rate determing step

[yangxu]

hello everyone, I was trying to do a practice exam last night and come upon this question about rate determining step, I'm confused on how you can tell which step is the rate determining step from a table that gives you rates and concentrations for individual reactant, here's the question:

 

consider the following data for the reaction:

 

H2O2 + 3I + 2H -> I3 + 2H2O

 

[H2O2] [H] Rate

I. 0.100M 5.00E-3M 1.00E-2M 0.137M/s

II. 0.100M 1.00E-2M 1.00E-2M 0.268M/s

III. 0.200M 1.00E-2M 1.00E-2M 0.542M/s

IV. 0.400M 1.00E-3M 2.00E-2M 0.108M/s

 

Two mechanisms are proposed:

 

I. H2O2 + I -> H2O + OI

OI + H -> HOI

HOI + I + H -> I2 + H2O

I2 + I -> I3

 

II. H2O2 + I + H -> H2O + HOI

HOI + I + H -> I2 + H2O

I2 + I -> I3

 

which of the following describes a particularly correct mechanism?

a. mechanism I with first step as the rds

b. mechanism I with second step as the rds

c. mechanism II with first step as the rds

d. mechanism II with second step as rds

e. none of the above

 

the answer is a, but I don't know how it's obtained, I tried to solve for the rate law in the format of Rate = k[A]etc. but I still can't tell which step is rate determining, could someone please help? thanks a lot!

[wolfson]

B + B ==> B-B (slow)

 

B-B + A ==> AB2 (fast)

 

A + B ==> A-B (fast)

 

A-B + B ==> AB2 (slow)

 

A + B2 + C ==> AB + BC

 

Rate=k[A] [C]

 

2A + B ==> A2B

 

Rate=k[A]^2

[yangxu]

thank you so much wolfson!

[wolfson]

As always anytime