C. Y. Yeung (CHW, 2009)

p.01

Energy Profile Energy Profile and and

Reaction MechanismReaction MechanismEnergy Profile Energy Profile andand

Reaction MechanismReaction Mechanism

How does the reaction

take place???

p.02

A + B A + B C C

How does a reaction occur?How does a reaction occur?

K.E. K.E. E Eaa

collide in right collide in right orientationorientation

A ++ B CSingle StagSingle Stag

e Rxne Rxn

Multi-Stage Multi-Stage RxnRxnA ++ B CIntermediateIntermediate

How to know that …. ?How to know that …. ?

p.03

From theFrom the

ChemicalChemical && DifferentialDifferential Rate Eqns Rate Eqns !!

e.g. 2A + B e.g. 2A + B 3C 3C

If after Kinetics Studies, it was found that If after Kinetics Studies, it was found that …… Rate = k[A][B]Rate = k[A][B]

i.e. The reaction is a 2-step reaction!i.e. The reaction is a 2-step reaction!

Step 1:Step 1: A + B A + B Intermediate Intermediate (r.d.s.)(r.d.s.)

Step 2:Step 2: Intermediate + Intermediate + AA C C

** ** a “A” molecule and a “B” molecule are a “A” molecule and a “B” molecule are involved in the involved in the “rate-determining step”“rate-determining step”..

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e.g. 2A + B e.g. 2A + B 3C 3C

If after Kinetics Studies, it was found that If after Kinetics Studies, it was found that …… Rate = k[A]Rate = k[A]22

i.e. The reaction is a 2-step reaction!i.e. The reaction is a 2-step reaction!

Step 1:Step 1: A + A A + A Intermediate Intermediate (r.d.s.)(r.d.s.)

Step 2:Step 2: Intermediate + Intermediate + BB C C

** ** Two “A” molecules are involved in the Two “A” molecules are involved in the “rate-determining step”“rate-determining step”..

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e.g. 2A + B e.g. 2A + B 3C 3C

If after Kinetics Studies, it was found that If after Kinetics Studies, it was found that …… Rate = k[B]Rate = k[B]

i.e. The reaction is a 2-step reaction!i.e. The reaction is a 2-step reaction!

Step 1:Step 1: B B Intermediate Intermediate (r.d.s.)(r.d.s.)

Step 2:Step 2: Intermediate + Intermediate + 2A2A C C

** ** Only one “B” molecule is involved in the Only one “B” molecule is involved in the “rate-determining step”“rate-determining step”..

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e.g. 2A + B e.g. 2A + B 3C 3C

If after Kinetics Studies, it was found that If after Kinetics Studies, it was found that …… Rate = k[A]Rate = k[A]22[B][B]

i.e. The reaction is a Single-step i.e. The reaction is a Single-step reaction!reaction!

2A + B 2A + B 3C 3C

** ** ALL the molecules are involved in the ALL the molecules are involved in the “rate-determining step”“rate-determining step”..

p.072A + B 2A + B 3C 3C Energy ProfileEnergy Profile ?? ??

Rate = k[A]Rate = k[A]22[B][B]

Rate = k[B]Rate = k[B]

Rate = k[A]Rate = k[A]22

Rate = k[A][B]Rate = k[A][B]

intermediate

Transition state

Transition state

higher Ea: r.d.s. (slower step)

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p. 79 Q.9 (1999 --- Differential Rate Eqn. p. 79 Q.9 (1999 --- Differential Rate Eqn. and Energy Profil and Energy Profil

e)e)(a)(a) Rate = k[A]Rate = k[A]

(c)(c) A + B A + B product product

i.e. i.e. Only a “A” is involved in Only a “A” is involved in r.d.s.r.d.s.

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Catalyst Catalyst changeschanges the energy profile of rxn! the energy profile of rxn!

A + B A + B C CIf no catalyst … A + B If no catalyst … A + B C, with high E C, with high Eaa..

With catalyst (X)… With catalyst (X)…

A + A + XX intermediate intermediate, with a lower E, with a lower Eaa. (r.d.s.). (r.d.s.)

intermediate + B intermediate + B C + C + XX (becomes a 2-stage rx(becomes a 2-stage rxn)n)

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Energy ProfilesEnergy Profiles with / withoutwith / without CatalystCatalyst …… (slow) [r.d.s.](slow) [r.d.s.]

(fast)(fast)

H is not affected H is not affected by catalystby catalyst

Only 1 transition stateOnly 1 transition state

2 transition states2 transition states

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p. 79 Q.11 (2001 --- Differential Rate Eqn. p. 79 Q.11 (2001 --- Differential Rate Eqn. and Energy Profil and Energy Profil

e)e)

(a)(a) Rate = k [I(g)]Rate = k [I(g)]22 [Ar] [Ar]

(c)(c) 2 I 2 I I I22

i.e. i.e. Two “I” atoms and one “Ar” aTwo “I” atoms and one “Ar” atom are involved in r.d.s.tom are involved in r.d.s.

3 possible energy profiles!!3 possible energy profiles!!

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3 Different 3 Different Stories are Stories are possible!possible!

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I(g) + I(g) + Ar I(g) + I(g) + Ar I I22(g) + Ar*(g) + Ar*

(g)(g)Role of ArRole of Ar: : acts as a third body to absorb enacts as a third body to absorb energy from the colliding I(g) atoms.ergy from the colliding I(g) atoms.

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Step 1:Step 1: I(g) + Ar(g) I(g) + Ar(g) I-Ar(g) [fast] I-Ar(g) [fast]

Step 2:Step 2: I-Ar(g) + I I-Ar(g) + I I I22 + Ar(g) [slo + Ar(g) [slo

w]w]Role of ArRole of Ar: : homogeneous catalysthomogeneous catalyst

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Step 1:Step 1: 2I(g) + Ar(g) 2I(g) + Ar(g) I I22Ar(g) [slow]Ar(g) [slow]

Step 2:Step 2: II22Ar(g) Ar(g) I I22 + Ar(g) [fast] + Ar(g) [fast]

Role of ArRole of Ar: : homogeneous catalysthomogeneous catalyst

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Expt. 9 Activation EnergyExpt. 9 Activation Energy

5 Br5 Br-- + BrO + BrO33-- + 6 H + 6 H++ 3 Br 3 Br22 + 3H + 3H22OO

formed quickly!formed quickly!

3 Br3 Br22 ++

OHOH OHOHBrBr BrBr

BrBr

++ 3 HBr3 HBr

e.g. 3 moles are form e.g. 3 moles are form in 42 sin 42 s

All the 3 moles All the 3 moles of Brof Br22 reacts reacts

with phenol. with phenol. does not does not

bleach methyl bleach methyl red indicator.red indicator.

At 42.1s, a new BrAt 42.1s, a new Br22 is formed, which will not is formed, which will not

react with phenol, but bleach methyl red!react with phenol, but bleach methyl red!

e.g. 1 mole.g. 1 mol

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The time required for bleaching Methyl Red is The time required for bleaching Methyl Red is

recorded. recorded. data treatmentdata treatment ...? ...?

Temp./0C Time/srate/mol s-1

(if 3 mol of Br2 are formed)

25 92 3/92

30 56 3/56

41 30 3/30

52 9 3/9

i.e. time required i.e. time required , , rate rate rate rate 1/t 1/t

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rate rate 1/t 1/t k k 1/t 1/t

Therefore …Therefore …

ln k = – + lnAln k = – + lnAEEaa

RR11TT

ln (1/t) = – + lnAln (1/t) = – + lnAEEaa

RR11TT

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AssignmentAssignment

p.74 p.74 Q.11, 12 [due date: 2/3(Mon)] Q.11, 12 [due date: 2/3(Mon)]

Lab Report: Expt. 9 Determination of ELab Report: Expt. 9 Determination of Eaa

[due date: 3/3(Tue)] [due date: 3/3(Tue)]

Quiz on Chemical Kinetics (Ch. 13-15)Quiz on Chemical Kinetics (Ch. 13-15)

[9/3(Mon)] [9/3(Mon)]

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Next ….Next ….

Chemical Equilibria & KChemical Equilibria & Keqeq

(p. 88-100)(p. 88-100)