Unit 12: Kinetics,
Thermodynamics &
Equilibrium
Lesson 12.5: Potential
Energy Diagrams
Endothermic Reactions
General Formula:
A + B + Energy  C + D
a.
b.
c.
d.
The reactants absorb energy (heat)
This causes the temperature of the surroundings
to decrease.
decrease
The products have more energy than the reactants
(stored in their bonds).
The products are less stable than the reactants.
This is how explosives are made!
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A General Example
What Happens During a
Reaction?
For the reaction:
We know it involves reactants  products.
But what about energy?
A reaction can do one of two things with
energy:
1.
2.
96
HA = 40kJ
A+BC
HB = 20kJ
Hc = 110kJ
How much energy
gy is absorbed during
g this
reaction?
(110kJ – 60kJ)
What is ∆H?
(+ 50kJ)
Rewrite the equation to show the Conservation of
Energy:
A + B + 50kJ  C
Absorb it (endothermic)
Release it (exothermic)
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97
A Table I Example
A second visit to H
N2 (g) + O2 (g)  2NO (g)
H = enthalpy:
A measurement of the total amount of energy stored
in a substance (units = kJ/mole).
∆H = +182.6 kJ/mole
(+) ∆H = Endothermic Reaction.
Only changes in H ( ∆H), can be measured
This means that Nitrogen and Oxygen need to
absorb 182.6 kJ of energy per mole of NO that
will be formed.
For endothermic reactions ∆H is positive.
For exothermic reactions ∆H is negative.
NO is an unstable compound.
∆H values of many reactions are listed on Reference
Table I.
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98
1
Potential Energy Diagrams
Exothermic Reactions
A graph that shows what happens to potential energy as
a reaction occurs.
General Formula:
What has to happen for a reaction to occur?
A + B  C + D + energy
a.
b
b.
c.
d.
1.
The reactants release energy (heat)
This causes the temperature of the surroundings to
increase.
The products have less energy than the reactants
(stored in their bonds).
The products are more stable than the reactants.
2.
3.
The reactants have some amount of energy
stored in their bonds.
To get them to react, some energy is put in to the
reactants (the “activation energy”) to get them to
collide effectively. PE goes up until the reactants
become the “activated complex”.
Once the activated complex reacts, the PE
decreases to the energy of the products
This is what is left behind after an explosion!
99
A General Example
What goes in a PE diagram
For the reaction:
HA = 60kJ
102
A+BC
HB = 40kJ
1.
Hc = 30kJ
2.
How much energy is released during this
reaction?
30kJ – (60kJ + 40kJ)
What is ∆H?
- 70kJ
Rewrite the equation to show the
Conservation of Energy:
A + B  C + 70kJ
3.
4.
5.
6.
7.
Y-axis: PE (kJ)
X-axis: Reaction coordinate (unmeasured
time).
H of reactants
H of activated complex
H of products.
The Activation energy (arrow from H of
reactants to H of activated complex)
∆H: Arrow from (H of reactants to H of
products)
100
Potential Energy Diagram
A Table I Example
C (g) + O2 (g)  CO2 (g)
103
∆H = -393.5 kJ/mole
Activated Complex
(-) ∆H = Exdothermic Reaction.
Thi means th
This
thatt C
Carbon
b and
dO
Oxygen release
l
393.5 kJ of energy per mole of CO2 that will be
formed.
Products
Reactants
CO2 is a stable compound.
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2
An Endothermic PE Diagram
Any
Questions
?
Note:
1. PE increases. Temperature of surroundings will
decrease.
2. The Products have more energy stored in their
bonds than the reactants
What
now?
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108
An Exothermic PE Diagram
Note:
1. PE decreases. Temperature of surroundings will
increase.
2. The Products have less energy stored in their bonds
than the reactants
106
The Effect of A Catalyst
Catalysts increase the rate of a reaction by lowering the
activation energy.
Less energy to get to the activated complex = higher
frequency of effective collisions.
107
3