Ch14_Fall2011_fullsize-1

Ch14_Fall2011_fullsize-1 - Chapter 14 Chemical Kinetics...

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1 14.1 Factors That Affect Reaction Rates 14.2 Reaction Rates 14.3 Concentration and Rate 14.4 Change of Concentration with Time 14.5 Temperature and Rate 14.6 Reaction Mechanisms 14.7 Catalysis Chapter 14 Chemical Kinetics Mastering Chemistry for Ch. 14 due Wednesday, September 14 , 11:00 PM
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2 Important Questions/Goals • How fast do chemical reactions occur? Language Tool/skills Concepts • How can I control a chemical rate? • What factors affect reaction rates? • How does the reaction occur (what is the reaction mechanism)? • How should I think about chemical reactions?
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3 14.1 Factors that Affect Reaction Rates What factors affect the speed of a reaction? 1. Physical State eg. fine powder vs solid block vs gas vs liquid 2. Reactant Concentration eg. dilute vs concentrated 3. Temperature eg. hot vs cold 4. Catalysts eg. alcohol sensitivity related to aldehyde dehydrogenase These are macroscopic variables with submicroscopic causes. Next step: We need to be (1) quantitative and (2) to develop insight and understanding . Heated in air Heated in pure O 2
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4 14.2 Reaction Rates Reaction rate - change in the concentration of reactants or products per unit time. – Units are typically in M/s (molarity per second) • Consider the reaction A B average rate of appearance of B = B [ ] t average rate of disappearance of A =- A [ ] t
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5 Average rate for formation of products Concentration (c) Time (t) t c Average Rate= c/ t c/ t > 0 give a positive rate (increasing concentration with time) Formation of products
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6 t c Average Rate=- c/ t c/ t < 0 (concentration decreases with time) concentration time Loss of reactants Average rate for loss of reactants
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7 time = 0.0 s Example: Consider the hypothetical reaction 10.0 s 20.0 s 30.0 s Red 2Blue (each or 0.01 M) [ red ] = [ blue ] = 0.08 M 0.00 M 0.04 M 0.08 M 0.02 M 0.12 M 0.01 M 0.14 M
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8 Red 2Blue time (s) [ Red ] and [ Blue ] (M) 0.00 0.04 0.08 0.12 0.16 0.0 10.0 20.0 30.0 Graphical representation of the data
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9 0.0 s Consider the rate of the hypothetical reaction, 30.0 s Red 2Blue Significant Points 1. Rate is associated with the slope of concentration vs time plots. 2. Stoichiometry can influence the rate and must be accounted for in defining the rate of the reaction. 3. How does rate change with time ?
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Reaction Rate = the change in concentration per unit time taking into account stoichiometry and reaction direction. What precisely is the “ rate of a reaction”? [ ] = change in concentration t = change in time a, b, c, d = stoichiometric coefficients Note : •minus sign for reactants •plus sign for products so that the rate of the reaction is always positive. For
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Ch14_Fall2011_fullsize-1 - Chapter 14 Chemical Kinetics...

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