matriculation chemistry ( reaction kinetics ) part 4

8/14/2019 Matriculation Chemistry ( Reaction Kinetics ) part 4

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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


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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


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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 E

aand at correct orientation.

The activation energy (Ea) is the minimum energy that

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


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..is the minimumenergy is required

to initiate the

chemical reaction.

The activation energy (Ea)


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Importance of Orientation

Orientationis unimportant Orientationis important


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Importance of Orientation

Orientationis important


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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


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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- H for the reaction.

- Reactant molecules must

have enough energy to

overcome an energy barrierseparating products from

reactants, Ea.

H

Ea

Progress of reaction

Potential energy


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A Reaction Profile: exothermic reaction

Ea

H

Ea (reverse reaction)

Activated complex

Transition state

(Forward reaction)

CO(g) + NO2(g) CO

2(g) + NO(g)


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A Reaction profile: endothermic reaction

Ea

2NOCl 2NO + Cl2

H


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Example:

1. For the reaction A + B C + D , the enthalphy change ofthe forward reaction is + 21 kJ/mol. The activation energy ofthe 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 exorthermic reaction.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

matriculation chemistry ( reaction kinetics ) part 4

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