Reaction Mechanisms

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Reaction 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
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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.

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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
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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?
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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.
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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.

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Rate Laws, Reaction Mechanisms

• Rate law for overall reaction is determined
  experimentally.
• Rate law for elementary step follows from its
  molecularity.

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Reaction Mechanisms

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

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Reaction Mechanisms

• Unimolecular single reactant molecule
  bond-breaking only.
• What will the rate depend on?

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Reaction Mechanisms

• Bimolecular Two reactant molecules (collision).
• Rate of reaction depends on O3 and O

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Reaction Mechanisms

• Bimolecular How do we get a step that is
  bimolecular in A? rate k A2

Example formation of O2
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Rate Laws, Reaction Mechanisms

• Rate law for an overall reaction is determined
  experimentally.
• Rate law for an elementary step follows from its
  molecularity.

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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.

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Rate Laws and Reaction Mechanisms
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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.
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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.
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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
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Catalysis 01

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

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Catalysis 01

• Catalysts function by lowering the energy of
  activation, which increases the rate of reaction.

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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.

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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.