Question

The following mechanism has been proposed for the gas-phase reaction of chloroform (CHCl₃) and chlorine:
1) Cl₂ ⇔ 2 Cl (fast, reversible)
2) Cl + CHCl₃ → HCl + CCl₃ (slow)
3) Cl + CCl₃ → CCl₄ (fast)
What rate law does this mechanism predict? Choose from the list below and enter the letters for all the terms needed in the rate law. (So for a rate law of k[Cl₂] enter AC, and for a rate law of k[Cl₂][CHCl₃] enter ACE.)

A. k	G. [Cl]1/2	M. [Cl2]2
B. [HCl]	H. [CHCl3]1/2	N. [Cl]2
C. [Cl2]	I. [CCl3]1/2	O. [CHCl3]2
D. [Cl]	J. [HCl]1/2	P. [CCl3]2
E. [CHCl3]	K. [Cl2]1/2
F. [CCl3]	L. [HCl]2

Answer 1

From the given data -

For the first step: K_eq = [Cl]² / [Cl₂]                ..... (1)

Rate determing step is second step: rate law = k [Cl][CHCl₃]    ..... (2)

From equation:1    [Cl] = [ K_eq*[Cl₂] ]^1/2

If we substitute this in equation:2)

                              rate law = k [ K_eq*[Cl₂] ]^1/2 [CHCl₃]

                                           = k (K_eq)^1/2 [Cl₂]^1/2 [CHCl₃]

If we write whole k' = k (K_eq)^1/2

Rate law = k' [Cl₂]^1/2 [CHCl₃]

Thus answer is AKE