Kinetics – �Catalysis & �Reaction Mechanisms
Lecture 9.5
Catalysis =
- catalysts are not altered during a reaction, they serve to lower Ea and speed up the reaction by offering a different pathway for the reaction
Catalysis
2 types of catalysts:
1) Homogeneous Catalyst =
2) Heterogeneous Catalyst =
Reaction Mechanisms = a series of simple steps that lead the initial reactants to final products
- must be determined experimentally
- must agree with overall stoichiometry AND the experimentally determined rate law
2O3(g) 3O2(g)
Consider the following reaction:
The reaction mechanism consists of two steps:
O3(g) O2(g) + O(g)
O(g) + O3(g) 2O2(g)
2O3(g) 3O2(g)
Reaction Mechanisms
Intermediate =
Catalyst =
Consider the following two step reaction mechanism:
H2O2(aq) + Br2(aq) 2Br-(aq) + 2H+(aq) + O2(g)
H2O2(aq) + 2Br-(aq) + 2H+(aq) Br2(aq) + 2H2O(l)
1) What are the intermediates present? Why?
2) What is the catalyst? Why?
3) What is the overall reaction?
Reaction Mechanisms
A Products
The elementary steps may be:
Molecularity
Elementary Steps
Rate Expression
Unimolecular
Bimolecular
Termolecular
Rate = k[A]
A + B Products
Rate = k[A][B]
Rate = k[A]2
2A Products
A + B + C Products
Rate = k[A][B][C]
OR??
- the individual steps in the reaction mechanism are called elementary steps
Reaction Mechanisms
Ex:
Step 1
Step 2
- the slow step is called the
Ex: Step 1 above
- for a reaction mechanism to be plausible, the rate expression of the RDS AND ALL STEPS PROCEEDING IT must match that of the experimentally determined rate law for the overall reaction
- in a reaction mechanism consisting of several steps, one step may be much slower than any other
Reaction Mechanisms
1) Is this an acceptable mechanism? Justify your answer.