JPJC 2026 Reaction Kinetics Lecture Notes

2026 JPJC J1 H2 Chemistry (9476) 1 Reaction Kinetics Jurong Pioneer Junior College 2026 JC1 H2 Chemistry (9476) Physical Chemistry Topic 6: Reaction Kinetics Recommended Reference Books: 1. Chemistry in Context (6th Edition) by Graham Hill and John Holman 2. A-Level Chemistry (4th Edition) by E N Ramsden 3. Cambridge International as and A Level Chemistry by Peter Cann and Peter Hughes Learning Outcomes: Candidates should be able to: (a) explain and use the terms: rate of reaction ; rate equation; order of reaction; rate constant; half-life of a reaction; rate-determining step; activation energy; catalysis (b) construct and use rate equations of the form rate = k[A]m[B]n (limited to simple cases of single- step reactions and of multi -step processes with a rate -determining step, for which m and n are 0, 1 or 2), including: (i) deducing the order of a reaction by the initial rates method (ii) justifying, for zero - and first-order reactions, the order of reaction from concentration - time graphs (iii) verifying that a suggested reaction mechanism is consistent with the observed kinetics (iv) predicting the order that would result from a given reaction mechanism (v) calculating an initial rate using concentration data [integrated forms of rate equations are not required] (c) (i) show understanding that the half -life of a first -order reaction is independent of concentration (ii) use the half-life of a first-order reaction in calculations (d) calculate a rate constant using the initial rates method (e) devise a suitable experimental technique for studying the rate of a reaction, from given information (f) explain qualitatively, in terms of frequency of collisions, the effect of concentration changes on the rate of a reaction (g) show understanding, including reference to the Boltzmann distribution, of what is meant by the term activation energy (h) explain qualitatively, in terms both of the Boltzmann distribution and of collision frequency, the effect of temperature change on a rate constant (and hence, on the rate) of a reaction (i) (i) explain that, in the presence of a catalyst, a reaction has a different mechanism, i.e. one of lower activation energy, giving a larger rate constant (ii) interpret this catalytic effect on a rate constant in terms of the Boltzmann distribution (j) outline different modes of action of homogeneous and heterogeneous catalysis, including: (i) the Haber process (ii) the catalytic removal of oxides of nitrogen in the exhaust gases from car engines (iii) the catalytic role of atmospheric oxides of nitrogen in the oxidation of atmospheric sulfur dioxide (iv) catalytic role of Fe2+ in the I−/S2O8 2− reaction (k) describe enzymes as protein molecules that act as biological catalysts with high specificity (in the reactions that they catalyse and in their choice of substrates as exemplified by the lock-and-key model), temperature sensitivity and pH sensitivity [knowledge of the levels of structure of proteins and the details of the denaturation process are not required]

2026 JPJC J1 H2 Chemistry (9476) 2 Reaction Kinetics Content page page 1 INTRODUCTION 2 7 EXPERIMENTAL TECHNIQUES TO MEASURE RATE 24 2 HOW DO REACTIONS OCCUR? 2 7.1 Continuous Methods 24 2.1 Collision Theory 2 7.2 Initial Rate Methods 28 2.2 Orientation of Collision 3 8 FACTORS AFFECTING THE RATE OF REACTION 31 2.3 Activation Energy 3 8.1 Effect of Surface Area 31 3 EXPRESSING THE REACTION RATE 4 8.2 Effect of Concentration / Pressure 31 4 RATE EQUATION 6 8.3 Effect of Light 31 4.1 Orders of Reaction & Rate Constant 6 8.4 Effect of Temperature 32 4.2 Zero Order Reaction (involving a single reactant) 8 8.5 Effect of Catalyst 33 4.3 First Order Reaction (involving a single reactant) 9 8.5.1 Autocatalysis 34 4.4 Second Order Reaction (involving a single reactant) 11 8.5.2 Enzymes (Biological Catalysts) 35 5 DETERMINING THE ORDER OF REACTION (involving multiple reactants) 12 9 9.1 TYPE OF CATALYSIS Homogeneous Catalysis 37 37 5.1 Initial Rates Method 12 9.2 Heterogeneous Catalysis 38 5.2 Isolation Method 15 Annex A: Determine 1st Order Reaction using Successive OR Non-Successive Half-lives 40 6 RELATIONSHIP BETWEEN RATE EQUATION AND REACTION MECHANSIM 20 Annex B: Applications of Enzymes in Real World Context 41 Summary 42 1 INTRODUCTION a. Reaction kinetics is a study of the rates of chemical reactions and the factors affecting these rates. b. The kinetics data is of practical use to scientists in establishing plausible step -by-step molecular pathways leading from reactants to products (i.e. reaction mechanisms). c. With the knowledge of reaction mechanisms and the factors that affect the rates, the chemical reactions can be effectively controlled. 2 HOW DO REACTIONS OCCUR? 2.1 Collision Theory a. Chemical reactions take place because of molecular collisions. b. However, not every collision results in a reaction. For a reaction involving collision between two species to take place, reactant particles must: (i) collide with the correct orientation; (ii) collide with sufficient energy for bonds to break. Such collisions are called effective collisions. The greater the frequency of effective collisions, the faster the reaction. [reactant] or [product] / mol dm  3 time 0 formation of product consumption of reactant

2026 JPJC J1 H2 Chemistry (9476) 3 Reaction Kinetics 2.2 Orientation of Collision The figures below illustrate at molecular level, the necessity of collision with sufficient energy and in the correct orientation for the reaction of NO2Cl + Cl → NO2 + Cl2 to take place. Fig. 2.1 The two reactant particles collide with sufficient energy and in the correct orientation ( such that the Cl–Cl bond can form while the N–Cl bond breaks). This effective collision leads to products being formed. Figure 2.2 In a poorly oriented collision, the two colliding particles simply "bounce off" without reacting. 2.3 Activation Energy a. Even though particles are correctly orientated, they must collide with sufficient energy for a reaction to occur. Activation energy (Ea) is the minimum energy the reactants must possess before a reaction can occur. b. In any system at a particular temperature, the particles present will have a wide range of kinetic energies. In a gas, the distribution of kinetic energy among the gas particles is represented by the Maxwell-Boltzmann distribution graph (More elaboration on p.34-35). c. Since Ea is the energy barrier the reactant particles must overcome before they react, • high Ea  reaction proceeds slowly (only a small number of particles have sufficient energy to react) • low Ea  reaction proceeds very fast (a large number of the molecules have sufficient energy to react) d. Ea of a reaction depends on the reaction mechanism and is independent of temperature. NO2Cl Cl NO2 Cl2 Correctly oriented collision Poorly oriented collision The area under the entire curve is a measure of the total number of particles present. After an effective collision Only this number of particles, represented by the shaded area, have sufficient energy (i.e. energy equal to or more than Ea ) for effective collision Ea ' Ea number of particles kinetic energy / kJ mol−1 0

2026 JPJC J1 H2 Chemistry (9476) 4 Reaction Kinetics 3 EXPRESSING THE REACTION RATE Consider