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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 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.
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..is the minimum
energy is required
to initiate the
chemical reaction.
The activation energy (Ea)
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Importance of Orientation
Orientation is unimportant Orientation is important
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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.
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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 -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
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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)
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Product
Reactant
activated complex.
Ea
(reverse reaction)
Ea
(forward reaction)
DH
A Reaction Profile for endothermic process
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A Reaction profile: endothermic reaction
Ea
2NOCl 2NO + Cl2
DH
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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
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