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Chemical Kinetics

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Chemical Kinetics Chapter 16 Kinetics Reaction Rates Factors affecting rate Quantitative rate expressions Determination Factors Models for Rates Reaction Mechanisms ... – PowerPoint PPT presentation

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Title: Chemical Kinetics


1
Chemical Kinetics
  • Chapter 16

2
Kinetics
  • Reaction Rates
  • Factors affecting rate
  • Quantitative rate expressions
  • Determination
  • Factors
  • Models for Rates
  • Reaction Mechanisms
  • Effects of catalysts

3
Rates
  • Change in concentration of a reactant or product
    per unit time

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Factors affecting rates
  • Nature of the reactants
  • State of subdivision/surface area
  • Concentration
  • Temperature
  • Catalysts

7
Reactants
  • Complexity
  • Bond strengths
  • Etc.

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Concentrations as functions of time
16
Graph Concentration vs. time
17
Average Rate
  • Change of concentration in a time interval

18
Average Rate
Slope of line between two points on the graph
19
Instantaneous rate
  • Slope of tangent line at a point on the graph

20
Instantaneous Rate
0.009 M
375 s
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Initial Rate (t 0)
23
Initial rate
  • Slope of tangent line at time 0 (y intercept)

24
Rate Laws
  • k rate constant
  • m, n order

rate kNO2n
25
Introduction to Rate Laws
  • Reversible chemical reactions
  • Forward
  • Backward
  • Equilibrium

26
Introduction
  • Dominant Reaction
  • Rate Law
  • k, k specific rate constant
  • n order of reactant
  • can be zero, fractional, or negative

27
Method of Initial Rates
  • Unknown k, m, n
  • Initial rate instantaneous rate just after
    reaction is initiated

28
Initial Rates, NO2 decomposition
29
Order of Reaction
  • General
  • Substituting
  • Solution

30
Rate constant
  • Rate 1
  • 7.1 x 10-5 M s-1 -k0.01 M2
  • k 0.71 M-1 s-1
  • Rate 2
  • 2.8 x 10-4 M s-1 -k0.02 M2
  • k 0.70 M-1 s-1

31
You try
32
O2 2 NO ? 2NO2
33
Overall Order
  • Sum 1 2 3
  • 6
  • Overall order of reaction 6

34
Types
  • Differential
  • Rate dependence on concentration
  • Integrated
  • Concentration dependence on time

35
First Order Reactions
  • For aA ? products
  • Differential
  • Integrated

36
Half-life, first order reactions
  • Integrated law
  • Half-life
  • Half of initial reacted
  • At ½A0
  • Independent of A0

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Second Order Reactions
  • For aA ? products
  • Differential
  • Integrated

40
Half-life, second order reactions
  • Integrated law
  • Half-life
  • Half of initial reacted
  • At ½A0
  • Inversely proportional to A0

41
Zero Order Reactions
  • For aA ? products
  • Differential
  • Integrated

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Graphical Method
  • First order
  • Second order
  • Zero order
  • Straight line

44
First order
lnA0
slope -k
lnA
  • Plot
  • lnA vs. time

time
45
Second order
slope k
1 A
  • Plot
  • 1 vs. time
  • A

1 Ao
time
46
Zero order
A0
slope -k
A
  • Plot
  • A vs. time

time
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Summary
  • Conditions set so dominant forward reaction
  • Differential Rate Laws
  • rate as a function of concentration
  • method of initial rates
  • Integrated Rate Laws
  • concentration as a function of time
  • graphical method
  • Experimental data collection
  • Rate law types can be interconverted

50
Reaction Mechanism
  • Chemical equation Summary
  • Mechanism Series of elementary steps
  • Elementary Steps Reactions with rate laws
  • from molecularity
  • Molecularity Number of species that must
  • collide to produce reaction

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Reaction Mechanism
  • Proposed elementary steps must satisfy
    conditions
  • reasonable reactions
  • sum of steps overall balanced reaction
  • mechanism rate law experimental rate law

53
Intermediates
  • appear in steps
  • produced in one step
  • used in subsequent
  • not in overall equation

54
Rate-determining step
  • In a multi-step process
  • SLOWEST step
  • Determines overall reaction rate
  • Bottleneck

55
Model for Kinetics
  • Collision Theory
  • rate determined by particle collisions
  • collision frequency and energy
  • Transition State Theory
  • how reactants convert to products

56
Collision Theory (Bimolecular Collsions)
  • Z no. of bimolecular collisions per second
  • fa fraction with Ea
  • P fraction with correct orientation
  • Ea activation energy

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Arrhenius Equation
  • k rate constant
  • Ea activation energy (minimum required)
  • T absolute temperature
  • R universal gas constant
  • A orientation factor
  • Energy orientation requirements for reaction

64
Hydrolysis of an ester
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Transition State Theory
  • Ea and internal energy
  • Bonds breaking and forming
  • Atoms rearranging
  • Transition State
  • Unstable intermediate
  • At point of highest energy

69
forward reaction
reverse reaction
70
exothermic reaction
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I- CH3Cl ? Cl- CH3I
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Catalysts
  • Speed reaction
  • Are not consumed
  • Alternative pathway for reaction with lower Ea
  • Types
  • Homogeneous
  • Heterogeneous
  • Enzymes are biological catalysts

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Adsorption, activation, reaction, desorption
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