The gas phase reaction between hydrogen and iodine is proposed to occur as follows:

Question

The gas phase reaction between hydrogen and iodine is proposed to occur as follows:

.....step 1.....fast:......I₂  2 I

.....step 2.....slow:....H₂ + 2 I  2 HI

(1) What is the equation for the overall reaction? Use the smallest integer coefficients possible. If a box is not needed, leave it blank.

+

+

(2)

Enter the formula of any species that acts as a reaction intermediate? If none leave box blank:

(3) Complete the rate law for the overall reaction that is consistent with this mechanism.

Use the form k[A]^m[B]ⁿ... , where '1' is understood (so don't write it if it's a '1') for m, n etc.

Rate =

Answer 1

I₂ <==> 2I ... fast

H₂ + 2I ==> 2HI ... slow

(1). H₂ + I₂ ==> 2HI ... overall

(2). I

(3). Rate = k[H₂][I]² based on the slow step, BUT I is an intermediate and intermediates cannot appear in the rate law, so we need some manipulations:

From the fast step, we can write k₁[I₂] = k_-1[I]²

Solving for [I]² we have [I]² = k_1/k_-1 [I₂]

Substituting this into the rate law Rate = k[H₂][I]² we have ...

Rate =k[H₂][ k_1/k_-1 [I₂]

Combining rate constants we have our final rate law as ...

Rate = k'[H₂][I₂]