Chapter13_LEC - Chapter 13 Chemical Kinetics Kinetics study...

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Chapter 13 Chemical Kinetics
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Tro, Chemistry: A Molecular Approach 2 study of the factors that affect the speed & mechanism of the rxn. 4 factors that influence the speed of a reaction: nature of the reactants, temperature, catalysts, concentration Kinetics
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Tro, Chemistry: A Molecular Approach 3 Defining Rate rate change in quantity in a given period of time rate of your car has - mi/hr - ve sign - concentration of the reactants decreases. at some time the reaction stops, - reactants run out / system has reached equilibrium . Rate can be monitored by spectrophotometry time distance Speed = time [reactant] time [product] Rate time ion concentrat Rate - = = = AADZZRX0.swf
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Tro, Chemistry: A Molecular Approach 4 Reaction Rates The average rate - change in concentration divided by the change in time Rate=Con@t2-con@t1 / t2-t1 0.0905-0.1000/50 =-1.9x10 -4 Average rate = Note that the average rate decreases as the reaction proceeds. This is because as the reaction goes forward , there are fewer reactant molecules
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Tro, Chemistry: A Molecular Approach 5 Reaction Rates Conc. vs. t yields a curve like this. slope of a line tangent to the curve at any point is the instantaneous rate at that time .
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Tro, Chemistry: A Molecular Approach 6 All reactions slow down over time . Therefore, the best indicator of the rate of a reaction is the instantaneous rate near the beginning.
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Tro, Chemistry: A Molecular Approach 7 Reaction Rates & Stoichiometry In this reaction, the ratio of C 4 H 9 Cl to C 4 H 9 OH is 1:1. Thus, the rate of disappearance of C 4 H 9 Cl = rate of appearance of C 4 H 9 OH . Rate = =
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Tro, Chemistry: A Molecular Approach 8 What if the ratio is not 1:1 ? the change in the concentration of each substance is multiplied by 1/coefficient Therefore , Rate = 1 2 [HI] t =
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Tro, Chemistry: A Molecular Approach 9 To generalize, then, for the reaction a A + b B c C + d D Rate = 1 a [A] t = 1 b [B] t = 1 c [C] t 1 d [D] t =
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10 H 2 ( g ) + I 2 ( g ) 2 HI ( g ) Using [H 2 ], the instantaneous rate at 50 s is: s M 0.0070 Rate s 40 M 28 . 0 Rate = - - = Using [HI], the instantaneous rate at 50 s is: s M 0.0070 Rate s 40 M 56 . 0 2 1 Rate = =
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Tro, Chemistry: A Molecular Approach 11 Factors Affecting Reaction Rate Nature of the Reactants physical condition of the molecules small molecules - react faster than large molecules; gases - react faster than liquids /faster than solids; powdered solids -more reactive than “blocks” more surface area for contact with other reactants certain types of chemicals are more reactive than others e.g., the activity series of metals ions react faster than molecules - no bonds need to be broken
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Tro, Chemistry: A Molecular Approach 12 Temperature and Rate Generally, as temperature increases , so does the reaction rate.
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Chapter13_LEC - Chapter 13 Chemical Kinetics Kinetics study...

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