Enthalpy and Energy Diagrams
Objectives:
 Define enthalpy
 Define spontaneous
 Draw and label an energy diagram
 Explain the concept of activation energy and activated complex
 Predict spontaneity based on energy diagram and H
So far we’ve talked a lot about heat.
 Heat is the change in energy due to a temperature difference.
 Heat can be used to measure the energy in something
 Heat moves from areas of hot to areas of cold.
A change in energy can also be described as a change in enthalpy when the system is
at constant pressure (open to the atmosphere = constant pressure)
Enthalpy: a measure of the energy content of a substance. This includes kinetic and
potential. (It also includes pressure-volume energy but we will ignore this in general
chemistry).
Since we will always be dealing with constant pressure systems in chemistry (until we get
to the gas laws unit), we can say
q = H
Just like heat
 H > 0 = Endothermic
 H < 0 = Exothermic
 Units: Joules, J
A visual for the change in energy of a reaction:
Activated Complex
Reactants
Energy
Products
Reaction Coordinate
The diagram above shows that
 Reactants are at a higher energy state than the products
 The reaction is exothermic
 An in-between compound (activated complex), is formed
Reaction Coordinate: a step in the process. Reactants have to “morph” into the
products.
Reaction Mechanism: how the substances “morph” into products, the steps involved in
changing from reactant to product
Animations of reaction mechanisms
 http://chemistry.berkeley.edu/links/reactions.html
o Notice that there is a point where the middle molecule is not bonded to
either. This is higher energy and therefore qualifies as the “activated
complex”
 http://neon.cm.utexas.edu/academic/resources_movies/movies/iverson/main.ht
m
o Notice here that there is a point where all three molecules are bonded
together. This is high energy because all the electrons are getting too
close! Therefore, this qualifies as an activated complex as well.
Activation Energy
Energy
H < 0
Reaction Coordinate
The diagram above shows that
 The change in energy between reactants and products is the change in
enthalpy
 The reaction is exothermic
 The reaction requires some energy before it will start, Activation Energy: Ea
Example of Activation Energy:
1. Candles don’t burn until you ignite them
2. Matches don’t light until you use some of your own energy as you strike them
3. The food didn’t burn until it was lit
4. The Nitrogen triiodide (remember explosion video) doesn’t explode until it was
tickled
Analogy: Old Love, New Love
Reactants are like you and an old boyfriend/girlfriend…content until you break up. You
are in a state of agitation…activated complex…until you meet and bond with your new
boy/girl and you are happy…at a low energy state….a new product has been formed.
What would an endothermic reaction look like?
Activation Energy
Energy
H > 0
Reactants
Reaction Coordinate
ENDOTHERMIC REACTION
What is the likelihood of an endothermic reaction taking place?
 Not, likely
 Therefore we say it’s non-spontaneous
 Spontaneous: refers to the tendency of a reaction to occur
o Spontaneous does not refer to how fast
o Carbon  Diamond is spontaneous but takes forever!
o Rusting  spontaneous but takes a long time!


We already know that substances like to be in a lower energy state…
Therefore, exothermic reactions are usually spontaneous
Why would a reaction that creates substances at a higher energy ever happen?
Stay tuned…
1.
2.
3.
4.
5.
6.
Which are exothermic?
Which are endothermic?
Which require activation energy?
Which are spontaneous?
Which reaction could provide the most energy?
Which reaction would require the most
energy?
A
D
B
C
E