Chemical Kinetics

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Chemical Kinetics

• Factors Affecting Reaction Rates
• Rate Law
• Order of Reactions
• Reaction Mechanisms
• Catalysis

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Rates of reactions
Chemical Kinetics the area of chemistry
concerned with the speeds, or rates,
of chemical reactions Reaction rate
the change in the concentration of
reactants or products per unit time
(M/s) Consider a hypothetical reaction
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Rates of reactions
Consider the example
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Rates of reactions
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Rates of reaction
Instantaneous rate the rate at at a particular
moment in the reaction Consider the
reaction
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Rates of reaction
Determination of instantaneous rate
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Rates of reaction
Reaction Rates and Stoichiometry
Consider a hypothetical reaction
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Factors affecting reaction rates
Factors Affecting the rate of a reaction

• Nature of reactants
• Concentration of reactants
• Temperature at which the reaction occurs
• Presence of a catalyst

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Factors affecting reaction rates
Nature of Reactants

• The more readily molecules collide, the more
• rapidly they react.
• When reactants are in different phases, the
• reaction is limited to their area of
  contact.

Concentration of Reactants
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Factors affecting reaction rates
The Rate Law
Consider the reaction
where k rate constant m,n reaction
orders
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Factors affecting reaction rates
Orders of Reaction
Overall reaction order the sum of the orders
with respect to each reactant
in the rate law
Zero-order reaction a reaction whose overall
order equals zero First-order reaction
a reaction whose rate depends on the
concentration of a single reactant raised
to the first power Second-order reaction a
reaction whose rate depends on the
reactant concentration raised to the
second power or on the concentrations
of two different reactants, raised to the
first power
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Factors affecting reaction rates
Zero-Order Reaction
A ? products
Rate k A0
Rate k
k mol L-1 s-1
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Factors affecting reaction rates
First-Order Reaction
A ? products
lnAt -kt lnA0
k s-1
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Factors affecting reaction rates
First-Order Reactions
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Factors affecting reaction rates
Second-Order Reaction
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Factors affecting reaction rates
Half-life (t1/2) the time required for the
concentration of a reactant to drop to one
half of its initial value
First order
Second order
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Factors affecting reaction rates
Temperature of the reaction

• In general, the rate of a reaction increases
  as the temperature increases.
• The rate constant and the rate of the
  reaction approximately double for each 10C in T

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Factors affecting reaction rates
Collision Theory

• Molecules collide to react.
• The reaction rate is directly proportional to
  the
• number of collisions per second.
• As the concentration of the reactants
  increases,
• the number of collisions increases.
• As the temperature increases, the molecules
• collide more forcefully and more frequently.

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Factors affecting reaction rates
Collision Model
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Factors affecting reaction rates
Types of Collision
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Factors affecting reaction rates
Activation Energy the minimum energy required
to initiate a chemical reaction
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Factors affecting reaction rates
Arrhenius Equation
where k rate constant Ea
activation energy R gas constant,
8.314 j/mol-K A Arrhenius factor
T absolute temperature
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Factors affecting reaction rates
Presence of catalyst
Catalyst a substance that changes the speed of
a chemical reaction but remains
unchanged in the reaction
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Factors affecting reaction rates
Homogeneous catalyst a catalyst that is present
in the same phase as the
reacting molecules
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Factors affecting reaction rates
Heterogeneous catalyst a catalyst that exists
in a different phase from the
reactant molecules
Enzyme an efficient biological catalyst
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Reaction mechanisms
Reaction mechanism the process by which a
reaction occurs Elementary step a process
that occur in a single step Molecularity
the number of molecules that participate as
reactants in an elementary step
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Factors affecting reaction rates
The rate law of any elementary step follows
directly from its molecularity.
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Reaction mechanisms
Multistep mechanism a process that occurs in a
sequence of elementary step
Overall equation the sum of all the elementary
steps in the reaction mechanism
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Reaction mechanisms
Net overall equation the simplified overall
equation where the intermediates
are eliminated Intermediate a
substance that is formed in one elementary
step and consumed in the next
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Reaction mechanisms
The rate law for the overall reaction of a
multistep mechanism is governed by the
rate-determining step.
Step 1
(slow)
(fast)
Step 2
Rate-determining step the slowest step in the
reaction