NJC 7 Kinetics H1 (Notes)

National Junior College SH1 H1 Chemistry REACTION KINETICS Content ● Simple rate equations; orders of reaction; rate constants ● Concept of activation energy ● Effect of concentration, temperature, and catalysts on reaction rate ● 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; 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, 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 conc-time graphs (iii) calculating an initial rate using concentration data (c) show understanding that the half-life of a first-order reaction is independent of concentration (d) explain qualitatively, in terms of frequency of collisions, the effect of concentration changes on the rate of a reaction (e) show understanding, including reference to the Boltz mann distribution, of what is meant by the term activation energy (f) 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 (g) (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 in terms of the Boltzmann distribution (h) outline the mode of action of heterogeneous catalysis, as exemplified by the catalytic removal of oxides of nitrogen in the exhaust gases from car engines (i) describe enzymes as biological catalysts which may have specific activity © 2022 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 H1 Chemistry 2 1 BASIC CONCEPT IN KINETICS 1.1 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 collide in a particular step of a reaction.

National Junior College SH1 H1 Chemistry 3 1.2 FACTORS AFFECTING RATE OF REACTION Learning Objectives: • explain qualitatively, in terms of frequency of collisions, the effect of concentration changes on the rate of a reaction 1.2.1 Concentration of Aqueous or Gaseous Reactants Rate of reaction increases as the concentration of reactants is increased. Increasing the concentration or pressure (for gases) is equivalent to increasing the amount of reactants particles per unit volume. This will increase the frequency of effective collisions between molecules and this increases the rate of reaction. The rate of a typical reaction is fastest at the beginning, where concentration of reactants is at their highest and gradually decreases as the reactants are used up and their concentration decreases. 1.2.2 Surface Area of Solid Reactant In a solid substance, only the particles on the surface can come into contact with a surrounding reactant. If the solid is in powdered form, then the surface area of contact increases dramatically, frequency of collision increases, number of effective collision increases. Hence, rate increases correspondingly.

National Junior College SH1 H1 Chemistry 4 BOLTZMANN DISTRIBUTION CURVE Learning objective: • show understanding, including reference to the Boltzmann distribution, of what is meant by the term activation energy Note for drawing Boltzmann distribution curve: 1) Graph must start from origin as there are no particles with zero energy. 2) Graph should taper off. 3) Shade the appropriate area under the curve to show proportion of particles with K.E. ≥ Ea. Boltzmann Distribution Curve shows energy distribution of reactant particles before reaction. Particles in a system have a range of kinetic energies. This energy distribution of particles can be represented using the Boltzmann distribution curve. In practice, only a certain fraction of the reactant particles has sufficient energy (K.E.  Ea) that result in reaction upon collision, as shown by the shaded area in the Boltzmann distribution curve. The greater the number of particles having kinetic energy  Ea, the greater the frequency of effective collision, the faster the rate of reaction

National Junior College SH1 H1 Chemistry 5 1.2.3 Temperature Learning objective: • 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 Increasing temperature of the reaction causes an increase in the rate of reaction. Rate approximately doubles for every 10 C increase in temperature. Worked example 1 C6H5CH2Cl undergoes a substitution reaction to form C 6H5CH2CN as shown in the reaction below. C6H5CH2Cl + CN– ⎯→ C6H5CH2CN + Cl– With the aid of Boltzmann distribution curve, predict and explain the effect on the rate of this reaction when the reaction temperature is decreased. When temperature of the reaction decreases, average kinetic energy of the reacting molecules decreases. The fraction of molecules with K.E. ≥ Ea decreases as shown in the Boltzmann distribution. The frequency of effective collisions decreases hence rate of reaction decreases. Note: The area under the curve is a measure of the total number of particles present. This area is the same for both curves.

National Junior College SH1 H1 Chemistry 6 1.2.4 Catalyst Learning objective: • 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 • interpret this catalytic effect on a rate constant in terms of the Boltzmann distribution A catalyst speeds up the rate of a chemical reaction by providing an alternative pathway with low