ASRJC H2 Chem 6. Reaction Kinetics

2024 JC1 H2 Reaction Kinetics 2024/ASRJC/Chemistry ANDERSON SERANGOON JUNIOR COLLEGE JC1 H2 CHEMISTRY REACTION KINETICS Content 1 Introduction 2 Expressing the reaction rate 2.1 Average, instantaneous, and initial reaction rates 3 Rate Equation (or Rate Law), Rate Constant & Order of Reaction 3.1 Rate equation or (Rate Law) 3.2 Order of reaction 3.3 Rate constant, k 4 Understanding Order of Reaction 4.1 Zero order reaction 4.2 First order reaction 4.3 Second order reaction 4.4 Pseudo-first order reaction 5 Determination of Rate Equation (or Rate Law) 6 Reaction Mechanism 7 Collision Theory and Factors Affecting Rate of Reaction 8 Catalysis (SDL) 9 Thermodynamic Stability versus Kinetic Stability (SDL)

2024 JC1 H2 Reaction Kinetics 2024/ASRJC/Chemistry Learning Outcomes Students must 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 an experimental technique for studying the rate of a reaction, from given information (f) explain qualitatively, in terms of collisions, the effect of concentration changes on the rate of a reaction (g) show understanding, including reference to the Boltzmann, 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 the 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 sulfur dioxide (iv) catalytic role of Fe3+ in the I–/S2O82– reaction (k) describe enzymes as biological catalysts which may have specific activity (i) explain the relationship between substrate concentration and the rate of an enzyme –catalysed reaction in biochemical systems References 1. Chemistry for Advanced Level, Cann and Hughes, Murray 2. Understanding Advanced Physical Inorganic Chemistry, Jeanne Tan and Kim Seng Chan 3. Chemistry, The Molecular Nature of Matter and Change (Fourth Edition), Silberberg, McGraw Hill 4. Chemistry & Chemical Reactivity (Sixth Edition), Kotz, Treichel and Weaver, Thomson 5. Chemistry The Central Science (Ninth Edition), Brown, LeMay, Bursten, Prentice Hall

2024 JC1 H2 Reaction Kinetics 2024/ASRJC/Chemistry 3 1 Introduction We have been taking a rather simple approach to chemical change, i.e.: reactants mix, and products are formed. A balanced equation is an essential quantitative tool for calculating product yields from reactant amounts, but it tells us nothing about how fast the reaction is. In energetics, we learnt about energy changes that accompany chemical reactions. While the value of the energy changes may indicate the direction of the chemical change, it does not tell us how fast a reaction proceeds. The rate of reaction can only be determined by experiment. Reactions occur at a wide range of rates. Some, like a neutralisation, a precipitation, or an explosive redox process, seems to be over as soon as the reactants make contact – in a fraction of a second. Others, such as the reactions involved in rusting or decaying, take a moderate length of time, from minutes to months or even thousands of years. Kinetics studies are important as it provide information on the rate and economic feasibility of a process in the industry. From there, an optimal set of conditions can be chosen to maximise the output in the shortest time frame. Knowing the mechanism will also allow the modification of processes or reactants, thereby increasing its effectiveness. For example, knowing how quickly a medicine acts or blood clots can make the difference between life and death; how long it takes for cement to harden or polyethylene to form can make the difference between profit and loss. In general, the rates of these diverse process / reactions depend very much on the same variables, most of which chemist can manipulate to maximise yields within a given time or to slow down an unwanted reaction. In this lecture, you will look at both the quantitative and qualitative aspects of reaction kinetics. • Reaction Kinetics is a study of the rates of chemical reactions, the factors which affect them and the mechanism by which chemical reactions occur. • The rate of chemical reaction varies greatly with different reactions and under different conditions. Factors that can affect the rate of a reaction include: ▪ temperature condition of the reaction, ▪ concentration of reactants (or pressure, for gaseous reactions), ▪ particle size of reactant, ▪ presence of catalyst. We will apply the Collision Theory to explain how these various factors affect the rates of chemical reactions (refer to Section 7). • We can measure the rate of a reaction by monitoring the rate of change of an observable property in the reaction, such as concentration of reactants or concentration of products, or mass, volume of gas etc. (refer to Section 10). • Kinetic experiments can also be performed to examine quantitatively how the various factors (e.g. change in concentration of a reactant) affect the rate of a reaction. (You will perform some kinetics experiments in the lab). • Quantitative data from these experiments is used to derive an important equation called the rate equation that is unique for a reaction under a set of conditions (refer to Section 5). • The rate equation allows us to determine the factors or conditions to increase (or decrease) the rate of a reaction, and most importantly, provides information on how the chemical reaction takes place at the molecular level, that is, information on the reaction mechanism (refer to Section 6).

2024 JC1 H2 Reaction Kinetics 2024/ASRJC/Chemistry 4 [A ]/ mol dm–3 t / s t1 t2 0 0 2 Expressing the Reaction Rate • The rate of reaction can be quantitatively expressed as the increase in concentration of a product per unit time or the decrease in concentration of a reactant per unit time. Rate = timeinchange ]product[inchange = timeinchange ]ntreacta[inchange− • The negative sign in the second expression reflects the fact that the concentration of the reactant is decreasing, hence including it will give a positive value for the rate. • Units for rate of reaction is mol dm–3 time–1 • Common units are mol dm–3 s–1 or mol dm–3 min–1 2.1 Average, Instantaneous, and Initial Reaction Rates Examining the rate of a real reaction reveals an important poi