Chapt13

Accounting

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C H A P T E R 13 Chemical Kinetics I N T R O D U C T I O N I N PREVIOUS CHAPTERS WE STUDIED BASIC DEFINITIONS IN CHEMISTRY , AND EXAMINED THE PROPERTIES OF GASES , LIQUIDS , SOLIDS , AND SO - LUTIONS . W E HAVE DISCUSSED MOLECULAR PROPERTIES AND LOOKED AT SEVERAL TYPES OF REACTIONS IN SOME DETAIL . I N THIS CHAPTER AND IN SUBSEQUENT CHAPTERS , WE WILL LOOK MORE CLOSELY AT THE RE - LATIONSHIPS AND LAWS THAT GOVERN CHEMICAL REACTIONS . H OW CAN WE PREDICT WHETHER OR NOT A REACTION WILL TAKE PLACE ? O NCE STARTED , HOW FAST DOES THE REACTION PROCEED ? H OW FAR WILL THE REACTION GO BEFORE IT STOPS ? T HE LAWS OF THERMODYNAMICS ( DIS - CUSSED IN C HAPTER 18 ) HELP US ANSWER THE FIRST QUESTION . C HEMICAL KINETICS , THE SUBJECT OF THIS CHAPTER , PROVIDES ANSWERS TO THE QUESTION ABOUT THE SPEED OF A REACTION . T HE LAST QUESTION IS ONE OF MANY ANSWERED BY THE STUDY OF CHEMICAL EQUILIBRIUM , WHICH WILL BE CONSIDERED IN C HAPTERS 14 , 15 , 16 , AND 18 . 13.1 THE RATE OF A REACTION 13.2 THE RATE LAW 13.3 THE RELATION BETWEEN REACTANT CONCENTRATION AND TIME 13.4 ACTIVATION ENERGY AND TEMPERATURE DEPENDENCE OF RATE CONSTANTS 13.5 REACTION MECHANISMS 13.6 CATALYSIS 507 Back Forward Main Menu TOC Study Guide TOC Textbook Website MHHE Website
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Chemical kinetics is the area of chemistry concerned with the speeds, or rates, at which a chemical reaction occurs . The word “kinetic” suggests movement or change; in Chapter 5 we defined kinetic energy as the energy available because of the motion of an object. Here kinetics refers to the rate of a reaction, or the reaction rate, which is the change in the concentration of a reactant or a product with time (M/s) . There are many reasons for studying the rate of a reaction. To begin with, there is intrinsic curiosity about why reactions have such vastly different rates. Some processes, such as the initial steps in vision and photosynthesis and nuclear chain re- actions, take place on a time scale as short as 10 12 s to 10 6 s. Others, like the cur- ing of cement and the conversion of graphite to diamond, take years or millions of years to complete. On a practical level, a knowledge of reaction rates is useful in drug design, in pollution control, and in food processing. Industrial chemists often place more emphasis on speeding up the rate of a reaction rather than on maximizing its yield. We know that any reaction can be represented by the general equation reactants 88n products This equation tells us that during the course of a reaction, reactants are consumed while products are formed. As a result, we can follow the progress of a reaction by moni- toring either the decrease in concentration of the reactants or the increase in concen- tration of the products. Figure 13.1 shows the progress of a simple reaction in which A molecules are converted to B molecules: A 88n B The decrease in the number of A molecules and the increase in the number of B mol- ecules with time are shown in Figure 13.2. In general, it is more convenient to express the reaction rate in terms of the change in concentration with time. Thus, for the re-
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