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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
7/29/2019 ITT Chng Ch 14 Chemical Equilibrium
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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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Le Chteliers Princile
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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Le Chteliers Princile
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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Le Chteliers Princile
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
Le Chteliers Princile
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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Le Chteliers Princile
Change Shift Equilibrium
Change Equilibrium
Constant
Concentration yes no
Iressure yes no
olume yes no