CHEMICAL
ENERGETICS
Energy Changes in Reactions
◦ As we know, some chemical reactions are capable of releasing large amounts of energy.
Quite recently we saw the raging fires in Australia that began in late 2019 and continued
into early 2020. This is a real-life example of an out of control chemical reaction.
◦ Thankfully, most of the reactions that we encounter can be controlled on a much smaller
scale.
◦ We have seen and discussed endothermic and exothermic reactions already but here is a
quick review:
Endothermic
Exothermic
breaking chemical bonds
making chemical bonds
absorbs/takes in energy
releases/gives out energy
feels “cold”
feels “hot”
𝚫H is positive
𝚫H is negative
(more on “𝚫H” in a bit...)
Energy Changes in Reactions
◦ In a reaction, we must have a combination of bonds first being broken
and then new bonds being made. Therefore, to know whether a
chemical reaction is exothermic or endothermic we must compare
the total energy given out to the total energy taken in.
◦ We can do this in two ways:
1) energy level diagrams
2) calculate the heat of reaction (𝚫H)
Energy Level Diagrams
◦ An energy level diagram is like a graph the represents the heat
changes happening in a chemical reaction.
◦ On the X-axis, you find the progress of the reaction over time, or the
reaction pathway
◦ On the Y-axis, you find the energy (heat energy 𝚫H) of the reactants
and products
◦ Arrows are used to represent whether the reaction is endothermic
(arrow pointing up) or exothermic (arrow pointing down)
Energy Level Diagrams
◦ In an exothermic reaction, the amount of energy in the reactants is
higher than the energy in the products. As a result, to form the
products, the reaction must lose heat in the process.
◦ Here is what a common energy level diagram for an exothermic
reaction might look like:
◦ 𝚫H is negative
Energy Level Diagrams
◦ This is not only an example of an exothermic reaction, but a
combustion reaction. Methane (CH4) is being burned to form carbon
dioxide and water.
◦ The products are more stable
than the reactants.
◦ Notice the black downward
pointing arrow between the
energy levels for the reactants
and products.
Energy Level Diagrams
◦ In an endothermic reaction, the amount of energy in the reactants is
lower than the energy in the products. As a result, to form the
products, the reaction must gain heat in the process.
◦ Here is what a common energy level diagram for an endothermic
reaction might look like:
◦ 𝚫H is positive
Energy Level Diagrams
◦ This is an example of an endothermic reaction. It is the reaction
between nitrogen and oxygen gases to form nitrogen monoxide (a
toxic gas).
◦ The reactants are more stable
than the products.
◦ Notice the black upward
pointing arrow between the
energy levels for the reactants
and products.
Energy Level Diagrams
Now that we have have a basic understanding of an energy level
diagram, we are going to change them just a little bit.
◦ The diagrams I showed you on the previous slides indicate that these
reactions (both endothermic and exothermic) happen spontaneously.
This is rarely true!
◦ Remember when we talked about activation energy? This is the
minimum amount of energy that reactions need to overcome to be
able to form the products.
◦ This usually means that we have to input some amount of energy to
get the reaction started.
exothermic
Energy Level Diagrams
Here are better energy level diagrams that
include the idea of activation energy.
Notice how the energy released or absorbed
doesn’t change – we still compare our reactants
to our products.
Once the reaction has met its required
activation energy, then the reaction will often
continue on its own.
endothermic
exothermic
Energy Level Diagrams
Let’s use the same combustion of methane
reaction from a previous slide to help us
understand.
If we set an open container of methane on the countertop, would it
immediately burst into flames? No! Even though there is oxygen in the air for
it to react with it would not do this (thankfully)!
For the combustion of methane to occur on its own, we must help it overcome
its activation energy by igniting it with a flame/heat energy. Once lit, the
exothermic reaction will continue to produce its own energy until all the
methane is burned away.
exothermic
Energy Level Diagrams
Some tips:
1) When asked to draw an energy level diagram
for an exothermic or endothermic reaction, you
*will* want to draw a graph that shows the
activation energy (similar to what you see here).
2) In that same diagram, you want to still make
sure you have either an up (endo) or down (exo)
pointing arrow between your reactants and products.
3) In case you need a trick to help you remember:
EXothermic = heat EXits;
ENdothermic = heat ENters
endothermic
Energy Level Diagrams
Remember that we have the ability
to lower the activation energy of some
reactions by using a catalyst.
The top graph represents no catalyst being used.
The bottom graph uses a catalyst.
Notice that we can’t completely remove the
activation energy or make the reaction
spontaneous, we can just lower it. This allows
us to have more successful collisions between
particles and a higher rate of reaction.
Energy Level Diagrams
o Example practice question:
Draw a reaction profile (same as an energy level diagram) for the
following reaction, which is exothermic.
Zn(s) + CuSO4(aq) à ZnSO4(aq) + Cu(s)
Energy Level Diagrams
o Example practice question:
Draw a reaction profile (same as an energy level diagram) for the
following reaction, which is exothermic.
Zn(s) + CuSO4(aq) à ZnSO4(aq) + Cu(s)
You should have drawn something
similar to this:
(I did not use a ruler so my lines are
are not very straight J)