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KineticsKinetics
Cartoon courtesy ofNearingZero.net©2011 University of Illinois Board of Trustees •
http://islcs.ncsa.illinois.edu/copyright
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
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
©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
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
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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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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
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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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Endothermic ReactionsEndothermic Reactions
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Exothermic ReactionsExothermic Reactions
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Endothermic Reaction withEndothermic Reaction witha Catalysta Catalyst
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Exothermic Reaction with a Exothermic Reaction with a CatalystCatalyst
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Decomposition of H2O2 by Various Catalysts
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Catalysts Increase the Number of Catalysts Increase the Number of Effective CollisionsEffective Collisions
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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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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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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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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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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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Combining these results
Introducing a proportionality constant k gives an overall rate law
R = k [H2][NO]2
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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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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.
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