Reaction Mechanisms Flashcards
Reaction Mechanism
Step by step sequence of elementary reactions by which an overall chemical reaction occurs
Elementary Steps
Each small step in the mechanism
Reaction Intermediate
- Produced in an elementary reaction
- Do not appear in the overall chemical reaction or rate law
- Short lived
Rate Determining Step
The slowest step in the reaction
How reaction intermediates work
They must be produced by one elementary reaction and then consumed by another
How is rate law determined?
- Based on the slowest step
- Should be the same as the one determined experimentally
What is molecularity?
- Describes the number of molecules that must collide for a reaction to occur
- Equal to the sum of stoichiometric coefficients of reactants in the elementary reaction
- Only for elementary steps
Elementary steps are either…
- Unimolecular
- Bimolecular
- Termolecular
Unimolecular
- A single molecule dissociates
- A –> Products
Bimolecular
- Two molecules collide
- Most common
Bimolecular examples
- 2A –> Products
- A + B –> Products
Termolecular
- Simultaneous collision of 3 molecules
- Rare
Termolecular examples
- 3A –> Products
- 2A + B –> Products
- A + B + C –> Products
Catalyst
Any species present at the beginning as a reactant and a product at the end
What do the peaks on the graph represent?
Activated complexes
What do the valleys on the graph represent?
Intermediate species
How is the slowest step on the graph represented?
One with the highest activation energy
How to find the # of elementary steps on the graph?
Count the # of peaks
How to find the overall delta H on the graph?
Difference in Ep of products and reactants
How to find the reverse delta H
- Same number as overall delta H
- Flip the sign
