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”..
p.04
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”..
p.05
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”..
p.06
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)
p.08
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.
p.09
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)
p.10
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
p.11
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!!
p.12
3 Different 3 Different Stories are Stories are possible!possible!
p.13
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.
p.14
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
p.15
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
p.16
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
p.17
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
p.18
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
p.19
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)]
p.20
Next ….Next ….
Chemical Equilibria & KChemical Equilibria & Keqeq
(p. 88-100)(p. 88-100)
