7/27/2019 Lec. 5-Collision Theory

1/13

11.2 Collision Theory

At the end of the lesson the students should be

able to:1. explain reaction rates in terms of collision

theory.

2. identify factors affecting the effectiveness of

collision.

3. define activation energy.

4. Define and state the characteristics of an

activated complex

Objectives

7/27/2019 Lec. 5-Collision Theory

2/13

Collision Theory is the theory to explain the rate of

chemical reactions. It is based on;

1- molecule must collide to react2- molecules must possess a certain minimum kinetic energy

(activation energy) to initiate the chemical reaction.

Collision Theory

Rate Number of effective collisions

time

7/27/2019 Lec. 5-Collision Theory

3/13

3- molecule must collide in the right orientation in order

for the reaction to occur.

Only effective collisions cause formation of product;collisions of molecules with Ea and at correct orientation.

The activation energy (Ea) is the minimum energy that

must be supplied or required by collisions for a reaction tooccur.

7/27/2019 Lec. 5-Collision Theory

4/13

..is the minimum

energy is required

to initiate the

chemical reaction.

The activation energy (Ea)

7/27/2019 Lec. 5-Collision Theory

5/13

Importance of Orientation

Orientation is unimportant Orientation is important

7/27/2019 Lec. 5-Collision Theory

6/13

7/27/2019 Lec. 5-Collision Theory

7/13

Transition State Theory

The configuration of the atoms of the

colliding species at the time of the collision is

called the transition state.

Species formed at transition state is called

activated complex.

7/27/2019 Lec. 5-Collision Theory

8/13

Very unstable i.e. It has a short half-life.

Its potential energy is greater than reactants or

products.

The activated complex and the reactants are in

chemical equilibrium. It decomposes to form products or reactants.

Characteristics of Activated Complex

7/27/2019 Lec. 5-Collision Theory

9/13

Reactant

product

- A reaction profile shows

potential energy plotted as afunction of the progress of the

reaction.

- The difference in potential

energies between the

products and the reactants is -DH for the reaction.

- Reactant molecules must

have enough energy to

overcome an energy barrierseparating products from

reactants, Ea.

DH

Ea

Progress of reaction

Potential energy

7/27/2019 Lec. 5-Collision Theory

10/13

A Reaction Profile: exothermic reaction

Ea

DH

Ea (reverse reaction)

Activated complex

Transition state

(Forward reaction)

CO(g) + NO2(g) CO

2(g) + NO(g)

7/27/2019 Lec. 5-Collision Theory

11/13

Product

Reactant

activated complex.

Ea

(reverse reaction)

Ea

(forward reaction)

DH

A Reaction Profile for endothermic process

7/27/2019 Lec. 5-Collision Theory

12/13

A Reaction profile: endothermic reaction

Ea

2NOCl 2NO + Cl2

DH

7/27/2019 Lec. 5-Collision Theory

13/13

Example:

1. For the reaction A + B C + D , the enthalphychange of the forward reaction is + 21 kJ/mol. Theactivation 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

2. Draw a potential energy diagram for an exorthermicreaction. Indicate on the drawing:

a) Potential energy of the reactants and the products

b) The activation energy for the forward and the reversereaction

c) The heat of the reaction