1. Chapter 13
Rates of Reaction
Dr. S. M. Condren

2. Reaction Rates average rate the rate over a specific time interval
instantaneous rate
the rate for an infinitely small interval
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3. Integrated Rate Laws A ---> products rate = - (D[A]/Dt) = k[A]m
average rate
rate = - (d[A]/dt) = k[A]m
instantaneous rate
Dr. S. M. Condren

4. Average vs. Instantaneous Rate
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5. 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
Dr. S. M. Condren

6. Rate Determining Step slowest step in a multistep mechanism
the step which determines the overall rate of the reaction
Dr. S. M. Condren

7. Rate Law
an expression which relates the rate to the concentrations and a specfic rate constant
Dr. S. M. Condren

8. 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
Dr. S. M. Condren

9. 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 NO2(g) + 2F(g) ---> 2NO2F(g) fast
A, B, C
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10. 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
Dr. S. M. Condren

11. Order of Reaction
Dr. S. M. Condren

12. Dr. S. M. Condren

13. Graphical Determination of Order of Reaction
This plot of ln[H2O2] vs. time produces a straight line, suggesting that the reaction is first order.
Dr. S. M. Condren

14. 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
Dr. S. M. Condren

15. Unimolecular Reaction
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16. Bimolecular Reaction
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17. 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

18. 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

19. Activation Energy the minimum energy required to start a reaction
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20. Temperature & Reaction Rate
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21. 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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22. 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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23. Reaction Mechanism
A set of elementary reactions which represent the overall reaction
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24. HCs = unburned hydrocarbons
Catalytic Converter
catalyst
H2O(g) + HCs > CO(g) + H 2(g) (unbalanced)
catalyst
2 H2(g) NO(g) > N2(g) H2O(g)
catalyst
CO(g) + O2(g) > CO2(g) (unbalanced)
catalyst = Pt-NiO
HCs = unburned hydrocarbons
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25. 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
Dr. S. M. Condren

26. 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
Dr. S. M. Condren

27. 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.
Dr. S. M. Condren