Chemical Kinetics and Reaction Dynamics (eBook)

Preface 8
Contents 10
1 Elementary 15
1.1 Rate of Reaction 15
1.2 Rate Constant 17
1.3 Order and Molecularity 18
1.4 Rate Equations 20
1.5 Half-life of a Reaction 22
1.6 Zero Order Reactions 24
1.7 First Order Reactions 26
1.8 Radioactive Decay as a First Order Phenomenon 31
1.9 Second Order Reactions 34
1.10 Third Order Reactions 42
1.11 Determination of Order of Reaction 44
1.12 Experimental Methods of Chemical Kinetics 53
2 Temperature Effect on Reaction Rate 60
2.1 Derivation of Arrhenius Equation 60
2.2 Experimental Determination of Energy of Activation and Arrhenius Factor 62
2.3 Potential Energy Surface 64
2.4 Significance of Energy of Activation 65
3 Complex Reactions 69
3.1 Reversible Reactions 69
3.2 Parallel Reactions 73
3.3 Consecutive Reactions 77
3.4 Steady-State Treatment 80
3.5 Chain Reactions 81
4 Theories of Reaction Rate 93
4.1 Equilibrium and Rate of Reaction 93
4.2 Partition Functions and Statistical Mechanics of Chemical Equilibrium 94
4.3 Partition Functions and Activated Complex 96
4.4 Collision Theory 97
4.5 Transition State Theory 103
4.6 Unimolecular Reactions and the Collision Theory 114
4.7 Kinetic and Thermodynamic Control 123
4.8 Hammond’s Postulate 124
4.9 Probing of the Transition State 125
5 Kinetics of Some Special Reactions 129
5.1 Kinetics of Photochemical Reactions 129
5.2 Oscillatory Reactions 134
5.3 Kinetics of Polymerization 138
5.4 Kinetics of Solid State Reactions 149
5.5 Electron Transfer Reactions 153
6 Kinetics of Catalyzed Reactions 156
6.1 Catalysis 156
6.2 Theories of Catalysis 159
6.3 Characteristics of Catalytic Reactions 160
6.4 Mechanism of Catalysis 161
6.5 Activation Energies of Catalyzed Reactions 163
6.6 Acid Base Catalysis 164
6.7 Enzyme Catalysis 166
6.8 Heterogeneous Catalysis 170
6.9 Micellar Catalysis 173
6.10 Phase Transfer Catalysis 179
6.11 Kinetics of Inhibition 182
7 Fast Reactions 189
7.1 Introduction 189
7.2 Flow Techniques 190
7.3 Relaxation Method 193
7.4 Shock Tubes 195
7.5 Flash Photolysis 196
7.6 ESR Spectroscopic Technique 197
7.7 NMR Spectroscopic Techniques 197
8 Reactions in Solutions 199
8.1 Introduction 199
8.2 Theory of Absolute Reaction Rate 199
8.3 Influence of Internal Pressure 201
8.4 Influence of Solvation 201
8.5 Reactions between Ions 201
8.6 Entropy Change 203
8.7 Influence of Ionic Strength (Salt Effect) 204
8.8 Secondary Salt Effect 206
8.9 Reactions between the Dipoles 207
8.10 Kinetic Isotope Effect 209
8.11 Solvent Isotope Effect 211
8.12 Hemmett Equation 212
8.13 Linear Free Energy Relationship 213
8.14 The Taft Equation 214
8.15 Compensation Effect 215
9 Reaction Dynamics 218
9.1 Molecular Reaction Dynamics 218
9.2 Microscopic-Macroscopic Relation 219
9.3 Reaction Rate and Rate Constant 221
9.4 Distribution of Velocities of Molecules 223
9.5 Rate of Reaction for Collisions with a Distribution of Relative Speeds 223
9.6 Collision Cross Sections 224
9.7 Activation Energy 227
9.8 Potential Energy Surface 230
9.9 Classical Trajectory Calculations 243
9.10 Potential Energy Surface and Classical Dynamics 248
9.11 Disposal of Excess Energy 253
9.12 Influence of Rotational Energy 254
9.13 Experimental Chemical Dynamics 255
Suggested Readings 261
Index 265

5 Kinetics of Some Special Reactions (p. 115)

5.1 Kinetics of Photochemical Reactions

There are certain reactions in which the activation may be carried out by electromagnetic radiations in visible and ultraviolet region having wavelength approximately between 100 to 1000 nm. These reactions are called photochemical reactions. A photon of radiation, referred to as quantum with energy h? (? being its frequency), is primary unit of radiation. When a photon from high energy electromagnetic radiations such as X- and ?-ray is used, the chemical processes are then called radiolytic reactions. The photochemical reactions are governed by two basic principles, viz. the Grotthus-Draper law and Einstein law of photochemical equivalence.

5.1.1 Grotthuss-Draper Law

This law states that only those radiations which are absorbed can be effective in producing the chemical change.

Although photochemical reaction is a result of absorption of light, it may not always lead to chemical change. Sometimes the absorption of photon may only increase the thermal energy or it may be reemitted (fluorescence).

5.1.2 Einstein Law of Photochemical Equivalence

In 1912, Einstein extended the concept of quantum theory of radiation to photochemical processes and stated that ‘each quantum of radiation absorbed by molecule activates one molecule in the primary step of a photochemical process’. This is known as Einstein law of photochemical equivalence. According to this law, if every molecule so activated in primary absorption directly decomposes, the number of molecules taking part in chemical change would be equal to the number of quanta of energy absorbed. However, a molecule activated photochemically may initiate a sequence of reactions so that many reactant molecules through a chain mechanism would undergo chemical change. In such cases the number of molecules undergoing chemical changes will be many times of the quanta of energy absorbed.