1. Chemical Kinetics Lesson 2
Chapter 12
Chemical Kinetics
Lesson 2

2. 12.2 Rate Laws: An Introduction
Reaction Rates
Rate Laws: An Introduction
Determining the Form of the Rate Law
The Integrated Rate Law
Reaction Mechanisms
12.6 A Model for Chemical Kinetics
12.7 Catalysis
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3. Summary of the Rate Laws
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4. Zero order First order Second order 4.7 M 3.7 M 2.0 M Exercise
Consider the reaction aA  Products. [A]0 = 5.0 M and k = 1.0 x 10–2 (assume the units are appropriate for each case). Calculate [A] after 30.0 seconds have passed, assuming the reaction is:
Zero order
First order
Second order
a) 4.7 M
[A] = –(1.0×10–2)(30.0) + 5.0
b) 3.7 M
ln[A] = –(1.0×10–2)(30.0) + ln(5.0)
c) 2.0 M
(1 / [A]) = (1.0×10–2)(30.0) + (1 / 5.0)
4.7 M
3.7 M
2.0 M
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5. Most chemical reactions occur by a series of elementary steps.
Reaction Mechanism
Most chemical reactions occur by a series of elementary steps.
An intermediate is formed in one step and used up in a subsequent step and thus is never seen as a product in the overall balanced reaction.
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6. NO2(g) + CO(g) → NO(g) + CO2(g)
A Molecular Representation of the Elementary Steps in the Reaction of NO2 and CO
NO2(g) + CO(g) → NO(g) + CO2(g)
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7. Elementary Steps (Molecularity)
Unimolecular – reaction involving one molecule; first order.
Bimolecular – reaction involving the collision of two species; second order.
Termolecular – reaction involving the collision of three species; third order.
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8. Rate-Determining Step
A reaction is only as fast as its slowest step.
The rate-determining step (slowest step) determines the rate law and the molecularity of the overall reaction.
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9. Reaction Mechanism Requirements
The sum of the elementary steps must give the overall balanced equation for the reaction.
The mechanism must agree with the experimentally determined rate law.
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10. Decomposition of N2O5
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11. Step 2: NO2 + NO3 → NO + O2 + NO2 (slow)
Decomposition of N2O5
2N2O5(g)  4NO2(g) + O2(g)
Step 1: N2O NO2 + NO3 (fast)
Step 2: NO2 + NO3 → NO + O2 + NO2 (slow)
Step 3: NO3 + NO → 2NO2 (fast)
2( )
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12. The reaction A + 2B  C has the following proposed mechanism:
Concept Check
The reaction A + 2B  C has the following proposed mechanism:
A + B D (fast equilibrium)
D + B  C (slow)
Write the rate law for this mechanism.
rate = k[A][B]2
rate = k[A][B]2
The sum of the elementary steps give the overall balanced equation for the reaction.
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13. Molecules must collide to react. Main Factors: Activation energy, Ea
Collision Model
Molecules must collide to react.
Main Factors:
Activation energy, Ea
Temperature
Molecular orientations
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14. Energy that must be overcome to produce a chemical reaction.
Activation Energy, Ea
Energy that must be overcome to produce a chemical reaction.
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15. Transition States and Activation Energy
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16. Change in Potential Energy
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17. For Reactants to Form Products
Collision must involve enough energy to produce the reaction (must equal or exceed the activation energy).
Relative orientation of the reactants must allow formation of any new bonds necessary to produce products.
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18. The Gas Phase Reaction of NO and Cl2
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19. R = gas constant (8.3145 J/K·mol) T = temperature (in K)
Arrhenius Equation
A = frequency factor
Ea = activation energy
R = gas constant ( J/K·mol)
T = temperature (in K)
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20. Linear Form of Arrhenius Equation
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21. Linear Form of Arrhenius Equation
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22. Exercise
Chemists commonly use a rule of thumb that an increase of 10 K in temperature doubles the rate of a reaction. What must the activation energy be for this statement to be true for a temperature increase from 25°C to 35°C?
Ea = 53 kJ
Ea = 53 kJ
ln(2) = (Ea / J/K·mol)[(1/298 K) – (1/308 K)]
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23. A substance that speeds up a reaction without being consumed itself.
Catalyst
A substance that speeds up a reaction without being consumed itself.
Provides a new pathway for the reaction with a lower activation energy.
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24. Energy Plots for a Catalyzed and an Uncatalyzed Pathway for a Given Reaction
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25. Effect of a Catalyst on the Number of Reaction-Producing Collisions
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26. Heterogeneous Catalyst
Most often involves gaseous reactants being adsorbed on the surface of a solid catalyst.
Adsorption – collection of one substance on the surface of another substance.
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27. Heterogeneous Catalysis
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28. Heterogeneous Catalyst
Adsorption and activation of the reactants.
Migration of the adsorbed reactants on the surface.
Reaction of the adsorbed substances.
Escape, or desorption, of the products.
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29. Exists in the same phase as the reacting molecules.
Homogeneous Catalyst
Exists in the same phase as the reacting molecules.
Enzymes are nature’s catalysts.
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30. Homogeneous Catalysis
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