Chapter 14 Chemical Kinetics

# Chapter 14 Chemical Kinetics - CHEM 1AA3: Intro. Chemistry...

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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 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 ? ? ? Chem 1AA3 Chem 1AA3 3 Preface (3) How fast is the reaction? 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) Chem 1AA3 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 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 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 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 = = Chem 1AA3 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 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 0.0 0.5 1.0 1.5 2.0 t = 1 min [G] = 1 M Chem 1AA3 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) 0.0 0.5 1.0 1.5 2.0 0.0 0.5 1.0 1.5 2.0 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
Chem 1AA3 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.) Chem 1AA3 14 14-2.

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## This note was uploaded on 05/16/2011 for the course CHEM 1a03 taught by Professor Landry during the Winter '08 term at McMaster University.

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Chapter 14 Chemical Kinetics - CHEM 1AA3: Intro. Chemistry...

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