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
(2) Enter the formula of any species that acts as a reaction intermediate?
(3) Complete the rate law for the overall reaction that is consistent with this mechanism.

Answer 1

Answer:

1) The overall chemical equation will be :

$2NO+Br_2\rightarrow  2NOBr$

2) The species which acting as a intermediate is $NOBr_2$.

3) The rate law for the overall reaction : $R=K[NO]^2[Br]$

Explanation:

1)  Step 1 : Fast:

$NO+ Br_2\rightarrow NOBr_2$ ..[1]

Step 2: Slow:

$NOBr_2 +NO\rightarrow 2 NOBr$...[2]

The overall chemical equation will be :

[1] + [2]

$2NO+Br_2\rightarrow  2NOBr$

2) Intermediate are those chemical species are which formed in between the process of chemical reaction , but do not appears in overall reaction. They act as a product and reactant in the process where chemical reaction taking place in more than one step.

The species which acting as a intermediate is $NOBr_2$.

3) Rate law for the overall reaction that is consistent with this mechanism:

When the reaction is taking place in more than one  step than the rate of the reaction is determined by the step in which reaction progress is slow.

So, for the given mechanism the rate of the overall chemical creation will be determined by step 2:

Step 2: Slow:

$NOBr_2 +NO\rightarrow 2 NOBr$...[2]

Rate of the reaction :

$R=k[NOBr_2][NO]$..[3]

From step 1 :

$K_c=\frac{[NOBr_2]}{[NO][Br_2]}$

$[NOBr_2]=K_c\times [NO][Br_2]$

Putting value of $[NOBr_2]$ rate expression [3]:

$R=k\times k_c[NO][NO][NO]=K[NO]^2[Br]$

Where : $K=k\times k_c$

The rate law for the overall reaction : $R=K[NO]^2[Br]$