11.2 Collision Theory and Transition State Theory

11.2 COLLIS ION THEORY AND 11.2 COLLIS ION THEORY AND

TRANSITION STATE THEORYTRANSITION STATE THEORY

CH EM I S T RY U N I TCH EM I S T RY U N I T

S K 0 2 7S K 0 2 7

OBJECTIVES:OBJECTIVES:

CHAPTER 11 : REACT ION K INET ICSCHAPTER 11 : REACT ION K INET ICS

11 . 2 COLL I S I ON THEORY AND TRANS I T I ON S TATE THEORY11 . 2 COLL I S I ON THEORY AND TRANS I T I ON S TATE THEORY

1.Collision theory.

Explain the requirements for effective collision:

i. minimum energy

ii. Correct orientation

S K 0 2 7 C H EM I S T RYS K 0 2 7 C H EM I S T RY

ii. Correct orientation

2. Transition state theory

i. Define activation energy of a reaction with

reference to the energy profile diagram.

ii. Define activated complex.

iii. State the characteristics of an activated

complex.



o Collision Theory is the theory to explain the rate of chemical reactions. It is based on;

1- molecule must collide to react

Collision Theory

1- molecule must collide to react

2- molecules must possess a certain minimum

kinetic energy (activation energy) to initiate the

chemical reaction.

3- molecule must collide in the right orientation in

order that the collisions will result in a reaction.




o Only effective collisions cause formation of product that is collisions of molecules with Ea and at correct orientation.

Collision Theory

orientation.

Rate ∝ Number of effective collisionstime




Activation Energy

The activation energy (Ea) is the min. kinetic energy that

molecules must posses in order for a chemical reaction to occur.

The Ea is shown on reaction profiles diagrams, illustrate the role

of the Ea as an energy barrier that must be overcome by the

reactants before they can form products. reactants before they can form products.

R

P

Ea

∆H = -ve

Potential energy

Progress of reaction

Exothermic reaction

R

PEa

∆H = +ve

Potential energy

Progress of reaction

Endothermic reaction



Activation Energy

BA C

BA C

BA C

BA CBA C

BA C

Two slow-moving molecules collide

They just bounce off

Chemically unchanged

(ineffective collision)

Two fast-moving molecules collide

Lead to bond making and bond breaking

New product formed.

(effective collision)

BA C

BA C



B

A B

A B

A

B

A

Orientation factor/Stearic factor : is how reacting molecules

are oriented relative to each other.

Collision that cannot produce

chemical change

(ineffective collision)

Collision that lead a reaction

(Effective collision)

B

A

B

A

A AB B

B

A B

A

BA B A



Transition State Theory

o The transition state theory describes what

happens to the reactant molecules prior to

their change into products. their change into products.

o An intermediate stage lies between the

reactants and the products is transition state.

o The transitional species with partial bonds is

activated complex.




2NOCl → 2NO + Cl2 ∆H =+ve

A Reaction profile

Ea reverse

NO-Cl + Cl-NO

∆HActivated complex

[NO---Cl---Cl---NO]

Ea reverse

Ea forward

2NO + Cl2



A Reaction Profile

CO(g) + NO2(g) → CO2(g) + NO(g) ∆H = -ve

CO + NO2

Activated complex

[CO---O---NO]

CO2 + NO



• Very unstable, It has a short half-life (exists

at the instant the reacting system has the

highest potential energy.

• Its potential energy is greater than reactants

Characteristics of an Activated Complex

• Its potential energy is greater than reactants

or products.

• In an activated complex, the bonds in the

reactant molecules are in the process of

breaking while the new bonds in the product

molecules are starting to form.




• The activated complex and the reactants are in

chemical equilibrium.

Characteristics of an Activated Complex

• It decomposes to form products or reactants.




1. Define the term activation energy

2. Draw a potential energy diagram for an

endothermic reaction. Indicate on the

drawing

Example:

drawing

i. the potential energy of reactants

ii. the potential energy of the products

iii. the energies of activation for the

forward and reverse reactions

iv. the heat of reaction.




Solution:

…..is the minimum

energy is required

to initiate the

The activation energy (Ea)

to initiate the

chemical reaction

and produce the

product. It must be

supplied by

collisions.



Exercise:

1. For the reaction A + B → C + D

the enthalpy change of the forward reaction is the enthalpy change of the forward reaction is + 21 kJ/mol. The activation energy of the forward reaction is 84 kJ/mol.

a) What is activation energy of the reverse

reaction?

b) Sketch the reaction profile of this reaction


11.2 Collision Theory and Transition State Theory

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i s t ry chapter

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reaction kinetics1

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