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