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Kinetics Cartoon courtesy of 2011 University of Illinois Board of Trustees

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Collision Theory Orientation of reactants must allow formation of new bonds. Collisions must have enough energy to produce the reaction (must equal or exceed the activation energy). ©2011 University of Illinois Board of Trustees
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Kinetics Kinetics Cartoon courtesy of NearingZero.net ©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright
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Page 1: Kinetics Cartoon courtesy of   2011 University of Illinois Board of Trustees

KineticsKinetics

Cartoon courtesy ofNearingZero.net©2011 University of Illinois Board of Trustees •

http://islcs.ncsa.illinois.edu/copyright

Page 2: Kinetics Cartoon courtesy of   2011 University of Illinois Board of Trustees

Chemical KineticsChemical KineticsThe area of chemistry that concerns The area of chemistry that concerns reaction ratesreaction rates..

However, only a small fraction of collisions produces a reaction. Why?

Key Idea: Molecules must collide to react.

©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright

©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright

Page 3: Kinetics Cartoon courtesy of   2011 University of Illinois Board of Trustees

Collision TheoryCollision Theory

Orientation of Orientation of reactants must reactants must allow formation allow formation of new bonds.of new bonds.

Collisions must Collisions must have enough have enough energy to energy to produce the produce the reaction (must reaction (must equal or exceed equal or exceed the activation the activation energy).energy).

©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright

Page 4: Kinetics Cartoon courtesy of   2011 University of Illinois Board of Trustees

©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright

©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright

Page 5: Kinetics Cartoon courtesy of   2011 University of Illinois Board of Trustees

Factors Affecting RateFactors Affecting Rate

TemperatureIncreasing temperature always

increases the rate of a reaction. Surface Area

Increasing surface area increases the rate of a reaction Concentration

Increasing concentration USUALLY increases the rate of a reaction

Presence of Catalysts

All increase number of effective collisions

©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright

Page 6: Kinetics Cartoon courtesy of   2011 University of Illinois Board of Trustees

CatalysisCatalysis•CatalystCatalyst: A substance that speeds up : A substance that speeds up a reaction without being consumeda reaction without being consumed•EnzymeEnzyme: A large molecule (usually a : A large molecule (usually a protein) that catalyzes biological protein) that catalyzes biological reactions.reactions.•Homogeneous catalystHomogeneous catalyst: Present in : Present in the same phase as the reacting the same phase as the reacting molecules.molecules.•Heterogeneous catalystHeterogeneous catalyst: Present in : Present in a different phase than the reacting a different phase than the reacting moleculesmolecules..

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Page 7: Kinetics Cartoon courtesy of   2011 University of Illinois Board of Trustees

Reaction RateReaction RateThe change in concentration of a The change in concentration of a reactant or product per unit of timereactant or product per unit of time

2 1

2 1

[ ] [ ]A at timet A at timetRatet t

[ ]ARatet

©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright

Page 8: Kinetics Cartoon courtesy of   2011 University of Illinois Board of Trustees

Activation EnergyActivation EnergyThe minimum energy required to transform

reactants into the activated complex

(The minimum energy required to produce an effective collision)

Flame, spark, high temperature, radiation are all sources of activation energy

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Page 9: Kinetics Cartoon courtesy of   2011 University of Illinois Board of Trustees

Endothermic ReactionsEndothermic Reactions

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Page 10: Kinetics Cartoon courtesy of   2011 University of Illinois Board of Trustees

Exothermic ReactionsExothermic Reactions

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Page 11: Kinetics Cartoon courtesy of   2011 University of Illinois Board of Trustees

Endothermic Reaction withEndothermic Reaction witha Catalysta Catalyst

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Page 12: Kinetics Cartoon courtesy of   2011 University of Illinois Board of Trustees

Exothermic Reaction with a Exothermic Reaction with a CatalystCatalyst

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Page 13: Kinetics Cartoon courtesy of   2011 University of Illinois Board of Trustees

Decomposition of H2O2 by Various Catalysts

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Page 14: Kinetics Cartoon courtesy of   2011 University of Illinois Board of Trustees

Catalysts Increase the Number of Catalysts Increase the Number of Effective CollisionsEffective Collisions

©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright

Page 15: Kinetics Cartoon courtesy of   2011 University of Illinois Board of Trustees

Rate Laws for ReactionsRate Laws for Reactions

Rate Laws are determined experimentallyRate Laws are determined experimentally

Relationship between concentration of one Relationship between concentration of one reactant and the ratereactant and the rate

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Page 16: Kinetics Cartoon courtesy of   2011 University of Illinois Board of Trustees

2NO2NO22(g) (g) 2NO(g) + O 2NO(g) + O22(g)(g) Reaction Rates:

2. Can measure appearance of products

1. Can measure disappearance of reactants

3. Are proportional stoichiometrically

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Page 17: Kinetics Cartoon courtesy of   2011 University of Illinois Board of Trustees

Rate Laws

Consider the systemConsider the system

If the concentration of NO is held constant and If the concentration of NO is held constant and the concentration of Hthe concentration of H22 is varied, the is varied, the experiment determines how the rate of the experiment determines how the rate of the reaction varies with respect to the reaction varies with respect to the concentration of Hconcentration of H22

O(g)2H (g)N 2NO(g) (g)2H 222

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Page 18: Kinetics Cartoon courtesy of   2011 University of Illinois Board of Trustees

The following results were found experimentally…

doubling the concentration of H2 doubled the rate and tripling the concentration of H2 tripled the rate

Therefore the concentration of H2 is directly related to the rate of the reaction and can be written

R is proportional to [H2]

The reaction is first order with respect to hydrogen (the exponent is 1)

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Page 19: Kinetics Cartoon courtesy of   2011 University of Illinois Board of Trustees

When the concentration of H2 was held constant…

Doubling the concentration of NO increased the rate of the reaction four times

Tripling the concentration of NO increased the rate of the reaction nine times

Therefore the rate of the reaction is directly related to the square of the concentration of NO

R is proportional to [NO]2

The reaction is second order with respect to NO (the exponent is 2)

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Page 20: Kinetics Cartoon courtesy of   2011 University of Illinois Board of Trustees

Combining these results

Introducing a proportionality constant k gives an overall rate law

R = k [H2][NO]2

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Page 21: Kinetics Cartoon courtesy of   2011 University of Illinois Board of Trustees

Reaction MechanismReaction Mechanism The series of steps by which a

chemical reaction occurs. A chemical equation does not tell us

how reactants become products It is a summary of the overall process.

Example:

6CO 6H O C H O O2 2light

6 12 6 2 6

has many steps in the reaction mechanism

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Page 22: Kinetics Cartoon courtesy of   2011 University of Illinois Board of Trustees

Rate-Determining StepRate-Determining StepIn a multi-step reaction, the In a multi-step reaction, the slowest step is the rate-determining step.. It It therefore determines the rate of therefore determines the rate of reaction.reaction.

©2011 University of Illinois Board of Trustees • http://islcs.ncsa.illinois.edu/copyright


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