Chapter 16


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16-1 CHAPTER 16 KINETICS: RATES AND MECHANISMS OF CHEMICAL REACTIONS 16.1 Changes in concentrations of reactants (or products) as functions of time are measured to determine the reaction rate. 16.2 Rate is proportional to concentration. An increase in pressure will increase in number of gas molecules per unit volume. In other words, the gas concentration increases due to increased pressure, so the reaction rate increases . Increased pressure also causes more collisions between gas molecules. Why does the increased collision rate not necessarily increase reaction rate? 16.3 The addition of more water will dilute the concentrations of all solutes dissolved in the reaction vessel. If any of these solutes are reactants, the rate of the reaction will decrease . 16.4 An increase in solid surface area would allow more gaseous components to react per unit time and thus would increase the reaction rate. 16.5 An increase in temperature affects the rate of a reaction by increasing the number of collisions, but more importantly the energy of collisions increases. As the energy of collisions increase, more collisions result in reaction (i.e., reactants products), so the rate of reaction increases. 16.6 The second experiment proceeds at the higher rate. I 2 in the gaseous state would experience more collisions with gaseous H 2 . 16.7 The reaction rate is the change in the concentration of reactants or products per unit time. Reaction rates change with time because reactant concentrations decrease, while product concentrations increase with time. 16.8 a) For most reactions, the rate of the reaction changes as a reaction progresses. The instantaneous rate is the rate at one point, or instant, during the reaction. The average rate is the average of the instantaneous rates over a period of time. On a graph of reactant concentration versus time of reaction, the instantaneous rate is the slope of the tangent to the curve at any one point. The average rate is the slope of the line connecting two points on the curve. The closer together the two points (shorter the time interval), the more closely the average rate agrees with the instantaneous rate. b) The initial rate is the instantaneous rate at the point on the graph where time = 0, that is when reactants are mixed. 16.9 The calculation of the overall rate is the difference between the forward and reverse rates. This complication may be avoided by measuring the initial rate, where product concentrations are negligible, so the reverse rate is negligible. Additionally, the calculations are simplified as the reactant concentrations can easily be determined from the volumes and concentrations of the solutions mixed.
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16-2 16.10 At time t = 0, no product has formed, so the B( g ) curve must start at the origin. Reactant concentration (A( g )) decreases with time; product concentration (B( g )) increases with time. Many correct graphs can be drawn. Two examples are shown below. The graph on the left shows a reaction that proceeds nearly to completion, i.e., [products] >> [reactants] at the end of the reaction. The graph on the right shows a reaction that does not proceed
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This note was uploaded on 01/18/2009 for the course CHEM 1C taught by Professor Roland during the Spring '08 term at UCSC.

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