Question

Explain the criteria that have to be met in order for a mechanism to be valid:

1. The set of elementary steps must add to give the overall reaction.

2. Each step must be physically reasonable.

3. The theoretical mechanism must give the experimentally determined rate law.

Answer 1

1. The sum of each elementary step in a reaction mechanism must yield the overall reaction equation.

Consider the following reaction:

2A (g) + B2(g) ==> 2AB(g)

The overall equation suggests that two A molecules collide with an B2 molecule, forming AB. However, because termolecular reactions are extremely rare, this reaction most likely consists of two or more elementary steps.

The reaction consists 2 elementary steps :

(1) 2A (g) ==> A2

(2) A2 (g) + B2 (g) ==>2 AB (g)

Note that the two steps here add to the overall reaction equation, as the intermediate A2 cancels.

2. Each step must be physically reasonable.

Each step have its activation energy , thus energy and orientation requirements must be met.

3. The theoretical mechanism must give the experimentally determined rate law.

in other words, The mechanism must be consistent with the observed kinetics for the overall reaction. The slowest step in the mechanism is called the rate determining or rate-limiting step. The molecularity of this step must conform with molecularity/ Order in rate law.