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Dr. S. M. Condren
Chapter 13
Rates of Reaction
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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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Dr. S. M. Condren
Integrated Rate Laws
A ---> products
rate = - ([A]/t) = k[A]m
average rate
rate = - (d[A]/dt) = k[A]m
instantaneous rate
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Dr. S. M. Condren
Average vs. Instantaneous Rate
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Dr. S. M. Condren
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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Dr. S. M. Condren
Rate Determining Step
• slowest step in a multistep mechanism
• the step which determines the overall rate of the reaction
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Dr. S. M. Condren
Rate Law
• an expression which relates the rate to the concentrations and a specfic rate constant
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Dr. S. M. Condren
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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Dr. S. M. Condren
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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Dr. S. M. Condren
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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Dr. S. M. Condren
Order of Reaction
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Dr. S. M. Condren
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Dr. S. M. Condren
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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Dr. S. M. Condren
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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Dr. S. M. Condren
Unimolecular Reaction
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Dr. S. M. Condren
Bimolecular Reaction
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Dr. S. M. Condren
Collision Rate ModelThree 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.
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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
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Dr. S. M. Condren
Activation Energy
the minimum energy required to start a reaction
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Dr. S. M. Condren
Temperature & Reaction Rate
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Dr. S. M. Condren
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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Dr. S. M. Condren
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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Dr. S. M. Condren
Reaction Mechanism
• A set of elementary reactions which represent the overall reaction
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Dr. S. M. Condren
Catalytic Converter
catalyst2 H2(g) + 2 NO(g) ------> N2(g) + 2 H2O(g)
catalyst
H2O(g) + HCs -------> CO(g) + H 2(g) (unbalanced)
catalystCO(g) + O2(g) --------> CO2(g) (unbalanced)
catalyst = Pt-NiOHCs = unburned hydrocarbons
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Dr. S. M. Condren
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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Dr. S. M. Condren
Chain Mechanismscombustion 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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Dr. S. M. Condren
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.