M2 Kinetics - S olomons Study Notes General Chemistry CHE...

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Unformatted text preview: S olomons Study Notes General Chemistry CHE 132 Spring 2009 Midterm 2 Solomon Weiskop PhD [ Kinetics ] These Study Notes cover the material of Ch. 13 Study Notes & Practice Problems are available to print out by registering at www.solomonlinetutor.com Solomon Weiskop PhD Solomons CHE 13 2 Tutoring Copyright 2009 1 1. INTRODUCTION In Thermodynamics we learnt that ? < 0 Spontaneous In other words, if G is negative the reaction goes. However, knowing that a reaction goes tells us nothing about how fast or how slow it goes. That is, it tells us nothing about the rate of the reaction. It turns out that the rate of a reaction is not determined by Thermodynamics but rather by Kinetics. Consider a generic reaction Reactants Products + ? ? + ? One normally begins a reaction with Reactants but no Products. Then, as the reaction goes, concentrations of Reactants decrease (as they react and thus get used up) ? < ? < and concentrations of Products increase (as they get produced) ? > ? > In Kinetics, we describe how fast or how slow these changes happen by giving the rate of change of concentration : for a Reactant: ? = ? ? for a Product: + ? = + ? ? ? = elapsed time 2 In chemistry, by convention all rates are made to be positive . So we stick in the extra ( ) for Reactants to cancel the minus sign that you get because ? ? < ? . Since ? ? > ? no extra minus sign is needed for Products. units : ? M= mole L ? ? M t = mole tL Note: t= some time unit (i.e. sec, msec, sec, min, hr etc.). There is no standard time unit in Kinetics. Thats because there is a wide range of time scales for chemical reactions. Some reactions are very fast and occur on a sec time scale (or even faster). Some reactions are very slow and occur on a much longer time scale, even 1000s of years. 3 2. RELATIVE RATES (governed by Stoichiometry) Consider the reaction ? ? + ?? ? ??? ? () As this reaction goes, the Reactants ( N 2 , H 2 ) react and so get used up (their concentrations decrease) while the Product ( NH 3 ) gets produced (its concentration increases). ? ? ? ? ?? ? What about their Relative Rates ? ? ? gets used up at a certain rate. ? ? also gets used up at a certain rate. Do they get used up at the same rate or at different rates? And what about ?? ? ? It is getting produced at a certain rate. How does this rate compare to the rate for ? ? or ? ? ?...
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This note was uploaded on 11/14/2010 for the course CHE 131 taught by Professor Kerber during the Spring '08 term at SUNY Stony Brook.

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M2 Kinetics - S olomons Study Notes General Chemistry CHE...

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