Chapter 14 - Full Notes

Chapter 14 - Full Notes - CHEM 1AA3: Intro. Chemistry II...

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Department of Department of 1 Chemistry Chemistry CHEM 1AA3: Intro. Chemistry II Chapter 14: Chemical Kinetics N.B. - equations in the slides are identified by number (in bold). If the same equation appears in the text, the text's equation number is also given (e.g., 14.1). Chem 1AA3 Chem 1AA3 2 Preface The 3 big questions in any chemical reaction: (1) What are the products? (2) What is the equilibrium position? A G H J ? ? ?
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Chem 1AA3 3 Preface (3) How fast is the reaction? Chem 1AA3 Chem 1AA3 4 Preface Why we measure rates: (1) predict/control reactions: - industrial syntheses - environmental reactions (e.g., smog formation, ozone layer breakdown) (2) monitor biological or chemical systems: - clinical diagnostics (e.g., liver damage) - polymerization (e.g. strength of plastics)
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Chem 1AA3 5 Preface (3) understand reaction mechanisms: - reaction order (e.g., S N 1 vs S N 2) - structure/function relationships (e.g. rate vs. leaving group ability in S N 2) (4) tell the temperature: - count rate of crickets chirping: T (°C) = 4 3 s 25 in chirps # + Chem 1AA3 Chem 1AA3 6 14-1. The rate of a chemical reaction rate = change in concentration of reactants and products over time •e . g . A G( 1 ) • 1 min after starting reaction, [G] = 1 M • What is the rate of the reaction over 1 min?
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Chem 1AA3 7 14-1. The rate of a chemical reaction average rate = 1 M / 1 min. time (min) 0.0 0.5 1.0 1.5 2.0 [G] (M) 0.0 0.5 1.0 1.5 2.0 t = 1 min [G] = 1 M A G Chem 1AA3 Chem 1AA3 8 14-1. The rate of a chemical reaction ( 2 ) • rates are always positive put a negative sign in front of [A]: ( 3 ) • stoichiometry is important: A 2G ( 4 ) ( 5 ) t [G] rate average = t [A] - rate average = t [G] 2 1 t [A] - rate average = =
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Chem 1AA3 9 14-1. The rate of a chemical reaction • In general: a A + b B g G + h H( 6 , 14.5) ( 7 , 14.2) Key concepts: - rate is change in concentration over time - rates are always positive - stoichiometry matters t [H] 1 t [G] 1 t [B] 1 - t [A] 1 - rate average = = = = h g b a Chem 1AA3 Chem 1AA3 10 14-2. Measuring reaction rates • The food colouring expt showed that rate can change during a reaction. • Many reactions could give an average rate of 1 M/min for 1 min. time (min) 0.0 0.5 1.0 1.5 2.0 [G] (M) 0.0 0.5 1.0 1.5 2.0 t = 1 min [G] = 1 M
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Chem 1AA3 11 • average rate = 1 M/min over 1 min for all 3 curves • average rate provides limited information 14-2. Measuring reaction rates time (min) 0 . 00 . 51 . 01 . 52 . 0 [G] (M) 0.0 0.5 1.0 1.5 2.0 Chem 1AA3 Chem 1AA3 12 instantaneous rate ( v ) : – the exact reaction rate at a defined time v = tangent to the line when plotting [G] vs. time. 14-2. Measuring reaction rates time (min) 0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 v = 0.69 M/min @ t = 1 min
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Chem 1AA3 13 14-2. Measuring reaction rates • measuring [A] or [G] over a finite time interval, t, gives the average rate • we want the instantaneous rate, v , the rate over an infinitely short time interval • taking an infinitely short time interval is the same as differentiating an equation in calculus (N.B. - " v " is an italicized letter vee (not the Greek letter nu). " v " comes from velocity , the other common term for rate. The textbook just writes out "rate of reaction" every time.)
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Chapter 14 - Full Notes - CHEM 1AA3: Intro. Chemistry II...

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