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Unless otherwise stated, all images in this file have been reproduced from: Blackman, Bottle, Schmid, Mocerino and Wille, Chemistry, 2007 (John Wiley) ISBN: 9 78047081 0866
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Unless otherwise stated, all images in this file have been reproduced from: Blackman, Bottle, Schmid, Mocerino and Wille, Chemistry , 2007 (John Wiley) ISBN: 9 78047081 0866 . CHEM1002 [Part 2]. A/Prof Adam Bridgeman (Series 1) Dr Feike Dijkstra (Series 2) Weeks 8 – 13 - PowerPoint PPT Presentation
Transcript of Unless otherwise stated, all images in this file have been reproduced from:
Unless otherwise stated, all images in this file have been reproduced from:
Blackman, Bottle, Schmid, Mocerino and Wille, Chemistry, 2007 (John Wiley)
ISBN: 9 78047081 0866
Slide 2/17
e CHEM1002 [Part 2]
A/Prof Adam Bridgeman (Series 1)Dr Feike Dijkstra (Series 2)
Weeks 8 – 13
Office Hours: Monday 2-3, Friday 1-2Room: 543ae-mail: [email protected]: [email protected]
Slide 3/17
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Chemical Kinetics I
• The rate of change of concentration of a reactant or a product is the rate of the reaction divided by the corresponding stoichiometric coefficient in the chemical reaction
• The rate law shows how the rate of the reaction depends on the concentration of how reactant
• The order of the reaction with respect to each reactant is determined from experimental data
• The order of the reaction with respect to each reactant is not given by the corresponding stoichiometric coefficient in the chemical reaction
• The rate constant (including its units) is found from experimental data
Summary of Last Lecture
Slide 4/17
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Lecture 16• Chemical Kinetics• Rate of Reaction• Rate Laws• Reaction Order• Blackman Chapter 14, Sections 14.1 - 14.3
Lecture 17• Half lives• The Temperature Dependence of Reaction Rates• Catalysis• Blackman Chapter 14, Sections 14.4 - 14.6
Chemical Kinetics II
Slide 5/17
e Concentration - Time Relationships• For the 1st order reaction A B, the rate law is
dt= k[A]rate =
-d[A]
• To find how [A] varies with time, this is integrated:
[A]0
= -kt[A]
ln
concentration at start (t = 0)
Slide 6/17
e
• For a first order reaction, so
Half Life (t1/2)
• The half life of a reaction is the time required for the concentration to fall to half its initial value.
[A]0
= [A]
21
t1/2 = ln 2 / k [A]0
= -kt[A]
ln
Slide 7/17
e Half Life
2 N2O5 = 4NO2 + O2
t1/2 = 24 minconstant – 1st order
Slide 8/17
e Collision theory
• Molecules must collide to reactcollision frequency increases with temperaturemolecules must be correctly orientatedmolecules must have sufficient energy to react
• Molecules must collide to reactcollision frequency increases with temperaturemolecules must be correctly orientatedmolecules must have sufficient energy to react
• The minimum energy that molecules must have to react is called the activation energy (Ea)
Slide 9/17
e Energy in Chemical Reactions
Ea forward
DH (forward)
transition state(highest energy point)
(exothermic)
Slide 10/17
e Multistep Reactions
• Each elementary step in a reaction has a separate activation energy
• The step with the largest activation energy is the rate determining step.
Slide 11/17
e Multistep Reactions
NOBr2 + NO 2NOBr slowNO + Br2 NOBr2 fast equilibrium
energy,DH
reaction coordinate
2NO + Br2NOBr2 + NO
Ea (1) Ea (2)
transition state (1)
transition state (2)
DrH
2NOBr
Ea (1) < Ea(2) so step 2is rate determining
Slide 12/17
e Arrhenius Equation
k = Ae -Ea/RT
• Describes the temperature dependence of the rate
• Ea is the activation energy – the minimum amount of energy that the reacting molecules must possess for the reaction is to be successful
• A is the pre-exponential factor or the “A factor” – depends on the collision frequency and orientation factor
Slide 13/17
e Arrhenius Equation
k = Ae -Ea/RT
lnk = lnA – Ea/RT
• For a typical chemical reaction, k doubles for every 10 °C (10 K) increase in temperature
Slide 14/17
e Catalysts• A catalyst increases the rate of a chemical
reaction without itself being changed • A catalyst provides an alternative reaction pathway
of lower activation energy
with catalyst
Slide 15/17
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Do not effect the position of the equilibrium
with catalyst
Do not change Keq
Catalysts Do not effect how favourable reaction is
Do effect how fast the reaction is
Slide 16/17
e• The enzyme provides a surface for the reaction• This surface stabilizes the transition state, lowering the
activation energy• The enzyme helps transform the transition state to
product
B
BA catalytic surface
A
Enzyme = Biological Catalyst
Slide 17/17
e Summary: Chemical Kinetics II
Learning Outcomes - you should now be able to:
• Be able to perform calculations using half lives• Be able to draw reaction coordinate diagrams• Explain why reaction rate increase with
temperature• Explain what catalysts do and how they do it• Embarrass your lecturer with a very high mark in
the exam
Very Best of Luck...
