Chapter 15Chemical Kinetics
Rates of Chemical Reactions
The Effect of Temperature on Reaction
Other Factors Affecting the Rates of reactions
d[NO]dt
Rate Expression
2( g) ( g) ( g)2NOCl 2NO Cl
Slope of the tangent lineInitial Rate
Exercise 1
Time(min) [CH3COOPh] (mol/L)
0 0.55
0.25 0.42
0.5 0.31
0.75 0.23
1 0.17
1.25 0.12
1.5 0.085
Time(min) [CH3COOPh] (mol/L)
0 0.55
0.25 0.42
0.5 0.31
0.75 0.23
1 0.17
1.25 0.12
1.5 0.085
Hydrolysis of Phenyl Acetate at 5deg.C
0
0.1
0.2
0.3
0.4
0.5
0.6
0 0.25 0.5 0.75 1 1.25 1.5
minutes
Mo
l/Lit
re
Exercise 1
Exercise 2
2 NO(g) + 2 H2(g) N2(g) + 2 H2O(g)
Reactant Concentration (mol/L) Rate of Appearance of N2 (mol/L·s)
[NO] [H2]
Run 1: 0.420 0.122 0.136
Run 2: 0.210 0.122 0.0339
Run 3: 0.210 0.244 0.0678
Run 4: 0.105 0.488 0.0339
Use in graphical methodsPlot for shape of rate response
Integrated Rate Expressions
d RRate=- =k
dt
Zero Order Rate Expression
d RRate=- =k R
dt
First Order Rate Expression
Conc Vs Time
Half Life
2d RRate k R
dt
0
0.2
0.4
0.6
0.8
1
1.2
0 1 2 3 4 5
Time
Conc
entra
tion
Second Order Rate Expression
Conc VS Time
Effect of Temperature on the Rate Constant
3 2 2NO+O NO +O
Increasing Concentration
The Reaction Coordinate
A + B AB A BdAB
A B A B
2 2NO +CO NO+CO
Temperature and Activation Energy
Ea
energy
reaction progress
Tlow
Ea'energy
reaction progress
Tlow
Thigh
products
reactants
a-ERTk=Ae
2( g) ( g) ( g)2NOCl 2NO Cl
Arrhenius Pre-exponential Factor
Exercise 3
Temp(oC) K(1/Ms)
283 3.52*10-7
356 3.02*10-5
393 2.19*10-4
427 1.16*10-3
508 3.95*10-2
2 2( g) ( g) ( g)2 HI H I
Exercise 3T(oC) T(K) k 1/T ln k
283 556 3.52E-07 0.001799 -14.8596
356 629 3.02E-05 0.00159 -10.4077
393 666 2.19E-04 0.001502 -8.42644
427 700 1.16E-03 0.001429 -6.75934
508 781 3.95E-02 0.00128 -3.23145
lnk = -22398/T + 25.303
-16
-14
-12
-10
-8
-6
-4
-2
0
0.0012 0.0014 0.0016 0.0018 0.002
1/T
ln k
Effect of Catalyst on the Rate Constant
Catalytic Hydrogenation
CH2=CH2 + H2 CH3-CH3
Elementary Step Molecularity Rate Equation
A prod. Unimolecular Rate = k[A]
A + B prod. Bimolecular Rate = k[A]·[B]
A + A prod. Bimolecular Rate = k[A]2
2 A + B prod. Termolecular Rate = k[A]2·[B]
Reaction Mechanisms
The Bimolecular Substitution Reaction
[Br····CH3····Cl]-
BrCH3
The Unimolecular Substitution Reaction
Br- + [CH3]+
BrCH3
+ BLOCK
[CH3]+
This reaction has two transition states
The Unimolecular Mechanism Requires an Intermediate
Exercise 4
Exercise 4
Time (min) [PhAc] (mol/L) ln[PhAc] 1/[PhAc]
0.00 0.55 -0.59783700 1.81818182
0.25 0.42 -0.86750057 2.38095238
0.50 0.31 -1.17118298 3.22580645
0.75 0.23 -1.46967597 4.34782609
1.00 0.17 -1.77195684 5.88235294
1.25 0.12 -2.12026354 8.33333333
1.50 0.085 -2.46510402 11.7647059
Slope=-k = -1.244 min-1
Exercise 4
Concepts from Chapter 15
Rate constants Rate lawsIntegrated rate equations
Zero-order, first-order and second-order reactionsHalf life, t1/2
Collision theoryActivation energy and the Arrhenius equation(s)CatalystsReaction co-ordinates, intermediates and transition statesReaction mechanisms and elementary stepsMolecularity - rate equations for elementary steps Rate determining stepsSupporting/disproving a proposed mechanism using kinetic data