1. Reaction Mechanisms Overall Reaction: A → Z
Reaction mechanism - sequence of molecular events, or elementary reaction steps, that defines the pathway from reactants to products.
Overall Reaction: A → Z
Reaction Mechanism: A → B → C → D → Z

2. Reaction Mechanisms
Each individual step in a mechanism is called an elementary step (or reaction).
An elementary step describes how individual atoms or molecules change. It generally involves the forming or breaking of 1 or 2 bonds.
An overall reaction describes the reaction stoichiometry of the balanced chemical equation. It may be the result of many bonds breaking and forming.

3. Elementary Steps Most elementary steps are one of two types:
1. Single Reactant – one reactant forms two products (or rearranges into one new product).
Example:
2. Two Reactants – two reactants collide to form new product(s):
Example:

4. Reaction Mechanisms
The balanced chemical equation for the reaction of nitrogen dioxide with carbon monoxide:
NO2(g) + CO(g)  NO(g) + CO2(g) Overall
What is the mechanism for this reaction?
(What series of elementary steps will give this overall transformation?)
Does NO2 collide with CO and transfer an atom? Does NO2 first split apart to NO and O?

5. Reaction Mechanisms NO2(g) + NO2(g)  NO(g) + NO3(g) Elementary
NO3(g) + CO(g)  NO2(g) + CO2(g) Elementary
An elementary reaction is a an individual molecular event (one step) that involves the forming and/or breaking of chemical bonds.
In the first elementary step, NO2 molecules collide and an oxygen atom is transferred. One bond is broken, one bond is formed.

6. Reaction Mechanisms NO2(g) + NO2(g)  NO(g) + NO3(g) Elementary
NO3(g) + CO(g)  NO2(g) + CO2(g) Elementary
NO2(g) + CO(g)  NO(g) + CO2(g)
The elementary steps must sum up properly to give correct stoichiometry for the overall chemical reaction.

7. Rate Laws, Reaction Mechanisms
Rate law for overall reaction is determined experimentally.
Rate law for elementary step follows from its molecularity.

8. Reaction Mechanisms
Molecularity: the number of molecules (or atoms) on the reactant side of the chemical equation for an elementary step.
Unimolecular:
Example:

9. Reaction Mechanisms
Unimolecular: single reactant molecule bond-breaking only.
What will the rate depend on?

10. Reaction Mechanisms Bimolecular: Two reactant molecules (collision).
Rate of reaction – depends on O3 and O

11. Reaction Mechanisms
Bimolecular: How do we get a step that is bimolecular in A? rate = k [A]2
Example: formation of O2

12. Rate Laws, Reaction Mechanisms
Rate law for an overall reaction is determined experimentally.
Rate law for an elementary step follows from its molecularity.

13. Rate Laws and Reaction Mechanisms
The rate law of each elementary step follows its molecularity.
The slowest elementary step in a multistep reaction is called the rate-determining step.
The overall reaction cannot occur faster than the speed of the rate-determining step.
The rate of the overall reaction is therefore determined by the rate of the rate-determining step.

14. Rate Laws and Reaction Mechanisms

15. The Arrhenius Equation 01
Collision Theory: A bimolecular reaction occurs when two correctly oriented molecules collide with sufficient energy.
Collision Theory Requirements 1. 2. 3.

16. The Arrhenius Equation 01
Collision Theory: A bimolecular reaction occurs when two correctly oriented molecules collide with sufficient energy.
Activation Energy (Ea): The potential energy barrier that must be surmounted before reactants can be converted to products.

17. The Arrhenius Equation 02
Sufficient Energy - reactants must get up and over the energy “hump” in order to form products. This hump is the energy of activation

18. Catalysis 01
A catalyst is a substance that increases the rate of a reaction without being consumed in the reaction.

19. Catalysis 01
Catalysts function by lowering the energy of activation, which increases the rate of reaction.

20. Catalysis 03
Homogeneous Catalyst: Exists in the same phase as the reactants. Example – both in solution
Heterogeneous Catalyst: Exists in different phase to the reactants. Example – a gas passing over a solid cataylst.

21. The Arrhenius Equation 03
The Arrhenius equation tells us that as the temperature of a system increases, the percentage of collisions with sufficient energy increases.