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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
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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 FeikeDijkstra (Series 2) Weeks 8 – 13 Office Hours: Monday 2-3, Friday 1-2 Room: 543a e-mail:adam.bridgeman@sydney.edu.au e-mail:feike.dijkstra@sydney.edu.au
Summary of Last Lecture • 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
Chemical Kinetics II • 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
For the 1st order reaction A B, the rate law is [A] ln = -kt [A]0 concentration at start (t = 0) Concentration - Time Relationships -d[A] = k[A] rate = dt • To find how [A] varies with time, this is integrated:
The half life of a reaction is the time required for the concentration to fall to half its initial value. [A] 1 = 2 [A]0 [A] ln = -kt [A]0 Half Life (t1/2) • For a first order reaction, • so t1/2 = ln 2 / k
Half Life 2 N2O5 = 4NO2 + O2 t1/2 = 24 min constant – 1st order
Molecules must collide to react collision frequency increases with temperature molecules must be correctly orientated molecules must have sufficient energy to react Collision theory • Molecules must collide to react • collision frequency increases with temperature • molecules must be correctly orientated • molecules must have sufficient energy to react • The minimum energy that molecules must have to react is called the activation energy(Ea)
Energy in Chemical Reactions transition state (highest energy point) Ea forward DH (forward) (exothermic)
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.
Multistep Reactions • NO + Br2 NOBr2 fast equilibrium • NOBr2 + NO 2NOBr slow Ea (1) < Ea(2) so step 2is rate determining
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 Arrhenius Equation
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
Catalysts • A catalyst increases the rate of a chemical reaction without itself being changed • A catalyst provides an alternative reactionpathway of lower activation energy with catalyst
Catalysts • Do not effect how favourable reaction is • Do not effect the position of the equilibrium • Do not change Keq • Do effect how fast the reaction is with catalyst
B A Enzyme = Biological Catalyst • 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 A catalytic surface
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...