Reaction Mechanisms

Steps of Reactions using Rate Laws

What is a mechanism?• A reaction mechanism is a description of the path,

or sequence of steps, by which a reaction occurs.

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REACTION MECHANISM

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Example: Decomposition of O3

• ALL reactions = series of elementary steps whose sum is the overall balanced reaction.

• The oxygen atom, O, is called an intermediate.• Intermediate = is both created and destroyed in

the mechanism, does not appear in the net equation.

Elementary Steps

• Can use molecularity to write rate law• Unimolecular, Bimolecular, Termolecular

UNIMOLECULAR

BIMOLECULAR

Reaction MechanismsMust satisfy two requirements:

1. sum of elementary = balanced eqn2. mechanism = experimentally determined

rate lawTo calculate rate law from a mechanism you need to first know the rate limiting step.

rate limiting step

Rate Limiting Step• The rate limiting step determines the rate of the

reaction because it is the slowest step.• You can rationalize that a reaction can only go so

fast as its slowest step

Calculating Rate Law for Elementary Steps

• Rate of an elementary step = k [A] m • where m is the stoichiometric power from eqn

• The rate law for the first elementary step in the is ….. rate = k [O3].

• The rate law for the second elementary (bimolecular) step in the is ….. rate = k [O][O3]

Calculating Rate Law for Elementary Steps

What is the overall rate law?• THE SLOWEST step determines the rate law.

rate = k [O3]

How do you determine the rate determining step?

• 1. Determine the rate law. • 2. Using chemical intuition and requirements,

devise potential mechanisms.

• Mechanisms can never be proven absolutely but could be possibly correct!

Sample Exercise 14.14 p. 586

This example has a slow elementary step first! YAY! Easy!

Sample Exercise 14.15 p. 588

This example has a fast elementary step first! YAY! Substitute!