COMPREHENSIVE CHEMICAL KINETICS


COMPREHENSIVE
Section 1.

THE PRACTICE AND THEORY OF KINETICS

Volume I The Practice of Kinetics
Volume 2 The Theory of Kinetics
Volume 3 The Formation and Decay of Excited Species

Section 2.

DECOMPOSITION AND ISOMERISATION REACTIONS

Volume 4 The Homogeneous Decomposition of Inorganic and Organometallic
Compounds
Volume 5 The Homogeneous Decomposition of Organic Compounds

Section 3.

INORGANIC REACTIONS

Volume 6 Homogeneous Reactions of Compounds of Non-metals
Volume 7 Homogeneous Reactions of Metals and their Compounds

Section 4.

ORGANIC REACTIONS

Section 5.

POLYMERIZATION REACTIONS

Section 6.

OXIDATION AND COMBUSTION REACTIONS

Section 7.

SELECTED ELEMENTARY REACTIONS (2

(6 volumes)

(2 volumes)

Additional Sections
HETEROGENEOUS REACTIONS
SOLID STATE REACTIONS
KINETICS AND TECHNOLOGICAL PROCESSES

(2 volumes)

volumes)


CHEMICAL KINETICS
EDITED BY


C. H. BAMFORD
M.A., Ph.D., Sc.D. (Cantab.), F.R.I.C.,F.R.S.
Campbell-BrownProfessor of Industrial Chemistry,
University of Liverpool
AND

C . F. H. TIPPER
Ph.D. (Bristol), D.Sc. (Edinburgh)
Senior Lecturer in Physical Chemistry,
University of Licierpool

VOLUME

1

THE PRACTICE OF KINETICS

ELSEVIER P U B L I S H I N G C O M P A N Y
AMSTERDAM

- LONDON - N E W Y O R K
1969


P.O.

ELSEVIER P U B L I S H I N G COMPANY
335 J A N V A N G A L E N S T R A A T
B O X 211, A M S T E R D A M , T H E N E T H E R L A N D S


AMERICAN ELSEVIER PUBLISHING COMPANY, INC.
52 V A N D E R B I L T A V E N U E
N E W Y O R K , N E W Y O R K 10017

FIRST REPRINT 1975

LIBRARY OF CONGRESS C A R D NUMBER
STANDARD BOOK N U M B E R

WITH

161 ILLUSTRATIONS

68-29646.

444-40673-5

AND

32

TABLES.

C O P Y R I G H T @ 1969 B Y E L S E V I E R P U B L I S H I N G C O M P A N Y , A M S T E R D A M
A L L RIGHTS RESERVED

T H I S BOOK O R A N Y P A R T T H E R E O F M U S T N O T BE R E P R O D U C E D I N A N Y FORM
W I T H O U T T H E W R I T T E N P E R M I S S I O N OF T H E P U B L I S H E R ,
ELSEVIER P U B L I S H I N G COMPANY, AMSTERDAM, T H E N E T H E R L A N D S


PRINTED IN THE NETHERLANDS


C O M P R E H E N S I V E CHEMICAL KINETICS

ADVISORY BOARD
Professor S. W. BENSON
Professor F. S. DAINTON
Professor G. GEE
Professor P. GOLDFINGER
Professor G. S. HAMMOND
Professor w. JOST
Professor G . B. KISTIAKOWSKY
Professor V. N. KONDRATIEV
Professor K. J. LAIDLER
Professor M. MAGAT
Professor SIR HARRY MELVILLE
Professor G. NATTA
Professor R. G. w. NORRISH
Professor s. OKAMURA
Professor SIR ERIC RIDEAL
Professor N. N. SEMENOV
Professor z. G. S Z A B ~
the late Professor v. v. VOEVODSKII
Professor 0. WICHTERLE


Contributors to Volume 1

L. BATT


Department of Chemistry,
University of Aberdeen, Aberdeen,
Scotland

D. N. HAGUE

Department of Chemistry,
University of Kent,
Canterbury, England

D. MARGERISON
The Donnan Laboratories,
University of Liverpool,
Liverpool, England
D. SHOOTER

Heavy Organic Chemicals
Division, Imperial
Chemical Industries Ltd.,
Billingham, England
(Now Arthur D. Little, Inc.,
Acorn Park, Cambridge,
Mass., U.S.A.)

R. P. WAYNE

Physical Chemistry
Laboratory, University 01
Oxford, Oxford, England



The rates of chemical processes and their variation with conditions have been
studied for many years, usually for the purpose of determining reaction mechanisms. Thus, the subject of chemical kinetics is a very extensive and important
part of chemistry as a whole, and has acquired an enormous literature. Despite
the number of books and reviews, in many cases it is by no means easy to find
the required information on specific reactions or types of reaction or on more general topics in the field. It is the purpose of this series to provide a background reference work, which wilI enable such information to be obtained either directly,
or from the original papers or reviews quoted.
The aim is to cover, in a reasonably critical way, the practice and theory of kinetics and the kinetics of inorganic and organic reactions in gaseous and condensed
phases and at interfaces (excluding biochemical and electrochemical kinetics, however, unless very relevant) in more or less detail. The series will be divided into
sections covering a relatively wide field; a section will consist of one or more volumes, each containing a number of articles written by experts in the various topics.
Mechanisms will be thoroughly discussed and relevant non-kinetic data will be
mentioned in this context. The methods of approach to the various topics will, of
necessity, vary somewhat depending on the subject and the author(s) concerned.
It is obviously impossible to classify chemical reactions in a completely logical
manner, and the editors have in general based their classification on types of chemical element, compound or reaction rather than on mechanisms, since views on
the latter are subject to change. Some duplication is inevitable, but it is felt that
this can be a help rather than a hindrance.
Since all kinetic work commences with the accumulation of experimental data,
this first volume deals with the methods used for determining the rates of “slow”,
“fast” and heterogeneous reactions, together with those for the detection and quantitative determination of labile intermediates. A chapter is also devoted to the processing of the primary data -where appropriate, with the aid of statistical methods.
Finally, the Editors wish to express their sincere appreciation of the advice
so readily given by the members of the Advisory Board.

Liverpool
November, 1968

C . H. BAMFORD

C . F. H. TIPPER



This Page Intentionally Left Blank


Contents

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

VII

Chapter I (L . BATT)

Experimental methods for the study of slow reactions . . . . . . . . . . . . .

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1 INTRODUCTION

2.

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

DETERMINATION OF A MECHANISM

A APPARATUS
3.1 Genera.

1


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6
6
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3.1.1 The vacuum linc . . . . . . . . . . . . . . . . . . . . . . . . .

3.1.2 Temperature control
. . . . . . . . . . . . . . . . . . . . . . .
3.2 Thermal systems . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2.1 Static method . . . . . . . . . . . . . . . . . . . . . . . . .
. . . .
3.2.2 Flow method . . . . . . . . . . . . . . . . .
3.2.3 Comparison offlow and static systems . . . . . . . . . . .
3.2.4 Analytical section (flow and static systems: . . .
. . .
3.3 Photochemical systems . . . . . . . . . . . . . . . . . . . . . .
. . . .
3.3.1 Introduction . . . . . . . . . . . . . . . .
3.3.2 The optical set-up . . . . . . . . . . . . . . . . . . . . . .
3.3.3 Sources of radiatioi: . . . . . . . . . . .
. . . . . .
. . . . . .
3.3.4 The production of “monochromatic” radiation

3.3.5 Uniform density filters . . . . . . . . . . . .
. .
3.3.6 Measurement o f the intensity of radiation . . . . . . . . .
. . . . . . . . . .
3.4 Radiochemical systems (radiation chemistry) . .
3.4.1 Introduction . . . . . . . . . . . .
. . . . . . . . . . .
3.4.2 Experimental techniquc . . . . . . . . . . . . . . . . . . . . . .
4

. ANALYSIS

. . . . . . . . . . .

4.1 Introduction . . . . . . .
4.2 Chemical methods . . . .
4.2.1 General . . . . . .
4.2.2 Radical traps . . . .
4.2.3 Chemical sensitisatiofi
4.2.4 Photosensitisation . .
4.2.5 Isotopes . . . . . .
4.2.6 Gaseous titrations . .

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X

CONTENTS

4.3 Gas chromatography . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.2 Carrier gases . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.3 Columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

4.3.4 Programmed temperature gas chromatography (PTGC) . . . . . . . .
4.3.5 Detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3.6 Product identification and trapping . . . . . . . . . . . . . . . .
4.3.7 Sampling and quantitative estimation . . . . . . . . . . . . . .
4.4 Spectroscopic methods . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.2 uv, visible and IR . . . . . . . . . . . . . . . . . . . . . . . . .
4.4.3 Electron spin resonance . . . . . . . . . . . . . . . . . . . . . .
4.4.4 Mass spectrometry . . . . . . . . . . . . . . . . . . . . . . . .
4.4.5 Optical pumping . . . . . . . . . . . . . . . . . . . . . . . . .
4.5 Other methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5.1 Polarography . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5.2 Polarimetry . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5.3 Conductance measurements . . . . . . . . . . . . . . . . . . . .
4.5.4 Thermal conductivity . . . . . . . . . . . . . . . . . . . . . . . .
4.5.5 Calorimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.5.6 Ultrasonic absorption measurements . . . . . . . . . . . . . . . .
4.5.7 Gas density balance . . . . . . . . . . . . . . . . . . . . . . . .
4.5.8 Quartz fibre manometer . . . . . . . . . . . . . . . . . . . . . .
4.5.9 Interferometry and refractometry . . . . . . . . . . . . . . . . .
4.5.10 Dielectric constant . . . . . . . . . . . . . . . . . . . . . . . . .
REFERENCES

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104


Chapter 2 (D. N . HAGUE)

Experimental methods for the study of fast reactions . . . . . . . . . . . . . 112
INTRODUCTION

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. FLASH PHOTOLYSIS AND PULSE RADIOLYSIS

1

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1.1 Flash photolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.1.1 Applications of flash photolysis . . . . . . . . . . . . . . . . . .
1.2 Pulse radiolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1.2.1 The hydrated electron . . . . . . . . . . . . . . . . . . . . . . .

2. s n o a TUBE AND ABIABATIC COMPRESSION
2.1 The hydrogen/oxygen reaction . . .

. CHEMICAL RELAXATION METHODS

3

4

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133
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142

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142

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149

3.1 The relaxation time . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Step.function. or transient methods . . . . . . . . . . . . . . . . . . . .
3.2.1 Temperature-jump . . . . . . . . . . . . . . . . . . . . . . . .
3.2.2 Applications of T-jump . . . . . . . . . . . . . . . . . . . . . . .
3.3 Stationary methods . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.1 Ultrasonic absorption . . . . . . . . . . . . . . . . . . . . . . .
3.3.2 Optical technique . . . . . . . . . . . . . . . . . . . . . . . . .
3.3.3 Ion-pair formation of metal salts . . . . . . . . . . . . . . . . .

. SPECTRAL LINE-BROADENING
4.1

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Nuclear magnetic resonance . . . . . . . . . . . . . . . . . . . . . . .
4.1.1 Applications Of NMR . . . . . . . . . . . . . . . . . . . . . . . .

4.2 Electron spin resonance and other spectral methods


XI

CONTENTS

. FLUORESCENCE QUENCHING . . . . . . . . . . . . . . . . . . . . . . . . . .
6. ELECTROCHEMICAL METHODS . . . . . . . . . . . . . . . . . . . . . . . . .
5


152

6.1 Polarography .

157
158

7.1 Flow methods in solution . . .
7.2 Flow methods in the gas phase .

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166

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7. FLOW METHODS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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8 . FLAMES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9. MOLECULAR BEAMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9.1 Gas-solid reactions . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

167
172
175
176

176

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Chapter 3 (D SHOOTER)

Experimental methods for the study of heterogeneous reactions . . . . . . . .

. GENERAL INTRODUCTION . . . . . . . . . . . . . . . . . . . .

1
2

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180

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Preparation of clean surfaces . . . . . . . . . . . . . . . . . . . .
High vacuum technique . . . . . . . . . . . . . . . . . . . . . .

THE SOLID-GAS INTERFACE

2.1 Adsorption

2.1.1
2.1.2
2.1.3 Measurement of adsorbed volume . . . . . . . . . . . . . . . . . .
2.1.4 Measurement of adsorbed weight . . . . . . . . . . . . . . . . . .

2.1.5 Measurement of work function . . . . . . . . . . . . . . . . . . .
2.1.6 Measurement of magnetic changes . . . . . . . . . . . . . . . . . .
2.1.7 Measurement of changes in electrical conductivity . . . . . . . . . . .
2.1.8 Electron diffraction . . . . . . . . . . . . . . . . . . . . . . . .
2.1.9 Spectroscopic measurements . . . . . . . . . . . . . . . . . . . . .
2.2 Catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.1 Catalyst preparation . . . . . . . . . . . . . . . . . . . . . . . .
2.2.2 Diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.3 Reactor design . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2.4 The use of isotopes . . . . . . . . . . . . . . . . . . . . . . . .
2.2.5 Mass spectrometry . . . . . . . . . . . . . . . . . . . . . . . .
2.3 Non-catalytic reactions . . . . . . . . . . . . . . . . . . . . . . . . .
2.3.1 Oxidation of metals . . . . . . . . . . . . . . . . . . . . . . . .
2.3.2 Dissociation reactions . . . . . . . . . . . . . . . . . . . . . . .
2.3.3 Transport reactions . . . . . . . . . . . . . . . . . . . . . . . .
2.3.4 Isotope exchange . . . . . . . . . . . . . . . . . . . . . . . . .
3. THE SOLID-LIQUID INTERFACE .
3.1 Adsorption from solution
3.2 Catalytic reactions . . .
3.3 Non-catalytic reactions .
4

. THE SOLID-SOLID
4.1
4.2
4.3
4.4
4.5

INTERFACE


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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
X-ray crystallography . . . . . . . . . . . . . . . . . . . . . . . . . .
Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Differential thermal analysis . . . . . . . . . . . . . . . . . . . . . . .
Electrical conductivity measurements . . . . . . . . . . . . . . . . . .


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CONTENTS

XI1

4.6 Magnetic measurements . . . . . . . . . . . . .
4.7 Dilatometry . . . . . . . . . . . . . . . . . .
4.8 Other methods . . . . . . . . . . . . . . . . .

5

. THE LIQUID-GAS

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INTERFACE

5.1 Introduction

5.2 Measurement of film pressure-film area characteristics
5.3 Surface potential measurements . . . . . . . . . .
5.4 Other methods . . . . . . . . . . . . . . . . .
REFERENCES

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Chapter 4 (R . P. WAYNE)

The detection and estimation of intermediates . . . . . . . . . . . . . . . . 279
1. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
279
1.1 Stable intermediates

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1.2 Free radicals and atoms . . . . . . . . . . . . . . . . . . . . . . . . .
1.3 Excited species . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1.41011s . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.


CLASSICALSPECTROSCOPY

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2.1 Emission spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Absorption spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . .
3.

MASSSPECTROMETRY

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3.1 Uncharged species . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Ionic species . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4 . ESR
5.

6.

'CHEMICAL' METHODS

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314

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324
324

324
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327

ELECTRICAL METHODS FOR CHARGED SPECIES

8. TRAPPED

294
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306'
308

Pressure measurement . . . . . . . . . . . . . . . . . . . . . . . . .
Thermal conductivity measurement . . . . . . . . . . . . . . . . . . . .
Calorimetry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Wrede-Harteck gauges . . . . . . . . . . . . . . . . . . . . . . . . .

REFERENCES

284
286
290

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MISCELLANEOUS PHYSICAL TECHNIQUES

6.1
6.2

6.3
6.4
7.

SPECTROMETRY

280
282
282
283

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RADICALS

328
331
336

Chapter 5 ( D. MARGERISON)

The treatment of experimental data . . . . . . . . . . . . . . . . . . . . . .

.

1 INTRODUCTION


2.

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343
343

TESTING THE DATA FOR CONSISTENCY WITH EQUATIONS OF THE TYPE

-d[A]/dr = k[A]"[B]b . . . . . . . . . . . . . . . . . . . . . . . . . . .
348
2.1 Reactions with known invariant stoichiometry
. . . . . . . . . . . . . . 349
2.1.1 Estimation of the orders a and b . . . . . . . . . . . . . . . . . . . 351
(a) Estimation of total order, a + b . . . . . . . . . . . . . . . . . . 351
(b) Estimation of the order. a . . . . . . . . . . . . . . . . . . . 358
360
(c) Estimation of the order. b . . . . . . . . . . . . . . . . . . . .


CONTENTS

XI11

2.1.2 Calculation of the rate coefficient . . . . . . . . . . . . . . . .
(a) Treatment of the data from a single run . . . . . . . . . . . .
(b) Treatment of the data from a series of runs . . . . . . . . . .
(c) A re-examination of the functions ffor) of Table 1 . . . . . . .
(d) Special procedures . . . . . . . . . . . . . . . . . . . . . . .
2.2 Reactions with unknown stoichiometry . . . . . . . . . . . . . . .


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.
.

.
.
.
.

. .

. TESTING THE DATA FOR CONSISTENCY WITH COMPLICATED RATE EXPRESSIONS . . . . .
4. ELUCIDATING THE DEPENDENCE OF THE RATE COEFFICIENT(S) ON TEMPERATURE. CONCEN3

TRATION OF CATALYST. AND IONIC STRENGTH

. . . . . . .
. . . . . . .
. . . . . .
. . . . . . .

4.1 Influence of temperature . . . . . . . .
4.2 Influence of the concentration of catalyst
4.3 Influence of ionic strength . . . . . . .

.
. . . .
. . . .

. . . .

.
.
.
.

.
.
.
.

.
.
.
.

.
.
.
.

. . . . . . . .
. . . . . . .
. . . . . . . .
. . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . .

360
362
367
386
387
393
394
403
404
406
406

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

407
401
408
409
419
420

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

422

Acknowledgement
REFERENCES

APPENDIX1
APPENDIX2
APPENDIX
APPENDIX

3
4


This Page Intentionally Left Blank


Chapter I

Experimental Methods for the Study of
Slow Reactions
L. BATT

1. Introduction

The labelling of reactions as “fast” or “slow” is obviously arbitrary, though
convenient, and for the purposes of this chapter it will be supposed that the latter
have half-lives of greater than several seconds (usually of the order of minutes).
The main emphasis will be placed on gas-phase systems, since the experimental
difficulties are generally more acute, and in fact the methods of following the
course of reactions in the liquid phase are usually (with appropriate modifications)
the same.
The primary aim of a chemical kinetic study is the determination of the mechanism by which a system changes in composition. This involves tests for heterogeneity as well as the determination of the order of a reaction in terms of the concentration of reactant or reactants, and an analysis of the products. Estimation
of the rate coefficient for a particular elementary step as a function of pressure
and temperature provides, in the first instance, Arrhenius parameters, which should

be rationalised in terms of the thermodynamics of the reverse process, and may
finally lead to particular bond energies.
The essential apparatus for pressure measurement and analysis, and other important aspects such as furnaces and temperature control, are reviewed for thermal, photochemical and radiochemical systems. The latter two also involve sources
of radiation, filters and actinometry or dosimetry. There are three main analytical techniques: chemical, gas chromatographic and spectroscopic. Apart from
the almost obsolete method of analysis by derivative formation, the first technique
is also concerned with the use of “traps” to indicate the presence of free radicals
and provide an effective measure of their concentration. Isotopes may be used for
labelling and producing an isotope effect. Easily the most important analytical
technique which has a wide application is gas chromatography (both GLC and
GSC). Intrinsic problems are those concerned with types of carrier gases, detectors,
columns and temperature programming, whereas sampling methods have a
direct role in gas-phase kinetic studies. Identification of reactants and products
have to be confirmed usually by spectroscopic methods, mainly IR and mass spectroscopy. The latter two are also used for direct analysis as may uv, visible and
ESR SpeCtrOSCOpy. NMR spectroscopy is confined to the study of solution reactions
References pp. 104-111


2

E X P E R I M E N T A L M E T H O D S FOR S L O W R E A C T I O N S

apart from the identification of derivatives. A combination of chemical and spectroscopic techniques occurs for example in the gas-phase titration for hydrogen
atoms by the addition of NO and spectroscopic estimation of the visible emission
from the hot nitroxyl. Apart from the above techniques, which are of general
application, there are other methods such as calorimetry, gas density measurement,
polarography, the damped oscillation of a quartz fibre, ultrasonics, conductivity
and refractive index measurements, which have limited application. However,
when the general techniques prove unsatisfactory, these techniques will have a
particular significance.


2. Determination of a mechanism
The essential experimental task for the gas kineticist is a determination of the
mechanism of a homogeneous chemical reaction. This must explain the order of
the reaction and the observed products. The first step is a test for heterogeneity,
achieved by drastically altering the surface area to volume ratio ( S / V )and the composition of the surface of the reaction vessel (RV) in which the reaction is studied.
For partly heterogeneous reactions, milder variations of ( S / V ) are obtained by
using spherical, cylindrical or octopus1-shaped RV's. In this case, a value for the
homogeneous rate ( W')is obtained from a graphical plot of the overall rate
against S/V, extrapolating to zero S/V. The problem is treated precisely by Hudson and Heicklen'. The order of the reaction (a) is determined from an observation of the variables in the equation

(w>

where Wi is the initial rate of the reaction at an initial concentration ci and k is
the rate coefficient. A similar equation may be used for a single run where the rate
is found as a function of time. However, the two orders are not always the same
for a given reaction. For the decomposition of acetaldehyde (AcH), Letort3 found
that the order with respect to initial concentration was 3 whereas that with respect
to time was 2. For reactions where there is more than one reactant, the isolation
method may be used, where one or more reactants are kept in great excess, while
the remaining is subjected to the variables in equation (A). However, in many cases
it does not yield reliable results4.
Provided that there is a change in the number of moles upon reaction, an obvious measure of the extent of a reaction is given by the change in pressure. The
latter has to be related to the stoichiometry of the reaction by quantitative analysis of the products and reactant or reactants and by material balance. Abnormal
pressure effects sometimes occur due to adiabatic reactions, unimolecular reactions which are in their pressure-dependent regions (particularly in flow systems)


2

D E T E R M I N A T I O N OF A M E C H A N I S M


3

and hot molecule or hot radical reactions. For free radical chain reactions the
mode of initiation and termination has to be determined, sometimes a difficult
task. The former may possibly be assessed from the use of a suitable radical trap.
Surface initiated and terminated reactions may be studied using sensitive calorimetric techniques. Some idea of the complexity of initiation and termination may
be realised by considering the thermal decomposition of AcH, which has been
recently reviewed by Laidler'. The rate of initiation is dependent on the square
of the AcH concentration but independent of the concentration of added inert
gases. Termination, involving the combination of methyl radicals, is pressuredependent whereas the kinetics of the decomposition of ethane under the same
conditions of temperature and pressure would predict the combination rate to
be pressure-independent. These conclusions are based on the possibly unsound
premise that H, and C,H, formation are direct measures of initiation and termination respectively. Information about the elementary steps that comprise a
mechanism is produced by the study of a sensitised decomposition such as
Me. + AcH

-+

CH,

+Ac.

(1)

Very often a particular reaction is studied in order to obtain its Arrhenius parameters. An example of this is given by reaction (l), although probably more interesting is the fate of the acetyl radical. The activation energy (Ef)
is related to
the standard heat of reaction AH" by the relationship,
A H o = E, -E,+ AnRT

(B)


where E,is the activation energy for the reverse process, An is the change in the number of moles upon reaction, and R and T have their usual meaning. AHo may be
separately determined from equation (C):
AHo = ZH,"(products)-ZHi(reactants)

(C>

If E, is zero, Efmay be related to a particular bond dissociation energy via equations (B) and (D). For the reaction
X.+CH,

-+

XH+Me.

(2)

where X. is an atom or a radical, the relevant bond dissociation energies are related
by equation (D):
AHo = D(Me-H)-D(X-H)

(D)

The pre-exponential factor (A,) is related to that for the reverse process (A,) by
References pp. 104-1 11


4

E X P E R I M E N T A L M E T H O D S FOR SLOW R E A C T I O N S


the expression,

ASTp, is the standard entropy change for the reaction in pressure units. If a change
in the number of moles occurs upon reaction as in reaction (3), and A , is in concentration units, AS;b, will have to be converted into concentration units. The

t-BuO. + Me. + Me,CO

(3 1

relation between K(p,and K(,,, the equilibrium constants for reaction (3) in pressure and concentration units respectively, leads to the result,
AS,",, = ASTp,-AnR In R T

(F)

where AS,",, is the standard entropy change in concentration units. In this case6,

AS" may be determined from the expression,
ASo = ZS"(Products) -Z:S"(Reactants)

(HI

Hence the rate of the reverse reaction may be calculated from equations (B) and
(G). Perhaps more important from the point of view of this chapter, this also
provides a check on the determined values for Efand Af and hence the proposed
mechanism. Laidler and Polanyi have reviewed empirical methods for predicting E, via various relationships between it and the heat of reaction'.
AH" and AS" may be calculated from values of HF and So listed in various
or calculated from Atom or Group Additivity Rules8314.
If the particular reaction studied is the unimolecular decomposition of a free
radical, such as (3), then the use of a "trap" will enable the effective concentration
of the radical to be measured. A radical trap will indicate the presence or absence

of a free radical reaction and may sometimes provide evidence for a partly or
entirely molecular reaction. Rate data for free radical reactions are derived assuming the occurrence of a steady state concentration of radicals. The time required to produce a steady state concentration of methyl radicals in the pyrolysis
of AcH is shown for various temperatures in Fig. 1. Realistic values for rate
coefficients may be obtained only if the time of product formation is long compared
to the time to achieve the steady state concentrations of the radicals concerned. Thus
deductions from the results from the bromination of isobutanel 5, neopentaneI6,
and toluene" have been criticised on the grounds that a steady state concentra-


2

D E T E R M I N A T I O N OF A M E C H A N I S M

5

Tiiie b e d

Fig. 1. Approach to the steady state in the pyrolysis of acetaldehyde; time-dependence of (Me)
for three temperatures; the dotted curve represents a 50-fold expansion of the 400" data; calculations of W. J. Probst. From ref. 9.

tion of the bromine atoms was not achieved under the chosen experimental conditions".
Two additional complications may be present in photochemical reactions, the
presence of hot molecules and hot radicals referred to earlier in thermal systems,
and the possible physical and chemical primary processes that may occur. Compounds such as biacetyl, 02,NO and olefins are particularly efficient quenchers
of electronically excited species. In accordance with the Wigner spin conservation
rule that the total spin in a quenching process is conserved, triplet state acetone
('A) is quenched very efficiently to the ground state ('A) by olefins ( 0 ) 1 9 .
3 ~ + 1 0

-+


(4)

1 ~ + 3 0

Larson and ONeal have used HBr as a diagnostic test for the presence of triplet
state acetone2'. Double trapping results in the production of isopropyl alcohol, uiz.
(5 1

'A+HBr

4

MezCOH+Br*

Me,COH

+HBr + Me2CHOH+Br.
or
RH

or

R.

Information about primary chemical processes and the ensuing secondary processes may be obtained by using suitable radical traps thus isolating molecular
primary processes. Direct information about primary processes may be obtained
by flash and matrix photolysis. Radical traps may also be used to good advanReferences p p . 104411



6

E X P E R I M E N T A L M E T H O D S F O R SLOW R E A C T I O N S

tage in radiochemical reactions. Here it is often very important to keep the extent
of conversion to 0.002 % or less to obtain valid information about the primary
processes*'. In addition to the processes that occur in photochemical and thermal systems, radiochemical primary and secondary processes result in the production of ions. Electrons may be removed by efficient electron traps such as NzO, SF,
and CCI,.

3. Apparatus
3.1 G E N E R A L

3.1.1 The vacuum line
General techniques for high vacuum systems have been covered in detail in
various reviews22, in particular conventional high vacuum taps and their lubri-

A

n

B

I

- Mercury

(d)

(el


Fig. 2. Various mercury cut-offs. From ref. 22c.


3

APPARATUS

7

Fig. 3. Ramsperger’s greaseless valve. From ref. 22c.

cants, used for the isolation of gases. A problem associated with the use of lubricants is their ready absorption of vapours and usually the inability to “bake” the
taps. The Echols type of tapz3 avoids these problems by using graphite as the lubricant and sealing with mercury. Apart from this, alternatives to the conventional
taps are various greaseless valves2’. The simplest of these are mercury cut-offs
shown in Fig. 2. There may be some difficulty if there is a large pressure differential
between either side of the U-tube. Polythene or teflon keys may be used with glass
tubing to form a tapz4. However, they are subject to degassing and may have their
own vapour absorption problems. This also applies to taps using elastomer diaphragms. Rapid operation of the latter type, either manually or electro-magnetically, has been achieved by Verdin”. A modification of the Bodenstein all-glass tapz6
is due to RamspergerZ7(Fig. 3). The tap is closed by seating a plug of AgCl into
a Pyrex tube, of which it wets the surface. Seating is achieved by a screw attached
to silver bellows coated with AgCl. This is an excellent tap provided no chemical
reaction occurs between its components and the gases involved. A similar tap has
References p p . 108-111


8

EXPERIMENTAL METHODS FOR SLOW REACTIONS
To H.V.


To$asstorage manifold (GSM.)
1-Gas inlet

i

L.V. = Low Vacuum
H.V. =High Vacuum

To LY

To MC Leod
gauge etc.
H.V.

4

I

To GSM.

?ig. 4. High-vacuum system for kinetic studies.

been constructed by Judge and Luckey2*. All-metal taps are available commercially which are bakeable, have rapid operation and provide good sealing; one
draw-back is their price, and once again chemical reaction may occur.
Details of a typical vacuum line for a gas phase kinetic study are given by MaccollZ3.A useful set-up is to have a manifold of large-diameter glass tubing leading from the pumping system, to which the different sections, storage and introduction, mixing and reaction vessels and analytical section, are connected (Fig. 4).
This manifold system has the advantage that each section may be evacuated independently of the others.

3.1.2 Temperature control

There are three general methods for maintaining a RV at a particular temperature: thermostats, furnaces and vapour baths29.

At temperatures close to 20+20", water is a suitable liquid for a thermostat.
At higher temperatures it is more convenient to use a fluid such as silicone oil,
or above 2W, molten metals. A typical furnace is shown in Fig. 5 . A silica former
is wound with nichrome wire such that the pitch decreases from one end to the centre and then increases again to the other end. Since heat losses are greatest at the
ends this provides a rough correction. An inconel tube also evens out the tempera-


3

9

APPARATUS
Platinum
resistance

Terminal

t herrnometer

block

I

Sliding
doors

Thermocouple
well
Loose
asbestos


Inconel
tube

I

silica
tube

"Si ndanyo 'I

Fig. 5. A typical furnace.

ture along the length of the furnace. Tappings are taken at regular intervals along
the winding. The wire is covered with asbestos paper and cement and placed in an
asbestos tube or box and packed into 3 in. of kieselguhr or other insulating material. Shunts are connected at the taps in the required places and adjusted until the
temperature profile at say 200" is f 0.5" along the length of the RV. The total
resistance of the furnace should be such as to allow a maximum output of f l Kw
depending on the temperature range to be covered. For photochemical or radiochemical studies the design has to be modified such that the furnace rests on an
optical bench and an uninterrupted path is provided for the source of radiation.
This is achieved by splitting the furnace in half longitudinally or laterally. The latter
is described in detail in the excellent treatise on photochemistry by Calvert and
Pithg. The former presents some difficulty because winding has to be carried out
over a semicircular cross-section but can be achieved by drilling holes or having
pegs at regular intervals at the extremities of the asbestos tube halves. The inconel
tube is replaced by two aluminium half-tubes with horizontal sections such that
one half sits on the other when they are slotted and clamped together. Entry tubes
to the RV may be led in at the join of the two halves. This design is particularly
useful for flash photolysis studies.
A circuit for heating and controlling the temperature of a thermostat or a furnace is shown in Fig. 6 . Just the right amount of current from a constant voltage

transformer and a variac transformer is fed into the furnace connected in series
with a small variable resistance && of the value of the furnace resistance, connected across a relay. For thermostats, the relay may be operated by a toluene/
mercury or a mercury thermo-regulator, depending on the temperature. Platinum
resistance thermometers or thermocouples, forming part of a Wheatstone bridge
network, are used to operate the relay for electric furnaces. This type of regulator
is available commercially. A strict control of current is required. In this way it is
N

References p p . 104-111


10

E X P E R I M E N T A L M E T H O D S FOR S L O W R E A C T I O N S
Reaction Furnace Circuit

I

''

r-3

R V Furnace

Fig. 6. Circuit for heating and controlling the temperature of a furnace.

possible to control at 200" C to 5 0.2" and at 600" C to & 2". Variations of this
type of control are described in ref. 22c.
Temperature measurement is almost invariably made using thermocouplesz2".
The latter must be constructed from fine wire30and have a fast response3' such

that even a very small temperature change may be measured precisely3'.

3.2

THERMAL SYSTEMS

3.2.1 Static method
(a) Reaction vessels

Reaction vessels are usually spherically or cylindrically shaped, vary in size from
200-1000 cm3, and have a thermocouple well in the centre. The tip of this well is
thin-walled and sometimes a drop of fluid, such as silicone oil, is used to provide
good thermal contact. The material most often used is Pyrex glass (maximum
temperature 600" C ) and fused silica (maximum temperature 1200" C ) . At room
temperature the two materials are practically impermeable to all gases except helium. At higher temperatures hydrogen diffuses through the glasses and particularly through silica3 At still higher temperatures oxygen and nitrogen may
diff me through silica34.The mechanism appears to involve adsorption at the glass
surface followed by passage through the glass. Silica reaction vessels should not
be heated to high temperatures when in contact with metals. Iron or nickel, for instance, will diffuse into the quartz and give rise to heterogeneous reactions. Many
gases are strongly adsorbed on glass surfaces and hence all glass apparatus, particularly the RV, should be well degassed at elevated temperatures. Both silica and
Pyrex glass are resistant to most gases at room temperature except HF. However,
HCI, HBr and probably HI are adsorbed on the surface and at high temperatures
react with the glass35.Adsorption occursvia dissolution in a thin layer of adsorbed

'.