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CHAPTER 16: CHEMICAL KINETICS (Part 1) Chemical Kinetics is the study of reaction rates and mechanisms of chemical reactions. The rate of a chemical reaction is a measure of the number of moles of a substance used up or formed in a unit volume of reaction mixture per unit of time. When a solution of silver nitrate is mixed with a solution of sodium chloride, a white precipitate of silver chloride is formed almost instantly. The reaction between Ag + and NaCl(aq) is an example for a very fast reaction. Many chemical reactions do not take place at such fast rates. What is meant by rate of a reaction? Consider a hypothetical reaction: A → Products Rate of the above reaction can be defined as the number of moles per liter of 'A' reacted in unit time. Unit of time can be second, minute, hour, days, etc. Higher the rate of a reaction, faster is the reaction. Consider a reaction, in which Reactants Products The rate of a reaction can be expressed either in terms of decrease in concentration of the reactant(s) or in terms of increase in concentration of the product(s). Rate = - concentration of reactant time or Rate = concentration of product time Consider the reaction, A B The rate of the reaction is given by Rate = - change in concentration of A change in time = - [A] t Or Rate = t [B] = in time change B of ion concentrat in change
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Examples : 1. In the reaction, 2HI H 2 (g) + I 2 (g), The molar concentration of HI changed from 3.00 M to 2.50 M in 100.0 s. What is the rate of the reaction? Rate of decrease of HI concentration Rate of disappearance of HI = - × 2.50 M HI - 3.00 M HI 100.0 s = 5.0 10 -3 mole HI L . s Rate of the increase of H 2 concentration 2HI H 2 (g) + I 2 (g) Initial 3.00 M 0 M 0 M After 100 s 2.50 M 0.25 M 0.25 M Rate of increase of H 2 concentration = 0.25 M H 2 - 0.0 M H 2 100.0 s = 2.5 10 -3 mole H 2 L . s × Rate of increase of H
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This note was uploaded on 09/13/2008 for the course CHEM 1412 taught by Professor Davis during the Spring '08 term at Alamo Colleges.

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