C. Y. Yeung (CHW, 2009)

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Maxwell-BoltzmaMaxwell-Boltzmann Distributionnn Distribution &&

Collision TheoryCollision Theory

Maxwell-BoltzmaMaxwell-Boltzmann Distributionnn Distribution &&

Collision TheoryCollision Theory

Interpretation of Rate of Gaseous Rxn Interpretation of Rate of Gaseous Rxn

at at Molecular LevelMolecular Level

No. of

Effective Collisions Rate of Rxn

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FactorsFactors affecting the Rate of Rxn …?affecting the Rate of Rxn …?

TemperatureTemperature Conc. / Conc. /

PressurePressure

Surface Area

Surface Area CatalystCatalyst

Kinetic energy

Kinetic energy of particles of particles

Frequency of Frequency of

Collisions Collisions

Areas of Contact

Areas of Contact No. of Collisions

No. of Collisions

Rxn occurs via

Rxn occurs via

a pathway with

a pathway with

a lower Ea lower Eaa

Effective Collisions

Collision TheoryCollision Theory

1. Molecules with K.E. Ea, and

2. Collide in right orientation.

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Conc. / Conc. /

PressurePressure

Frequency of Frequency of

Collisions Collisions

FrequencyFrequency of Collisions is of Collisions is Important Important !!

Molecules have different Molecules have different Kinetic Energy (K.E.)Kinetic Energy (K.E.)

During collisions, some molecules gain energy from the others, and During collisions, some molecules gain energy from the others, and some molecules loss energy to the others.some molecules loss energy to the others.

i.e. i.e. DistributionDistribution of K.E. of of K.E. of molecules in molecules in a systema system

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In the system, molecules have In the system, molecules have a range of K.E.a range of K.E.

Maxwell-Boltzmann DistributionMaxwell-Boltzmann DistributionMolecules with Molecules with “most probable speed”“most probable speed”

Area under curve Area under curve total no. of molecules total no. of molecules

No. of molecules with No. of molecules with K.E. K.E. E Eaa rxn occurs! rxn occurs!

Distribution of K.E. Distribution of K.E. at temperature = Tat temperature = T11

no. of molecules

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Distribution of K.E. atDistribution of K.E. at Different TemperatureDifferent Temperature??

Areas under curve 0Areas under curve 000C, 25C, 2500C and 100C and 10000C are C are the samethe same! ! (no. of molecules in system remains constatnt)(no. of molecules in system remains constatnt)

Temp.Temp., more molecules take part in rxn., more molecules take part in rxn.

rate of rxn rate of rxn ..

EEaa

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Explain how temp. affects the Rate of Rxn …Explain how temp. affects the Rate of Rxn …

Temp. Temp.

Collision Frequency Collision Frequency

Energy transfer between molecules Energy transfer between molecules becomes more frequent.becomes more frequent.

Wider distribution of K.E.Wider distribution of K.E.

No. of molecules have K.E. No. of molecules have K.E. E Ea a

No. of molecules take part in rxn No. of molecules take part in rxn

Rate of Rxn Rate of Rxn

p.07p. 77 Q.4 (1996 --- M-B Distribution)p. 77 Q.4 (1996 --- M-B Distribution)

As temp. As temp. , fraction of molecules with , fraction of molecules with high K.E. high K.E. . . Hence, Hence, force exerted by the collision of moleculesforce exerted by the collision of molecules

on the container wallon the container wall ..

p.08p. 78 Q.7(b) (1998 --- M-B Distribution)p. 78 Q.7(b) (1998 --- M-B Distribution)

As a higher temp., the portion towards high K.E. is As a higher temp., the portion towards high K.E. is much larger. much larger. More molecules would have sufficient More molecules would have sufficient energy to overcome Eenergy to overcome Eaa and to react. and to react. Increasing the Increasing the

number of effective reactant increases the rate of number of effective reactant increases the rate of reaction.reaction.

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CatalystCatalyst & M-B Distribution: & M-B Distribution:

Catalyst provides an Catalyst provides an alternative pathwayalternative pathway for the for the reaction to occur.reaction to occur.

Positive Catalyst Positive Catalyst provides a pathway provides a pathway with with lower Elower Eaa..

EEaa’ (catalyse’ (catalyse

d)d)EEaa (uncatalys (uncatalys

ed)ed)

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ConclusionConclusion

5 factors5 factors affecting the rate of reactions affecting the rate of reactions (with explanations in terms of frequency (with explanations in terms of frequency of of effective collisionseffective collisions. . [Collision Theory][Collision Theory]

Maxwell-Boltzmann Maxwell-Boltzmann DistributionDistribution of of MoleMolecular Speedscular Speeds and and Kinetic EnergyKinetic Energy

Explain how Explain how temperature changetemperature change and and ususe of catalyste of catalyst affect the rate of reaction w affect the rate of reaction with Maxwell-Boltzmann Distributionith Maxwell-Boltzmann Distribution

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Next ….Next ….

Single / Multi-stage Reaction & Single / Multi-stage Reaction & Energy Profile, Catalyst (p. 58-69)Energy Profile, Catalyst (p. 58-69)

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AssignmentAssignment

Pre-lab: Pre-lab:

Expt. 9 Determination of Activation EnergyExpt. 9 Determination of Activation Energy

[due date: 26/2(Thur)] [due date: 26/2(Thur)]

p. 53 Check point 15-1 p. 53 Check point 15-1 p. 58 Check point 15-2p. 58 Check point 15-2

[due date: 26/2(Thur)] [due date: 26/2(Thur)]