2. ln-cre2
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Lecture 2
Stoichiometry and Chemical
Reaction Equilibria
Departemen Teknik Kimia
Universitas Indonesia
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Stoichimetry, Extent of Reaction and
ConversionStiochiometry coefficients
a A+ b B c C + d DA + b/a Bc/a C + d/a D
= c/a + d/ab/a -1
A A + B B + CC + D D =0A = -1,B = - b/a,C = c/a,D = d/aI: coeff. stoichiometry 0iiAv
Extent ofreaction,
dn1/v1 = dn2/v2= dn3/v3= = dni/vi= d= (ni-ni0)/vi
Conversion, X
X = (ni0-ni)/ni0 = - (vi/nio)
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Examples:2HI22
IH rf=
kfCH2CI
2
rb=kbCHI
2and
Forward reaction is 1st order in CH2and CI
2( 2nd order overall)
Reverse reaction is 2nd order in CHI
Stoichiometric Equation
This describes theoverall reaction
but the reaction ordercannot
be deduced from it.
Compare the above reaction with the analogous nonelementaryreaction between H2and Br2 .
2HBr22
BrH 2
22
2
211
BrHBr
BrH
f CCk
CCk
r
k1
k2
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Chemical Reaction Equilibrium
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Kinetics vs. Themodynamics
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Chemical Reaction Equilibria
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Equilibrium state for given T and PCriteria :
(G)T,P = (G/)T, P= 0iiv
(G) : Gibbs free energy change, i: the chemical potential
0)ln()( , ii
oiiPT aRTGvG
- RT ln Ka= Go
aaaa
ab
BA
ad
D
ac
CRTG/
//
ln0
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Consider the general reversible reaction:
DcC dbBaA At equilibrium rA=0
Therefore:
Therefore:b
B
a
A
d
D
c
CC
bB
fA
CC
CCK
k
k
Thermodynamic equilibrium relationship
b
B
a
AfAfA CCkr
d
D
c
CbAbA CCkr 0 bAfAA rrr
rfA= rbAdD
c
CbA
b
B
a
AfA CCkCCk
Thermodynamic equilibrium constant
TTR
HTKTK RXCC
11exp)()(
1
1
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TTR
HTKTK RXCC
11exp)()(
1
1
KC
T
KC
T
Endothermic Exothermic
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All reactions are reversible in principle. The extent of reversibility depends on -G, the Gibbs
Free Energy change.
pRTInKG Where Kpis the equilibrium constant interms of partial pressures.
Chemical equilibrium
If Kp is large, reaction is essentially irreversible, whichmeans that the equilibrium position lies very far to the
product side.
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Equilibrium Constants
Pa KKK ai= fi/fi,o= iPi
CCCC
ab
BA
ad
D
ac
CCK /
//
CCCC
ab
BA
ad
D
ac
CCK /
//
Pa KKK
)(RTKK CP
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Impact of T, P, Inert changes
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Total pressure changePi=yi P
KP=KyP
d ln K/dT = Ho(T) /RT2
Temperature change
dTCHTH opoo
)(
plot log K vs 1/T , slope = -(Ho/R), (see Fig EC-1.1)Exo. Reaction (Honegative), K decreases with increasing TEndo. Reaction (Hopositive), K increases with increasing T
Ka=Kf/P [P/nA+ nB+nC+nD+ nI]c+d-a-b)(
dD
cC
bB
aA
nn
nnInert change
Impact of T, P, Inert changes
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Calculation ProcedureDetermine (Go) or (Ho).
a A + b B c C + d D maka:Xo= c. Xof,C+ d. X
of,D a. X
of,A b. X
of,B , X : G, H
Calculate KPat T
with Ho(T) = Ho(25oC) +CpdT2
)(ln
RT
TH
dT
Kd oP
)()(ln TGTKRT o
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Calculation ProcedureCombine equilibrium composition and Equilibrium
constant
b
B
a
A
d
D
c
C
aa
aaK
.
.iiaK
Express in Extent of reaction, composition or conversion
0 iiAv
oi
i
oi
ioiin
v
n
nnX
Solve analytically/graph or numerically
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Example 3-6 The reversible gas-phase decomposition of nitrogentetroxide, N2O4, to nitrogen dioxide, NO2, is to be carried out atconstant temperature. The feed consists of pure N2O4at 340 K and202.6 kPa (2 atm). The concentration equilibrium constant, Kc, at 340
K is 0.1 mol/dm3. BA 2
concentration equilibrium constant:at
equilibrium !!
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(a) Calculate the equilibrium conversion of N2O4in a constant-volume batch reactor.
atequilibrium !!
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(b) Calculate the equilibrium conversion of N2O4in a flowreactor.
atequilibrium !!
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for batch system
for flow system
(c) Assuming the reaction is elementary, express the rate ofreaction solely as a function of conversion for a flow systemand for a batch system.
Elementary reaction:
BA 2
(d) Determine the CSTR volume necessary to achieve 80% of
the equilibrium conversion.
CSTR design equation
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HW2
Scott Fogler P3-11A
P3-15B
P3-16B
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