A Reaction Mechanism is the complete description of how the reactants are converted to products.
A reaction mechanism is presented as a series of single-step changes called elementary process or reaction which expresses how molecules or ions react with each other.
Based on numbers of molecules involved in the elementary step, there are three kinds of elementary steps:
1. Unimolecular step (or process)
- an elementary process that involves the decomposition of a single molecule or ion.
- is always a first order reaction
A = B + C + D, Rate = k [A]
A* = X + Y, Rate = k [A*]
A* represents an excited molecule.
- an elementary process in which 2 molecules or ions collide.
A + A = B + C, Rate = k [A]2
A + B = X + Y, Rate = k [A] [B]
3. Trimolecular/Termolecular Process
- simultaneous collision of three molecules.
(3) O + O2 + N2 = O3 + N2
A + A + A = products, Rate = k [A]3
A + A + B = products, Rate = k [A]2 [B]
A + B + C = products, Rate = k [A] [B] [C]
Rate = k [O] [O2] [N2]
- is the slowest step in a chemical reaction.
- determines the rate of overall reactions.
Many reactions occur in a number of elementary steps.
ex.
The problem is to postulate a reaction mechanism which will confirm to the rate law and produce the net reaction.
(1) Slow: H2 + ICl ---> HI + HCl
(2) Fast: HI + ICl ---> I2 + HCl
Net: H2 + 2ICl ---> I2 + 2HCl
The rate law expressions for the two elementary steps are
Rate (1) = k1 [H2] [Cl] and Rate (2) = k2 [HI] [ICl]
Since the rate law of original equation is also the rate law of equation (1) in the reaction mechanism, then it is the slow step or the rate determining step.
Reaction intermediate/product
- the specie that is formed in one step of the reaction mechanism and consumed in a subsequent step.
- does not appear in the net equation for the overall.
- the specie that is introduced as a reactant and later regenerated without change in the subsequent step.
- like a reactant product it also does not appear in the net equation for the overall.
EXAMPLE 1
A mechanism proposed for the reaction between SnCl2 and FeCl3 is as follows.
SnCl2 + 2Cl ---> SnCl4
SnCl4 + FeCl3 ---> SnCl3 + FeCl2 + 2Cl
SnCl3 + 2Cl3 ---> SnCl4 + FeCl2
a. Write the overall chemical reaction.
b. What are the reaction intermediates of the reaction?
c. What specie(s) serve the catalyst?
Solution:
a. The overall reaction is the sum of the three equations
SnCl2 + 2Cl ---> SnCl4
SnCl4 + FeCl3 ---> SnCl3 + FeCl2 + 2Cl
SnCl3 + FeCl3 ---> SnCl4 + FeCl2
SnCl2 + 2FeCl3 ---> SnCl4 + FeCl2
b. The intermediate products or reaction intermediates are the specie(s) formed in one elementary step and consumed in a subsequent step. So the intermediates are SnCl4 and SnCl3.
c. The catalyst is Cl because it is introduced as a reactant and later regenerated without change in the subsequent step.
Sources:
Breaking Through Chemistry
by: Saranay M. Baguio & Rose Mary B. Butaran