Reaction mechanisms?

Question

Example 1: Consider the following reac<on:
2 NO(g) + Br2(g) → 2 NOBr(g)

(a) Write the rate law for the reaction, assuming it involves a single elementary reaction. (b) Is a single step mechanism likely for this reaction?

Example 2; A reaction is believed to occur in two steps:

O3 + NO2--> NO3 + O2

NO3 + NO2 --> N2O5

 (a) Write the overall reaction. (b) The experimental rate law is rate = k[O3][NO2]. What can you say about the relative rates of the two steps of the mechanism. 

 

 

Answer 1

Step 1:

Example 1:

(a)

The chemical equation is .

Single elementary reaction:

Every reaction is made up of one or more elementary steps.

Rate law of the elementary reaction depends on the concentration of reactants only.

For example, is a elementary reaction,

then rate law of the reaction is defined by .

Rate law of the reaction is .

Step 2:

(b)

The chemical equation is .

The reaction is a termolecular reaction.

A termolecular reaction requires the collision of three particles at the same place and time.

A termolecular reaction is a very rare reaction as the three molecules reacting at same time is unlikely.

Hence the single step mechanism for this reaction is unlikely.

Solution:

(a) Rate law of the reaction is .

(b) The single step mechanism for this is unlikely because it is a termolecular reaction.

Answer 2

Step 1:

Example 2:

(a)

The reaction is taken in two steps:

Add the two elementary reactions.

As appears in equal amount on the both sides of the reaction, it can be eliminated.

The net equation is .

Hence, the overall reaction is .

Step 2:

(b)

The step 1 reaction is slow reaction and step 2 reaction is the fast step reaction.

 The rate determining step is rate of reaction of step 1.

Rate law of the reaction of step 1 is .

Hence, the rate of reaction of the overall reaction is .

Solution:

(a) The overall reaction is .

(b) The rate of reaction of the overall reaction is .