Question

If the proposed mechanism for the reaction is

i. I2 → 2 I fast

ii. 2 I → I2 fast

iii. H2 + 2I → 2 HI slow

(a). Identify intermediate for the reaction.

(b). Derive the rate law for the reaction

Answer 1

Part A

The intermediate is $I$

PART B

The slowest elementary step is

$H_2+2I\to\>2HI$

Which is the rate - determining step

If this reaction occured in a single step

Its law would be

Rate $=K_2[I]^2[H_2].........(i)$

Considering equilibrium between

$I_2\>and\>2I$

The rate of forward reaction will be equal to the rate of the reverse reaction.

$K_1\>\>[I_2]=K_{-1}\>\>[I]^2......(ii)$

$[I]^2=\frac{K_1}{K_{-1}}\>[I_2].......(iii)$

Substituting $(iii)\>in\>(ii)$

Rate $=K_2(\frac{K_1}{K_{-1}}\>[I_2])\>\>[H_2])$

$=K[H_2]\>[I_2]$