Chapter 14 - Kinetics Chapter 14 Chemical Kinetics Factors...

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1 Chapter 14 Chemical Kinetics Kinetics • In kinetics we study the rate at which a chemical process occurs. • Besides information about the speed at which reactions occur, kinetics also sheds light on the reaction mechanism (exactly how the reaction occurs). Factors That Affect Reaction Rates • Physical State of the Reactants – In order to react, molecules must come in contact with each other. – The more homogeneous the mixture of reactants, the faster the molecules can react. Factors That Affect Reaction Rates • Concentration of Reactants – As the concentration of reactants increases, so does the likelihood that reactant molecules will collide. Factors That Affect Reaction Rates • Temperature – At higher temperatures, reactant molecules have more kinetic energy, move faster, and collide more often and with greater energy. Factors That Affect Reaction Rates • Presence of a Catalyst – Catalysts speed up reactions by changing the mechanism of the reaction. – Catalysts are not consumed during the course of the reaction.
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2 Reaction Rates Rates of reactions can be determined by monitoring the change in concentration of either reactants or products as a function of time. Reaction Rates In this reaction, the concentration of butyl chloride, C 4 H 9 Cl, was measured at various times. C 4 H 9 Cl( aq ) + H 2 O( l ) → C 4 H 9 OH( aq ) + HCl( aq ) Reaction Rates The average rate of the reaction over each interval is the change in concentration divided by the change in time: C 4 H 9 Cl( aq ) + H 2 O( l ) → C 4 H 9 OH( aq ) + HCl( aq ) Average rate = [C 4 H 9 Cl] t Reaction Rates • Note that the average rate decreases as the reaction proceeds. • This is because as the reaction goes forward, there are fewer collisions between reactant molecules. C 4 H 9 Cl( aq ) + H 2 O( l ) → C 4 H 9 OH( aq ) + HCl( aq ) Reaction Rates • A plot of [C 4 H 9 Cl] vs. time for this reaction yields a curve like this. • The slope of a line tangent to the curve at any point is the instantaneous rate at that time. C 4 H 9 Cl( aq ) + H 2 O( l ) → C 4 H 9 OH( aq ) + HCl( aq ) Reaction Rates • All reactions slow down over time. • Therefore, the best indicator of the rate of a reaction is the instantaneous rate near the beginning of the reaction. C 4 H 9 Cl( aq ) + H 2 O( l ) → C 4 H 9 OH( aq ) + HCl( aq )
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3 Reaction Rates and 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 is the same as the rate of appearance of C 4 H 9 OH. C 4 H 9 Cl( aq ) + H 2 O( l ) → C 4 H 9 OH( aq ) + HCl( aq ) Rate = - [C 4 H 9 Cl] t = [C 4 H 9 OH] t Reaction Rates and Stoichiometry • What if the ratio is not 1:1? 2 HI(
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This note was uploaded on 12/07/2011 for the course CHEM 4:12 taught by Professor Larsen during the Fall '08 term at University of Iowa.

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Chapter 14 - Kinetics Chapter 14 Chemical Kinetics Factors...

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