slide 1 reaction mechanisms reaction mechanism - sequence of molecular events, or elementary...
TRANSCRIPT
Slide 1
Reaction MechanismsReaction Mechanisms
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
Slide 2
Reaction MechanismsReaction 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.
Slide 3
Elementary StepsElementary 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:
Slide 4
Reaction MechanismsReaction 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?
Slide 5
Reaction MechanismsReaction 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.
Slide 6
Reaction MechanismsReaction 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.
Slide 7
Rate Laws, Reaction MechanismsRate Laws, Reaction Mechanisms
Rate law for overall reaction is determined experimentally.
Rate law for elementary step follows from its molecularity.
Slide 8
Reaction MechanismsReaction Mechanisms
Molecularity: the number of molecules (or atoms) on the reactant side of the chemical equation for an elementary step.
Unimolecular:
Example:
Slide 9
Reaction MechanismsReaction Mechanisms
Unimolecular: single reactant molecule bond-breaking only.
What will the rate depend on?
Slide 10
Reaction MechanismsReaction Mechanisms
Bimolecular: Two reactant molecules (collision).
Rate of reaction – depends on O3 and O
Slide 11
Reaction MechanismsReaction Mechanisms
Bimolecular: How do we get a step that is bimolecular in A? rate = k [A]2
Example: formation of O2
Slide 12
Rate Laws, Reaction MechanismsRate Laws, Reaction Mechanisms
Rate law for an overall reaction is determined experimentally.
Rate law for an elementary step follows from its molecularity.
Slide 13
Rate Laws and Reaction MechanismsRate Laws and Reaction Mechanisms
• 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.
• The rate law of each elementary step follows its molecularity.
Slide 14
Rate Laws and Reaction MechanismsRate Laws and Reaction Mechanisms
Slide 15
The Arrhenius Equation01The Arrhenius Equation01
Collision Theory: A bimolecular reaction occurs when two correctly oriented molecules collide with sufficient energy.
Collision Theory Requirements
1.
2.
3.
Slide 16
The Arrhenius Equation01The Arrhenius Equation01
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.
Slide 17
The Arrhenius Equation02The Arrhenius Equation02
Sufficient Energy - reactants must get up and over the energy “hump” in order to form products. This hump is the energy of activation
Slide 18
A catalyst is a substance that increases the rate of a reaction without being consumed in the reaction.
Catalysis 01Catalysis 01
Slide 19
Catalysts function by lowering the energy of activation, which increases the rate of reaction.
Catalysis 01Catalysis 01
Slide 20
Catalysis 03Catalysis 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.
Slide 21
The Arrhenius Equation03The Arrhenius Equation03
The Arrhenius equation tells us that as the temperature of a system increases, the percentage of collisions with sufficient energy increases.