Phyprn7 Kinetics Part1 and 2

8/3/2019 Phyprn7 Kinetics Part1 and 2

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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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Chemical Kinetics

HOW RAPIDLY AND BY WHAT MECHANISM DOES A

REACTION TAKE PLACE?

-depends on

a. Nature of the reactantsb. Concentration of reactantsc. Temperature


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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Chemical Kinetics

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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


Chemical Kinetics

THE LAW OF MASS ACTION

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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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Chemical Kinetics

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THE LAW OF MASS ACTION rate is proportional to the

concentration of the reactants


rate = k [A]x [B]y [C]z

Rate Equation or

Rate law


Chemical Kinetics

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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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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.


Chemical Kinetics

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RATE LAW

Determination of rate law for a reactioncan be done by carrying out a series of

EXPERIMENT



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Chemical Kinetics

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RATE LAW


Chemical Kinetics

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RATE LAW

Integrated Rate Law


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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


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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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.


Chemical Kinetics

Determination of Reaction OrderUsing Integrated Rate Laws


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Chemical Kinetics

Determination of Reaction OrderUsing Integrated Rate Laws

What is the order of the reaction?


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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Chemical Kinetics

Determination of Reaction Order Using the Half-Life

Zero Order

First Order

Second Order

Half-life


Chemical Kinetics


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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


Chemical Kinetics


8:29 PM 22


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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


Chemical Kinetics


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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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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


The Collision Theory


Chemical Kinetics



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Chemical Kinetics


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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

Phyprn7 Kinetics Part1 and 2

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