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 reactionKeq

1/2 [Br2]1/2

increase [react]

bimolecular elementary steps

increase rate of reaction

increase T increase rate of reactionincrease number of collisionsincrease force of collisions

minimum energy required for reaction:

activation energy

T1

T2

Ea

# molecules

Kinetic Energy

T2 > T1

= Ea

Arrhenius Equation

T dependence of a rate constant, k

k = k a) increasesb) decreasesk

R = gas constant (8.314 x 10-3kJ/K mol)T = temperature (K)z = collision frequencyp = steric factor (<1)

with Tz p e-Ea/RT

with Eaa) decreasesb) increases

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)

A-B + C

A + B-C

A...B...Cactivated complex

P.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

Br2(l) + C5H12(l) C5H11Br(l) + HBr(l)

Hrxn

Reaction coordinate

P.E.Br2 + C5H12

C5H11Br + HBr

Hrxn < 0

Ea

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