18.6-8 Kinetics-Catalysis-2020 (1).pdf - 2020/4/30 18.6...

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Chapter18:ChemicalKinetics
18.1RatesofChemicalReactions
18.2RateLaws
18.3ReactionMechanisms
18.4ReactionMechanismsandRate
18.5EffectofTemperatureonReactionRates
18.6MolecularTheoriesofElementaryReactions
18.7ReactionsinSolution
18.8CatalysisCumulativeExercise: SulfiteandSulfateKineticsinAtmosphericChemistry
Contents
18.6MolecularTheoriesofElementaryReactions
18.7ReactionsinSolution
18.8CatalysisCumulativeExercise
Contents
18.6MolecularTheoriesofElementaryReactions
18.7ReactionsinSolution
18.8CatalysisCumulativeExercise:SulfiteandSulfateKineticsinAtmosphericChemistry
Catalysis andActivationEnergy
• Perhapsthemostinterestingaspectofchemical
kineticsisthestudyofcatalysis
– itrelatesto manypracticalapplications
• Catalysisisto increasethereactionrate throughthe
additionofasubstance( catalyst )thatis unchanged in
thereaction
• Catalysis lowertheactivationenergy ofareaction
Catalysts
• Catalystschangereactionratesbyprovidingan
alternativereactionpathwaywithadifferent
activationenergy.
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Catalysts
• Asubstancethat
speedsupareaction
withoutbeing
consumeditself
• Providesa new
pathway with alower
activationenergy for
thereaction
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EffectofaCatalystontheNumberofEffectiveCollisions
TypesofCatalysis
• Homogeneous catalystsExistinthe
samephase asthereactingmolecules
• Heterogeneous catalystsexistin
differentphases thanthereactants.
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Homogeneous Catalysis
• Existsinthesamephaseasthereacting
molecules
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Homogeneous Catalysis: ExampleI
• Thefirstexample :
• Thisreactionisslowintheabsenceofacatalyst
• I
‐
canbeacatalystforthisreaction,tomakeitfast
– Insteadofgoinginabimolecularreactionwheretwohydrogen
peroxideshavetocrashintoeachother,there’s anentirely
differentpathway (loweractivationenergy) thatinvolves I ‐ ,to
formwaterand hypoiodide .
– Then,thehydrogenperoxidereactswithhypoiodide toform
waterand I - .
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2 2 2 2
2
2 2
2H O ( ) 2H O( ) + O (g)
Rate = k[H O ]
k aq  
Homogeneous Catalysis: ExampleI
• Now,thereactionhastwosteps
• Iodide and Hypoiodide donotappearinthe
balancedoverallreaction
• The catalyst endsupunchangedbytheoverall