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8/3/2019 Phyprn7 Kinetics Part1 and 2
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8/21/20
Dr. Cynthia B. FabianDepartment of Chemical Engineering
College of Engineering
August 15-17, 2011
PHYSICAL PRINCIPLES 2
PHYPRN2
Physical Principles 2
Chemical Kinetics
TopicsChemical Kinetics
A. Rate of ReactionB. Order and MolecularityC. Empirical Rate EquationsD. Analysis of Kinetic Results
1. Integral Method2. Half-life3. Differential Method
E. Effect of Temperature on Reaction Rates1. The Arrhenius Equation2. Collision theory
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Physical Principles 2
Chemical Kinetics
HOW RAPIDLY AND BY WHAT MECHANISM DOES A
REACTION TAKE PLACE?
-depends on
a. Nature of the reactantsb. Concentration of reactantsc. Temperature
Physical Principles 2
Chemical Kinetics
The reaction rate can be defined either as the increasein the concentration of a product per unit time or as the
decrease in the concentration of a reactant per unit time.
4
aA +bB cC +dD
Reaction Rates
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Physical Principles 2
Chemical Kinetics
5
THE LAW OF MASS ACTION
The rate of anyreaction is at eachinstant proportionalto the concentrationraised to a powerequal to the numberof molecules ofeach species
participating in theprocess.
aA +bB +cC mM +nN +oO
rate = k [A]a [B]b [C]c
Physical Principles 2
Chemical Kinetics
THE LAW OF MASS ACTION
6
aA +bB +cC mM +nN +oO
r ate = k [A]a [B]b [C]c1. really applies only to each of the individual
intermediate steps in a chemical reaction
2. actual overall rate expression for a givenchemical reaction can be determined reliably
only by experiment and often is a verycomplicated equation, appearing to bear no
relation to the overall chemical equation
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Physical Principles 2
Chemical Kinetics
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THE LAW OF MASS ACTION rate is proportional to the
concentration of the reactants
aA +bB +cC mM +nN +oO
rate = k [A]x [B]y [C]z
Rate Equation or
Rate law
Physical Principles 2
Chemical Kinetics
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aA +bB +cC mM +nN +oO
rate = k [A]x [B]y [C]z
RATE LAW
The values of the exponentsx, y and z determine the reaction order
with respect to A, B and C, respectively. The sum of the exponents
defines theoverallreaction order.
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Physical Principles 2
Chemical Kinetics
Reaction order and Molecularity
Molecularity of any reaction depends on whetherone, two, or more molecules are involved in therate determining steps.
Unimolecular: A productsBimolecular: 2A products
A + B productsTermolecular: A + 2 B products
Reaction order shows the dependence of anobserved rate on the concentration of thereactants.
Physical Principles 2
Chemical Kinetics
10
RATE LAW
Determination of rate law for a reactioncan be done by carrying out a series of
EXPERIMENT
rate = k [A]x [B]y [C]z
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Physical Principles 2
Chemical Kinetics
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RATE LAW
Physical Principles 2
Chemical Kinetics
12
RATE LAW
Integrated Rate Law
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Physical Principles 2
Chemical Kinetics
Integrated Rate LawA products
Zero-Order Reactions
In the rare case that a reaction of a single reactant is zero order (the rateis independent of the concentration of the reactant), the rate law for theforward reaction is
Physical Principles 2
Chemical Kinetics
Integrated Rate Law
A productsFirst-Order Reactionsis one whose rate depends on the concentration of a single
reactant raised to the first power.
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Physical Principles 2
Chemical Kinetics
Integrated Rate LawA products
Second-Order Reactions
is one whose rate depends on the concentration of a singlereactant raised to the second power or on the concentrations of twodifferent reactants, each raised to the first power.
Physical Principles 2
Chemical Kinetics
Determination of Reaction OrderUsing Integrated Rate Laws
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Physical Principles 2
Chemical Kinetics
Determination of Reaction OrderUsing Integrated Rate Laws
What is the order of the reaction?
Physical Principles 2
Chemical Kinetics
R = 0.895
0.00E+00
1.00E-03
2.00E-03
3.00E-03
4.00E-03
5.00E-03
6.00E-03
7.00E-03
8.00E-03
9.00E-03
0 100 200 300 400 500 600
[A]
t
R = 0.973
-7
-6
-5
-4
-3
-2
-1
0
0 100 200 300 400 500 600
ln [A]
t
y = 0.540x + 124.9
R = 1
0.00E+00
5.00E+01
1.00E+02
1.50E+02
2.00E+02
2.50E+02
3.00E+02
3.50E+02
4.00E+02
4.50E+02
0 100 200 300 400 500 600
1/[A]
t
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Physical Principles 2
Chemical Kinetics
Determination of Reaction Order Using the Half-Life
Zero Order
First Order
Second Order
Half-life
Physical Principles 2
Chemical Kinetics
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Physical Principles 2
Chemical Kinetics
Reaction Rates and Temperatures
Integrated with Ea is a constant
Integrated with Ea is a constant
between the limits k = k1 and T= T1and k= k2 and T= T2
Physical Principles 2
Chemical Kinetics
Reaction Rates and Temperatures
8:29 PM 22
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Physical Principles 2
Chemical Kinetics
Reaction Rates and Temperatures
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rate =k [A]x [B]y [C]z
k= A e-Ea/RT
ARRHENIUS EQUATION
A is the frequency factor (or pre-exponential factor)
Ea is the activation energyR is the ideal gas constant
T is temperature in K
Physical Principles 2
Chemical Kinetics
Reaction Rates and Temperatures
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Molecules must be activated before they can react on coll ision
The energy that reactants must absorb in order to become activated andreacts is the energy of activation Ea
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Physical Principles 2
Chemical Kinetics
The fraction of collisions with an energy equal to or greater than theactivation energy, f
The fraction of collisions having proper orientation for conversion of reactantsto products is called the steric factor, p.
For a bimolecular reaction,
Where Z is a constant related to the collision frequency
The reaction rate is
and the rate law says
Reaction Rates and Temperatures
The Collision Theory
Physical Principles 2
Chemical Kinetics
Reaction Rates and Temperatures
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Physical Principles 2
Chemical Kinetics
Reaction Rates and Temperatures
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rate =k [A]x [B]y [C]z
k= A e-Ea/RT
ARRHENIUS EQUATION
A is the frequency factor (or pre-exponential factor)
Ea is the activation energyR is the ideal gas constant
T is temperature in K