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Unformatted text preview: Chapter 13 The Rates of Reactions No physicochemical change takes place instantaneously, i.e., at an infinite rate. Most changes take place at a measurable rate. Many ionic reactions tend to be quite fast ( recall the precipitation reactions you have carried out in the lab ) while most reactions involving the formation and breaking of covalent bonds tend to be slow. In this chapter the focus of interest is the rate at which a reaction takes place. We would like to know, among other things, • How are we to measure the rate of a reaction? • What factors affect the rate of a reaction? • What can we learn from the rate of reaction about changes taking place at the molecular level? Some reasons why a chemist is interested in these matters are: • We would like to prepare substances in the shortest possible time. • If many reactions take place, a knowledge of the factors affecting the rate may enable us to increase the rate of the desired reaction. • It helps us get a better understanding of the reaction at the molecular level. How do we know a change is taking place? A chem icalchange producesnew substances . Most often this results in obvious changes in physical and chemical properties: change in color, production of new solids, evolution of gases, change of temperature, change in electrical conductivity etc. All these changes are a consequence of the primary change, viz., change in amounts (number of moles) of reactants and products . a3 Exercise 13.0.1 How does the amount of a reactant vary with time: ( increase, decrease )? Sketch a graph of amount with time. How does the amount of a product vary with time: ( increase, decrease )? Sketch a graph of amount with time. CHAPTER 13. TheRatesofReactions A brief digression into notation : The balanced equation for a chemical reaction plays a very important role in the description of rates of reactions. It is usually assumed that the reaction takes place from left to right as written, i.e., the reactants are on the left and products on the right. In the balanced equation N 2 (g) + 3H 2 (g) = 2NH 3 (g) the numbers associated with each species are referred to as coefficients , which we will abbreviate to coeff. or cft. Thus cft(H 2 )=3, and cft(NH 3 )=2. Since the amounts of reactants decrease with time, and those of products increase with time, it is often useful to give these coefficient a sign, a negative sign forcoefficients ofreactants and a positive sign forthose ofproducts . These signed coefficients are termed stoichiometric coefficients and denoted by the Greek symbol ν ( nu ). Thus ν (N 2 ) = − 1 , ν (H 2 ) = − 3 since N 2 and H 2 are reactants, and ν (NH 3 )=2. Changes in quantities will always be measured as the new or final value − the initial or old value . A finite change is denoted by the symbol Δ . Thus a finite change in Y will be denoted by Δ Y def = Y final − Y initial ....
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