ReviewReaction mechanism
Br2(l)
step 1 Br2 2 Br.h
step 2 Br. +step 3 C5H11
. +
overall Br2
C5H12 HBr + C5H11.
Br. C5H11Br
+ C5H12(l) C5H11Br(l) + HBr(l)
+ C5H12 C5H11Br + HBr
Br2(l) + C5H12(l) C5H11Br(l) + HBr(l)
step 1 Br2 2 Br.h
step 2 Br. +step 3 C5H11
. +
C5H12 HBr + C5H11.
C5H11BrBr.assume step 2 is rate determining(slow)
rate = k2 Br.= intermediate[Br.][C5H12]Keq =
[Br.]2
[Br2]
[Br.] =
rate = k2 k’[Br2]1/2 [C5H12]rate = k’[Br2]1/2 [C5H12]11/2 order reaction Keq
1/2 [Br2]1/2
increase [react]
bimolecular elementary steps
increase rate of reactionincrease T increase rate of reaction
increase number of collisionsincrease force of collisions
minimum energy required for reaction:
activation energy
T1
T2
Ea
# molecules
Kinetic Energy
T2 > T1
= Ea
Arrhenius EquationT dependence of a rate constant, k
k =
R = gas constant (8.314 x 10-3kJ/K mol)T = temperature (K)z = collision frequencyp = steric factor (<1)
z p e-Ea/RT
Ea = activation energy (kJ/mol)
p = steric factor
z = collision frequency
combine to give A
k =A e-Ea/RT
Arrhenius Equation
k = A e-Ea/RT
ln k =y = m x + b
ln (k2/ k1) =
plot ln k
- (Ea/R) (1/T)ln A- (Ea/R) (1/T) + ln A
v.s. 1/T slope = -Ea/R intercept = ln A
(Ea /R) (1/T1 - 1/T2)
P.E.C5H12
+ Br2
C5H11Br + HBr
step 2 Br. + C5H12 HBr + C5H11.
step 3 C5H11. + Br. C5H11Br
2Br.
+ C5H12
HBr + C5H11.
+ Br.
Ea
Ea
Ea
Br2(l) + C5H12(l) C5H11Br(l) + HBr(l)h
step 1 Br2 2 Br.h
Br2(l) + C5H12(l) C5H11Br(l) + HBr(l)
Hrxn
Reaction coordinate
P.E.Br2 + C5H12
C5H11Br + HBr
Hrxn < 0
Ea
Arrhenius Equationln k = - (Ea/R) (1/T) + ln A
ln K = - (Ho/R) (1/T) + So/RK = kf/kr
A orientation factor related to So
Ea related to Ho
A-B + C
A + B-C
A...B...Cactivated complexP.E. is at a maximumtransition state
A-B + C A...B...C A + B-C
P.E.
Reaction coordinate
Eaf
Eab
Hrxn
reactants
products
activated complex
P.E.
Reaction coordinate
Eaf
Eab
Hrxn
reactants
products
activated complex
exothermic Eab Eaf>
endothermic
products
reactants
Eaf
Eab
P.E.
Reaction coordinate
activated complex
exothermic Eab Eaf>
endothermic Eab Eaf<large Ea = slow rate
- catalyst + catalystlowers Eaf lowers Ear
Keq unchangedand H
faster forward reaction ( kf )faster reverse reaction ( kr )
kf kr
= Keq
