ## and 1 atm.

6.$


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)$*g+ , %-%*g" -%*g"# %)%- *l+ ! & 23"#.$ k@

s !negative or positive?

9ystem gives off heat

Eothermic

!< #

3"#.$ k@ are released for every 1 mole of methane

that is combusted at %4# and 1 atm.

6.$


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)%- *s+ )%- *l+ ! & 6.#1 k@

The stoichiometric coefficients always refer to the number

of moles of a substance


f you reverse a reaction/ the sign of !changes

)%- *l+ )%- *s+ ! & 26.#1k@

f you multiply both sides of the eDuation by a factor n/

then !must change by the same factor n.

%)%- *s+ %)%- *l+ ! & % 6.#1& 1%.# k@

6.$


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)%- *s+ )%- *l+ ! & 6.#1 k@

The physical states of all reactants and products must be

specified in thermochemical eDuations.


6.$

)%- *l+ )%- *g+ ! & $$.# k@

)ow much heat is evolved when %66 g of white

phosphorus *0$+ burn in air?

0$*s+, 4-%*g+ 0$-1#*s+ !& 2#1 k@

%66 g 0$1 mol 0$

1%." g 0$

#1 k@

1 mol 0$ & 6$># k@


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omparison of !and E

%Aa *s+ , %)%- *l+ %Aa-) *aq+ , )%*g+ ) & 26>.4 k@Fmol

E& ! 2 PV t %4 #/ 1 mole )%& %$.4 = at 1 atm

PV& 1 atm %$.4 = & %.4 k@

E& 26>.4 k@Fmol ( %.4 k@Fmol & 2>#.# k@Fmol

6.$


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The specific heat*s+ of a substance is the amount of heat *q+

reDuired to raise the temperature of one gramof the

substance by one degreeelsius.

The heat capacity*C+ of a substance is the amount of heat

*q+ reDuired to raise the temperature of a given quantity*m+

of the substance by one degreeelsius.

& m x s

)eat *q+ absorbed or releasedC

q& m x s x t

q& x t

t& tfinal2 tinitial

6.4


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)ow much heat is given off when an 36" g iron bar cools

from "$# to 4#?

sof Be & #.$$$ @Fg #

t& tfinal( tinitial& 4# ( "$# & 23"#

q& mst& 36" g #.$$$ @Fg # (3"# & 2$/### @

6.4


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onstant2:olume alorimetry

Ao heat enters or leaves5

qsys& qwater, qbomb, qrn

qsys& #qrn& 2 *qwater, qbomb+

qwater& m x s t

qbomb& $om$xt

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!eaction at onstant V

!G qrn

!& qrn


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onstant20ressure alorimetry

Ao heat enters or leaves5

qsys& qwater, qcal, qrn

qsys& #qrn& 2 *qwater, qcal+

qwater& m x s t

qcal& %alxt

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!eaction at onstant P!& qrn


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6.4


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Chemistry in Action:

Buel :alues of Boods and -ther 9ubstances

6)1%-6*s+ , 6-%*g+ 6-%*g+ , 6)%- *l+

) & 2%3#1 k@Fmol

1 cal & $.13$ @

1 al & 1### cal & $13$ @

9ubstance !%om$ustion*k@Fg+

pple 2%

Heef 23

Heer 21.4

Iasoline 2$


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Hecause there is no way to measure the absolute value of

the enthalpy of a substance/ must measure the enthalpy

change for every reaction of interest?

Establish an arbitrary scale with the standard enthalpy of

formation*)#+ as a reference point for all enthalpy

epressions.f

Standard enthalpy of formation*)#+ is the heat change

that results when one moleof a compound is formed from

its elementsat a pressure of 1 atm.

f

The standard enthalpy of formation of any element in itsmost stable form is Jero.

)#*-%+ & #f

)#*-

+ & 1$% k@Fmolf

)#*/ graphite+ & #f

)#*/ diamond+ & 1."# k@Fmolf 6.6


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6.6


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The standard enthalpy of reaction*)# + is the enthalpy of

a reaction carried out at 1 atm.rn

a , $H % , d8

)#rn d)#*8+f%)

#*+f& K , L 2 $)#*H+fa)

#*+fK , L

)#rn n)#*products+f& m)

#*reactants+f2

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Hess!s "aw# 7hen reactants are converted to products/

the change in enthalpy is the same whether the reaction

takes place in one step or in a series of steps.

*Enthalpy is a state function. t doesnt matter how you get

there/ only where you start and end.+


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*graphite+, 1F%-%*g+ - *g+

- *g+, 1F%-%*g+ -%*g+

*graphite+, -%*g+ -%*g+

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alculate the standard enthalpy of formation of 9%*l+

given thatC

*graphite+, -%*g+ -%*g+ )# & 2".4 k@rn

9*rhombic+, -%*g+ 9-%*g+ )# & 2%"6.1 k@rn

9%*l+, -%*g+ -%*g+, %9-%*g+ )# & 21#>% k@rn

1. 7rite the enthalpy of formation reaction for 9%

*graphite+, %9*rhombic+ 9% *l+

%. dd the given rns so that the result is the desired rn.

rn*graphite+, -%*g+ -%*g+ )# & 2".4 k@

%9*rhombic+, %-%*g+ %9-%*g+ )# & 2%"6.1%k@rn

-%*g+, %9-%*g+ 9%*l+, -%*g+ )# & ,1#>%k@rn,

*graphite+, %9*rhombic+ 9% *l+

)# & 2".4 , *%2%"6.1+ , 1#>% & 36. [email protected]


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HenJene *6)6+ burns in air to produce carbon dioide and

liDuid water. )ow much heat is released per mole of

benJene combusted? The standard enthalpy of formation

of benJene is $".#$ k@Fmol.

%6)6*l+, 14-%*g+ 1%-%*g+, 6)%- *l+

)#rn n)#*products+f& m)

#*reactants+f2

)#rn 6)#*)%-+f1%)

#*-%+f& K , L 2 %)#*6)6+fK L

)#rn & K 1%(".4 , 6(13>.6 L ( K %$".#$ L & 24"$6 k@

24"$6 k@

% mol& 2 %"> k@Fmol 6)6

6.6


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Chemistry in Action: Bomardier Beetle !efense

6)$*-)+%*aq+, )%-%*aq+ 6)$-%*aq+, %)%- *l+ !0& ?

6)$*-)+%*aq+ 6)$-%*aq+, )% *g+ !0& 1>> k@Fmol

)%-%*aq+ )%- *l+, M-% *g+ !0& 2"$.6 k@Fmol

)%*g+, M-%*g+ )%- *l+ !0& 2%36 k@Fmol

!0& 1>> 2 "$.6 ( %36 & 2%#$ k@Fmol

Eothermic5

Th h l f l i * H + i h h d


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The enthalpy of solution*Hsoln+ is the heat generated orabsorbed when a certain amount of solute dissolves in a

certain amount of solvent.

!soln& !soln 2 !components

6.>

7hich substance*s+ could be

used for melting ice?

7hich substance*s+ could be

used for a cold pack?

The 9olution 0rocess for Aal


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The 9olution 0rocess for Aal

!soln & 9tep 1 , 9tep % & >33 >3$ & $ k@Fmol 6 >

ITT Chng Ch 06 Thermochemistry

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