Chemistry: Mechanisms

Question

The following mechanism has been proposed for the gas phase reaction of nitrogen monoxide with bromine.

.....step 1.....fast:......NO + Br₂  NOBr₂

.....step 2.....slow:....NOBr₂ + NO  2 NOBr

(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

(1) sum the elementary steps:

2NO + Br₂ ==> 2NOBr ... overall reaction

(2) NOBr₂ is an intermediate

(3) The rate law will be determined by the SLOW step so would be

rate = k[NOBr₂][NO] HOWEVER NOBr₂ does not appear in the overall reaction. So, we must use step 1 and solve for [NOBr₂] and then substitute that into step 2 equation.

Since step 1 is reversible and is at equilibrium, we can set the rate of the forward reaction equal to the rate of the reverse reaction and obtain the following:

k1[NO][Br₂] = k2[NOBr₂] and then [NOBr₂] = k1/k2 [NO][Br₂]

For step 2 we have Rate = k3[NOBr₂][NO] and substituting for [NOBr₂] we have...

Rate = k3 * k1/k2[NO][Br₂][NO] and we can group the 3 rate constants as a single constant called k'

Finally,..

Rate = k' [NO]²[Br₂] and this is in agreement with the overall reaction