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Chapter 13
Rates of Reaction
Dr. S. M. Condren
Reaction Rates
average rate
• the rate over a specific time interval
instantaneous rate
• the rate for an infinitely small interval
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Integrated Rate Laws
A ---> products
rate = - (D[A]/Dt) = k[A]m
average rate
rate = - (d[A]/dt) = k[A]m
instantaneous rate
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Average vs. Instantaneous Rate
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For the reaction:
The dependence of the
concentration of H2 on
time is shown below.
Is the reaction rate
faster at point A or
point B?
A, B, both rates are the
same
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Rate Determining Step
• slowest step in a multistep mechanism
• the step which determines the overall rate of
the reaction
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Rate Law
• an expression which relates the rate to the
concentrations and a specfic rate constant
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Rate Law
Reaction rate = k [A]m [B]n
where
m => order with regard to A
n => order with regard to B
overall order = m + n
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The empirical rate law for the reaction
is Rate = k[NO2][F2].
Which of the following mechanisms is consistent with this rate law?
A) NO2 (g) + F2 (g) <=> NO2F(g) + F(g) fast
NO2(g) + F(g) --->NO2F(g) slow
B) NO2 (g) + F2(g) <==> NO2F(g) + F(g) slow NO2(g) + F(g) ---> NO2F(g) fast
C) F2(g) <==> F(g) + F(g) slow
2NO2(g) + 2F(g) ---> 2NO2F(g) fast
A, B, C
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Order of Reaction
• exponent of the concentration for a reactant
that implies the number of molecules of that
species involved in the rate determining
step
• first order, exponent equals one
• second order, exponent equals two
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Order of Reaction
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Graphical Determination of Order of Reaction
This plot of ln[H2O2] vs.
time produces a straight
line, suggesting that the
reaction is first order.
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A Nanoscale View:
Elementary Reactions
• unimolecular - rearrangement of a molecule
• bimolecular - reaction involving the
collision of two particles
• termolecular - reaction involving the
collision of three particles
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Unimolecular Reaction
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Bimolecular Reaction
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Collision Rate Model
Three conditions must be met at the
nanoscale level if a reaction is to occur:
• the molecules must collide;
• they must be positioned so that the reacting
groups are together in a transition state
between reactants and products;
• and the collision must have enough energy
to form the transition state and convert it
into products.
Dr. S. M. Condren
Transition State:
Activated Complex or
Reaction Intermediates
• an unstable arrangement of atoms that has
the highest energy reached during the
rearrangement of the reactant atoms to give
products of a reaction
Dr. S. M. Condren
Activation Energy
the minimum energy required to start a reaction
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Temperature & Reaction Rate
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Catalyst
• susbstance which speeds up the rate of a
reaction while not being consumed
Homogeneous Catalysis - a catalyst which is
in the same phase as the reactants
Heterogeneous Catalysis - a catalyst which is
in the different phase as the reactants
catalytic converter
– solid catalyst working on gaseous materials
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Which curve
illustrates the
effect of a catalyst
on the reaction
diagram, given
that it speeds up
the rate of a
reaction?
A, B, C, D
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Reaction Mechanism
• A set of elementary reactions which
represent the overall reaction
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Catalytic Converter
catalyst
H2O(g) + HCs -------> CO(g) + H 2(g)
(unbalanced)
catalyst
2 H2(g) + 2 NO(g) ------> N2(g) + 2 H2O(g)
catalyst
CO(g) + O2(g) --------> CO2(g)
catalyst = Pt-NiO
HCs = unburned hydrocarbons
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(unbalanced)
Chain Mechanisms
chain initiating step - the step of a mechanism
which starts the chain
chain propagating step(s) - the step or steps
which keeps the chain going
chain terminating step(s) - the step or steps
which break the chain
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Chain Mechanisms
combustion of gasoline in an internal
combustion engine
chain initiating step - additives which generate
free radicals, particles with unpaired
electrons
chain propagating step(s) - steps which
generate new free radicals
chain terminating step(s) - steps which do not
generate new free radicals
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Enzymes
any one of many specialized organic
substances, composed of polymers of
amino acids, that act as catalysts to
regulate the speed of the many
chemical reactions involved in the
metabolism of living organisms.
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