Kinetics study of the rates of reactions

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Kinetics study of the rates of reactions
Rates have units of amount of substance
consumed (or amount of substance produced) per
unit time e.g., M or Ms-1 s
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Kinetics study of the rates of reactions
What is a chemical reaction? (breaking/making new
bonds) What factors influence the rate of a
chemical reaction? - concentrations of reacting
species - temperature of reaction - presence of
a catalyst
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Kinetics study of the rates of reactions
What factors influence the rate of a chemical
reaction? - concentrations of reacting species
increase the of collisions between
molecules - temperature of reaction
increase the energy of collisions between
molecules - presence of a catalyst
catalysts act in many ways to increase reaction
rates, including orientation of reactants in a
more reactive conformation lower activation
energy for a reaction
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Average rates vs. instantaneous rates
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A rate law relates the reaction rate to the
concentration(s) of those species that influence
the rate of the reaction. Rate
kAmBn Where k the rate constant for
the reaction at some specified temperature
(i.e., k is a function of temperature),
and m and n are usually whole number
exponents. The rate law CAN NOT be derived from
the balanced chemical equation!
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Rate order is determined from the rate law
The order of the rate law is the sum of all the
exponent terms in the rate law
rate order 2 2 1 1 2
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This is key information
The rate law includes useful information re the
mechanism of the reaction. Knowing the order
of the rate law helps to define the reaction
mechanism.
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A first order rate gives a linear plot when
lnA is plotted vs. time
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First order and second order rates can be
distinguished by plotting the data
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First order and second order rates can be
distinguished by plotting the data
This is a second order reaction based on these
plots
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How does a change in concentration affect rate?
2NO(g) Cl2(g) ? 2NOCl(g)
Rate kAm
Rate kNO2Cl2 Rate kNO2Cl21 If NO
doubles, the rate increases by a factor of 4 If
Cl2 doubles, the rate increases by a factor of 2
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For first order reactions
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Collision Theory - frequency of collisions
(Z) - energy of collisions (f e-Ea/RT) -
orientation of collisions (p)
k Zpf Zpe-Ea/RT
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Temperature dependence of k
k Zpf Zpe-Ea/RT
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Transition State Theory extends Collision Theory
and introduces concept of an unstable (high
energy) activated complex The activation
energy Ea represents the difference in energy
between the reactant ground state and the
activated complex state.
k Ae-Ea/RT where A frequency factor or
Arrhenius parameter and is related to the terms
Z and p in collision theory.
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Transition State Theory The Energy diagram for
a reaction shows the relative differences in
energy for the reactant, product and transition
states.
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Transition State Theory The Energy diagram for
a reaction shows the relative differences in
energy for the reactant, product and transition
states.
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The following equation can be used to find k at
some temperature of interest, or Ea if the rate
constant is known at two different temperatures.
ln(k2/k1) (Ea/R)(1/T1 1/T2)
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Determining Reaction Mechanism If a reaction
occurs in two or more elementary steps, the
slowest step is referred to as the Rate
Determining Step (RDS).
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Determining Reaction Mechanism If a reaction
occurs in two or more elementary steps, the
slowest step is referred to as the Rate
Determining Step (RDS).
The determination of the rate law is different
depending on whether the RDS is the first step or
a later step (see p592-597)
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Catalysis A catalyst lowers the Ea of the RDS.
This will increase the value of k thus, the
reaction rate is accelerated.
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