ITT Chng Ch 14 Chemical Equilibrium

7/29/2019 ITT Chng Ch 14 Chemical Equilibrium

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Chemical Equilibrium


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Equilibriumis a state in which there are no observable

changes as time goes by.

Chemical equilibriumis achieved when:

the rates of the forward and reverse reactions are equal and

the concentrations of the reactants and products remain

constant

Physical equilibrium

H2O l!

Chemical equilibrium

"2O#g!

$#.$

H2O g!

2"O2g!


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"2O#g! 2"O2g!

%tart with "O2 %tart with "2O# %tart with "O2& "2O#

equilibriumequilibrium

equilibrium

$#.$


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$#.$

constant


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"2O#g! 2"O2g!

' #.() * $+,)K'-"O22

-"2O#

a/ 0 b1 cC 0 d

K'-Cc-d

-/a-1bLaw of Mass Action

$#.$


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K33 $

K44 $

5ie to the right 6avor products

5ie to the left 6avor reactants

Equilibrium 7ill

K'

-Cc-d

-/a-1ba/ 0 b1 cC 0 d

$#.$


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Homogenous equilibrium applies to reactions in which all

reacting species are in the same phase.

"2O#g! 2"O2g!

Kc'-"O2

2

-"2O#Kp'

"O2P2

"2O#P

a/ (g)0 b1 (g) cC (g)0 d (g)

$#.2

Kp' Kc(RT)n

n ' moles of gaseous products 8 moles of gaseous reactants

' c0 d)8 a0 b!

9n most cases

KcKp


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Homogeneous Equilibrium

CH)COOH aq!0 H2O l! CH)COO,aq!0 H)O

0 aq!

Kc'-CH)COO

,-H)O0

-CH)COOH-H2O-H2O ' constant

Kc=-CH)COO

,-H)O0

-CH)COOH' Kc-H2O

;eneral practice notto include units for the

equilibrium constant.

$#.2


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and -COCl2 '

+.$# M. Calculate the equilibrium constants Kcand K

p.

CO g!0 Cl2g! COCl2g!

Kc'

-COCl2

-CO-Cl2 '

+.$#

+.+$2 * +.+?# ' 22+

Kp' Kc(RT)n

n' $ 8 2 ' ,$ R' +.+@2$ T' 2=) 0 =# ' )#= A

Kp' 22+ * +.+@2$ * )#=!,$' =.=

$#.2


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Heterogenous equilibrium applies to reactions in which

reactants and products are in different phases.

CaCO)s! CaO s!0 CO2g!

Kc'-CaO-CO2

-CaCO)

-CaCO) ' constant

-CaO ' constant

Kc' -CO2 ' Kc*-CaCO)

-CaOKp' PCO2


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PCO 2 ' Kp

CaCO)s! CaO s!0 CO2g!

PCO 2 does not depend on the amount of CaCO)or CaO

$#.2


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Consider the following equilibrium at 2D? A:


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/ 0 1 C 0

C 0 E 0 6

/ 0 1 E 0 6

Kc'-C-

-/-1Kc'

-E-6

-C-

-E-6-/-1Kc'

Kc

Kc

Kc

Kc' KcKc *

9f a reaction can be e*pressed as the sum of

two or more reactions> the equilibrium

constant for the overall reaction is given by

the product of the equilibrium constants of

the individual reactions.

$#.2


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"2O#g! 2"O2g!

' #.() * $+,)K' -"O22

-"2O#

2"O2g! "2O#g!

K ' -"2O#

-"O22

' $K

' 2$(

7hen the equation for a reversible reactionis written in the opposite direction> the

equilibrium constant becomes the reciprocal

of the original equilibrium constant.

$#.2


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7riting Equilibrium Constant E*pressions

$. the concentrations can be e*pressed in Mor in atm.

2. pure liquids and solvents

do not appear in the equilibrium constant e*pressions.

). you must

specify the balanced equation and the temperature.

?. 9f a reaction can be e*pressed as a sum of two or morereactions> the equilibrium constant for the overall reaction is

given by the product of the equilibrium constants of the

individual reactions.

$#.2


17/29 $#.)

Chemical Ainetics and Chemical Equilibrium

/ 0 21 /12

kf

kr

ratef' kf-/-12

rater' kr-/12

Equilibrium

ratef' rater

kf-/-12' kr-/12

kf

kr

-/12

-/-12

' Kc'


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Calculating Equilibrium Concentrations

$. E*press the equilibrium concentrations of all species in

terms of the initial concentrations and a single unnownx>

which represents the change in concentration.

2. 7rite the equilibrium constant e*pression in terms of theequilibrium concentrations. Anowing the value of the

equilibrium constant> solve forx.

). Having solved forx> calculate the equilibrium

concentrations of all species.

$#.#

/t $2@++C th ilib i t t K ! f th ti


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/t $2@++C the equilibrium constant Kc! for the reaction

9s $.$ * $+,). 9f the initial concentrations are -1r2 ' +.+() M

and -1r ' +.+$2 M> calculate the concentrations of these

species at equilibrium.

1r2g! 21r g!

1r2g! 21r g!

5etxbe the change in concentration of 1r2

9nitial M!

Change M!

Equilibrium M!

+.+() +.+$2

,x 02x

+.+() ,x +.+$2 0 2x

-1r2

-1r2Kc' Kc'

+.+$2 0 2x!2

+.+() ,x' $.$ * $+,) %olve forx

$#.#


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Kc'+.+$2 0 2x!2

+.+() ,x' $.$ * $+,)

#x20 +.+#@x0 +.+++$## ' +.++++(D) 8 +.++$$x

#x20 +.+#D$x0 +.++++=#= ' +

ax20 bx0 c'+,b b28 #ac

2ax'

1r2g! 21r g!

9nitial M!

Change M!

Equilibrium M!

+.+() +.+$2

,x 02x

+.+() ,x +.+$2 0 2x

x' ,+.++$=@x' ,+.+$+?

/t equilibrium> -1r ' +.+$2 0 2x' ,+.++D Mor +.++@## M

/t equilibrium> -1r2 ' +.+(2 8x' +.+(#@ M

$#.#


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9f an e*ternal stress is applied to a system at equilibrium> the

system adFusts in such a way that the stress is partially offset

as the system reaches a new equilibrium position.

Le Chteliers Princile

Changes in Concentration

"2g!0 )H2g! 2"H)g!

/dd

"H)

Equilibrium

shifts left to

offset stress

$#.?


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Changes in Concentration continued

Change Shifts the Equilibrium

9ncrease concentration of products! leftecrease concentration of products! right

ecrease concentration of reactants!

9ncrease concentration of reactants! right

left

$#.?

a/ 0 b1 cC 0 d

/dd/ddGemove Gemove


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Changes in olume and Iressure

/ (g)0 1 (g) C (g)

Change Shifts the Equilibrium

9ncrease pressure %ide with fewest moles of gas

ecrease pressure %ide with most moles of gas

ecrease volume9ncrease volume %ide with most moles of gas

%ide with fewest moles of gas

$#.?


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Changes in


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uncatalyJed catalyJed

$#.?

Catalyst lowers Eafor bothforward and reverse reactions.

Catalyst does not change equilibrium constant or shift equilibrium.

/dding a Catalystdoes not change K

does not shift the position of an equilibrium systemsystem will reach equilibrium sooner


Ch i t I A ti


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Chemistry In Action

5ife at High /ltitudes and Hemoglobin Iroduction

Kc'-HbO2

-Hb-O2

Hb aq! 0 O2aq! HbO2aq!

Ch i t I A ti


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Chemistry In Action


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Change Shift Equilibrium

Change Equilibrium

Constant

Concentration yes no

Iressure yes no

olume yes no

ITT Chng Ch 14 Chemical Equilibrium

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