NJC Kinetics 2025 student version (Notes)

National Junior College SH1 H2 Chemistry 2025 REACTION KINETICS Content ● Simple rate equations; orders of reaction; rate constants ● Concept of activation energy ● Effect of concentration, temperature, and catalysts on reaction rate ● Homogeneous and heterogeneous catalysis ● Enzymes as biological catalysts 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 (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 the 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 SO 2 (iv) catalytic role of Fe2+ in the I−/S2O82– reaction (k) describe enzymes as a 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 © 2025 National Junior College All Rights Reserved. No part of this publication may be produced or transmitted in any form or by any means, electronic or mechanical, including photocopy, recording or any other information storage and retrieval system, without prior permission in writing from the copyright owner.

National Junior College SH1 H2 Chemistry 2025 2 1 DEVELOPMENT OF CHEMICAL KINETICS The concept of Chemical Kinetics started since 1700s and was developed by numerous experts of various nationalities in the field of Science and Mathematics to reach the understanding which we have today. The conceptual development started first with the focus on the finding of general laws for chemical reactions and relating kinetics to thermodynamics, followed by understanding of reactions through mechanism and transition states to describe multi-step chemical reaction. Below is a quick look at the progress in the development of Chemical Kinetics (or during those time, Chemical Dynamics).

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National Junior College SH1 H2 Chemistry 2025 4 What can we learn from these events leading up to the modern concept of Kinetics? Science is a pursuit to explain the natural phenomenon happening around us. Scientific knowledge and ideas are generated through observation of nature either through experiment or simply by daily encounters. Science is built upon evidences. Scientists built the general laws and concepts through gathering and evaluating of evidences. Scientists cannot just give a statement and call it a scientific knowledge. Evidences need to be sought through thorough studies in order to formulate a durable idea. Science is a complex social construct, is durable yet tentative. Science is not made by one man alone. As we can see from the development of the concept of Kinetics, scientists of different nationalities, different research interest and different abilities collectively contribute to the pool of knowledge over a long period of time, testing and building on each other’s hypothesis/proposal in order to derive general laws which accurately describe the natural state. Hence, what we have today is credible and is as close as we can know with our technology (durable). However, as new discoveries and advances are made, new evidences may appear which require us to re -evaluate what we have and adjust our knowledge accordingly (tentative).

National Junior College SH1 H2 Chemistry 2025 5 2 INTRODUCTION Chemical reactions are processes where starting materials are converted into specific products. For a chemical reaction to occur, three important questions should be asked: In this chapter, we will focus on learning about the concept of kinetics and its use in understanding chemical reactions. This will be done through studying kinetics qualitatively and quantitatively at two levels: (1) MACROSCOPIC LEVEL : addresses rates of reactions: what reaction rate means, how to determine a reaction rate and the rate equation experimentally, and how factors such as temperature, reactant concentration and the use of a catalyst influence rate. (2) PARTICULATE LEVEL: addresses the mechanism of the reaction, the detailed pathway taken by atoms and/or particles as a reaction proceeds. We need to learn at a particulate level, how does the distribution of energy before, during and after reaction affect the (i) energetic feasibility and (ii) kinetic feasibility (rate) of a chemical reaction?

National Junior College SH1 H2 Chemistry 2025 6 3 BASIC CONCEPT IN KINETICS 3.1 COLLISION THEORY (Understanding reaction at particulate level) For a reaction between two particles to occur two conditions must be met: 1) The particles must collide in the correct orientation. 2) The particles must collide with sufficient minimum energy (known as the activation energy) to bring about the necessary re-organisation of the bonds in the colliding particles. Assuming product to be , When particles collide at the correct orientation and sufficient energy, this collision is known as effective collision which will bring about a chemical reaction. The greater the frequency of effective collisions, the greater the rate of reaction. Hence, we can easily deduce that the experimental conditions required for increasing the number of effective collisions to increase reaction rate are: (i) Increase in temperature of the reacting particles (ii) Increase concentration of particles/ increasing surface area of solid particles (iii) Use of catalyst Note: The greater the number of particles involved in a single step, the more difficult it is to have all of them to collide at the correct orientation to affect a reaction. Hence usually, only a maximum of two particles will c