Physical Chemistry Third Edition

556 12 Chemical Reaction Mechanisms I: Rate Laws and Mechanisms

12.33 a.Find the rate law for the reaction

H++I−+H 2 O 2 
H 2 O+HIO

assuming the mechanism

(1) H++H 2 O 2 
H 3 O+ 2 (fast)

(2) H 3 O+ 4 +I−−→H 2 O+HIO

(slow, rate-limiting)

b.Find the rate law assuming the mechanism

(1) H++I−
HI

(2) H 2 O 2 +HI−→H 2 O+HIO

(slow, rate-limiting)

12.34The reaction

H 2 O 2 +3I−+2H+−→2H 2 O+I− 3

obeys the rate law^18

−

d[H 2 O 2 ]

dt

k 1 [H 2 O 2 ][I−]+k 2 [H 2 O 2 ][I−][H+]

The two terms presumably correspond to two competing

mechanisms. Propose a reasonable mechanism for the

first term, and two different reasonable mechanisms for

the second term.

12.35The hypothetical reaction

2A−→2B+C

obeys the rate law

rate

d[C]

dt

k

[A]^2

[B]

Propose a mechanism consistent with this rate law.

12.5 Chain Reactions

Chain mechanisms involve reactive intermediates that are calledchain carriers. A chain

mechanism usually contains aninitiationstep, in which chain carriers are formed; one or

morechain propagation stepsin which products are formed and in which chain carriers

are produced as well as being consumed; and achain termination stepin which chain

carriers are consumed without being replaced. Since chain carriers are produced as

well as being consumed, the reaction can continue without further initiation steps.

The following gas-phase reaction has been identified as a chain reaction:

H 2 +Br 2 −→2HBr (12.5-1)

The empirical rate law for the forward reaction in the presence of some HBr is

−

d[H 2 ]

dt



ka[H 2 ][Br 2 ]^1 /^2

1 +kb[HBr]/[Br 2 ]

(12.5-2)

wherekaandkbare temperature-dependent parameters. The accepted mechanism for

the forward reaction is^19

(1) Br 2 
2Br

(2) Br+H 2 
HBr+H

(3) H+Br 2 −→HBr+Br

(12.5-3)

Step 1 is the initiation step. The forward reactions of steps 2 and 3 are chain propagation

steps, producing the two chain carriers, Br and H. The reverse reaction of step 1 is the

termination step. The reverse reactions of steps 2 and 3 regenerate chain carriers but

consume the product. They are calledinhibition processes.

(^18) D. Benson,Mechanisms of Inorganic Reactions in Solution, McGraw-Hill, New York, 1968, p. 6ff.
(^19) K. J. Laidler,op. cit., p. 291ff (note 3).