Chemistry 12
Potential Energy Diagrams
Mrs. Purba
Transition State Theory
Explains what happens once the colliding particles
react...reactants become products
What is the transition or change from reactants to
products?
Kinetic energy (KE) of the reactants is transferred to
potential energy (PE) as the reactants collide (law of
conservation of energy).
Example: Bouncing Ball
KE is converted to PE which is stored in the ball as
it strikes the floor.
The PE is then converted to KE as the ball bounces
away from the floor.
Potential Energy Diagram:
Charts the potential energy of a reaction against the
progress of the reaction.
x-axis → time or progress of the reaction
y-axis → potential energy
“hill” or “bump” represents the activation barrier of
the reaction
Slow reaction, big “bump”…high activation barrier
Fast reaction, small “bump” …low activation
barrier.
Potential Energy Diagram (Ep) for an
Exothermic Reaction:
Activated Complex
Is the short-lived, unstable structure formed during a
successful collision between reactant particles.
In the activated complex, old bonds of the reactants are in
the process of breaking, and new bonds of the products
are in the process of being formed.
This structure breaks up to form product particles.
The Ea is the minimum energy required for a reaction
(successful collision) to occur, and is the energy required to
form the activated complex.
Potential Energy Diagram for an
Endothermic Reaction
Potential Energy Diagram (Ep) for a Fast
Endothermic & Exothermic Reaction
Note:
The small activation energy shown in the
diagrams for fast exothermic and fast
endothermic reactions.
With a small activation energy, more collisions
will have the required energy for a successful
collision and products will form at a faster
rate.
Problem 1:
The following reaction has an activation energy of 120
kJ and a ∆H of +113 kJ.
2NO2(g) → 2NO(g) + O2(g)
i) Draw and label a potential energy (activation
energy) diagram for this reaction.
ii) Calculate the activation energy for the reverse
reaction.
Problem 2:
The following hypothetical reaction has an activation
energy of 70 kJ and a ∆H of –130 kJ.
A+B→C+D
i) Draw and label a potential energy diagram for this
reaction.
ii) Calculate the activation energy for the reverse
reaction
Problem 3:
Analyse the activation energy
diagram
shown above right, for the
hypothetical
reaction:
2X + Y → W + 2Z
Answer the following questions:
i) What is the activation energy for
the forward reaction? The
reverse reaction?
ii) What is the value for ∆H for the
forward reaction?
You try:
1. The following hypothetical reaction has an
activation energy of 120 kJ and a ∆H of 80 kJ.
2A + B → 2C + D
i) Draw and label a potential energy diagram for this
reaction.
ii) Calculate the activation energy for the reverse
reaction.
2. Analyse the activation
energy diagram shown
below for the hypothetical
reaction:
E + 2F → G + H
i) What is the activation
energy for the forward
reaction? The reverse
reaction?
ii) What is the value of ∆H for
the forward reaction? The
reverse reaction?
iii) What is the energy of the
activated complex?
Textbook
Problems on page 476, #1- 4