Video Transcript
Chemical reactions typically
involve the release or absorption of energy. Reaction profiles are diagrams that
indicate the energy levels at various steps along the reaction pathway. In this video, we will learn how to
read reaction-profile energy diagrams and identify and describe the energy transfers
involved.
A reaction profile looks something
like this. This is the reaction profile for
the following chemical equation where hydrogen gas and oxygen gas combine to form
water. The 𝑥-axis of a reaction profile
represents the reaction coordinate or the progress of the reaction. Rather than being linked to a
specific unit of time, we can generally break down the reaction coordinate into a
beginning, middle, and end: when there are reactants, when there’s a transition
state in the middle, and when there are products.
The 𝑦-axis represents the energy
of the particles involved in the reaction. So the height of the line toward
the left gives us the energy of the reactants. The height of the line in the
middle gives us the energy of the transition state. And the height of the line on the
right gives us the energy of the products. We can quantify these amounts of
energy and use specific units. But for our purposes here, we will
mainly be looking at relative amounts of energy.
Another value of note is the
enthalpy change of the reaction, often shortened as △𝐻. This value gives us the change in
enthalpy or the change in total energy from the beginning to the end of the
reaction. In other words, it’s the difference
between the energy of the products and the energy of the reactants. In the reaction we see here, the
energy decreases from the beginning to the end of the reaction. That means that the △𝐻 is
negative. It also means that this is an
exothermic reaction. When an exothermic reaction occurs,
the lost energy will be released into the surroundings, mostly in the form of heat,
but often in other forms of energy in smaller amounts as well.
Conversely, if there’s an increase
in energy from the reactants to the products, that means that the enthalpy change
will be a positive value. It also means that it is an
endothermic reaction as energy has been absorbed from the surroundings to give the
particles involved more energy. It is also worth noting that we
call this value △𝐻 when the reaction runs at constant pressure. If the reaction instead runs at a
constant volume, for example, inside a propane tank, we call this value △𝐸 or the
change in internal energy of the system.
The next value of note is the
activation energy. We find this energy by taking the
difference between the energy of the transition state and the energy of the
reactants. And it is the energy needed to turn
the reactants into the highly unstable transition state before eventually becoming
the products. Because the activation energy is a
high amount of energy that must be reached before the reaction can proceed, we
sometimes call it an energy barrier. We can think of this sort of like a
ball rolling up and down a hill. If we push the ball up to the top
of the hill giving it energy, it can then roll down into the valley on the other
side.
The chemical formulas of the
products and the reactants included in a chemical equation give us a good idea of
what those molecules look like. The structure of the transition
state, however, is not clearly indicated. The transition state, also known as
an activated complex, is an extremely unstable high-energy state. It’s a necessary middle step toward
becoming the products, but it only exists for a tiny fraction of a second. Since they only exist for such a
short period of time, we don’t need to focus too much on what specifically an
activation complex looks like. But to give us an idea, it might
look something like this: an awkward and unstable arrangement of particles that look
something like the products and something like the reactants.
Since oxygen and hydrogen molecules
are extremely unlikely to spontaneously combine in this way, it takes a lot of
energy to bend, twist, break, or otherwise distort the bonds to create the
transition state on the way to creating the products. In fact, stability and energy are
inversely related. If a particle has more energy, it’s
more likely to move and change, making it less stable. Conversely, low-energy particles
are more stable. We can think back to our
ball-on-a-hill analogy to understand this relationship even better. A low-energy ball at the bottom of
the hill is likely to remain in that general area. But a high-energy ball’s position
at the top of the hill is unstable because it could easily roll down to find a new
position at the bottom of the hill.
So the transition state is an
example of something that is high energy and low stability. One way to provide the energy
necessary to overcome the energy barrier is by heating or igniting the reactants
with a flame. Note that not all reactions need
extra energy to proceed as there’s enough energy in the surroundings to make some
reactions occur spontaneously.
Another way to overcome the energy
barrier is with a catalyst. A catalyst is sort of like a
reaction helper. It arranges the particles in
particular ways to make it easier for them to react without directly participating
in the reaction. One catalyst for the
water-producing reaction we’ve looked at here is palladium. When hydrogen gas reaches the
surface of palladium, it weakens the bond between the hydrogen atoms. This allows the reaction to take
place more easily as less energy is required to overcome the energy barrier. As a result, the reaction profile
looks something like this, with a lower activation energy seen in the form of a
lower central peak.
Now that we’ve learned a bit about
reaction profiles and how to interpret them, let’s do some practice problems to
review.
Labeled in the diagram are the
chemical energies of three compounds, a through c. Which of the labeled compounds is
highest in energy? Which of the labeled compounds is
lowest in energy? Which of the labeled compounds is
most stable? Which of the labeled compounds is
least stable?
Based on the diagram included in
the question, this question is asking us to look at the energy and stability of the
three compounds. The axis of the diagram indicates
that the energy increases as we move up from low energy at the bottom to high energy
at the top. With this understanding in mind, we
can see that the labeled compound with the highest energy is b, the one furthest up
in the diagram, while the one with the lowest energy is a, the one furthest down in
the diagram.
The third and fourth questions ask
us to find the most and least stable of the three choices. Stability is not directly indicated
by the diagram. But if we know the relationship
between energy and stability, we can answer this question. In fact, energy and stability are
inversely related. Particles with more energy are more
likely to move and change, making them less stable. Particles without a lot of energy
don’t move around as much, making them more likely to stay the way they are or
making them highly stable. So, the choice with the highest
energy, compound b, is also the choice that is the least stable. Conversely, the choice with the
lowest energy, compound a, is also the choice that is the most stable.
To summarize our answers, we can
say that the compound with the highest energy is compound b. The compound with the lowest energy
is compound a. The compound that is the most
stable is also compound a. And the compound that is the least
stable is compound b.
The reaction-profile diagram for a
two-step chemical reaction is shown below. In step one, compound a reacts to
form compound b, and in step two, compound b reacts to form compound c. Which step has the highest
activation energy? Which step is an exothermic
reaction?
The reaction profile for this
two-step chemical reaction looks a little different than the more commonly seen
reaction profile of a single step reaction. But we can simply think of it as
two single reaction profiles joined together end to end. The first part of the question asks
which of these two steps has the highest activation energy. To find the activation energy, we
take the difference between the energy of the reactants and the energy of the
transition state. For the reaction profile, that
means finding the vertical difference between the starting point, a for step one and
b for step two, and the central peak in the step. If we visualize this on the
diagram, we can see that step one has a higher activation energy than step two. So our answer to the first part of
the question is that step one has the highest activation energy.
The next part of the question asks
us to find which step is an exothermic reaction. As a reminder, an exothermic
reaction is a reaction where energy is released into the surroundings. That means that the change in
enthalpy is negative and the energy of the products will be lower than the energy of
the reactants. So, as we move from a to b in step
one and b to c in step two, which step involves a release of energy? The answer is step two. We can see that the products c have
a lower energy level than the reactants b. This means that some of the
chemical energy from b has been released into the surroundings. The opposite is true for step
one. Since the energy increases as we
move from the reactants to the products, energy is absorbed from the surroundings,
making the reaction endothermic.
So, to answer the second part of
the question, we can say that step two is the exothermic reaction. Overall, this question relies on us
understanding the definitions of activation energy and exothermic reaction as well
as being able to recognize the portions of the reaction-profile diagrams that apply
to these concepts. In the end, we can say that the
step with the highest activation energy is step one and the step that is an
exothermic reaction is step two.
Now that we’ve done some practice
problems, let’s review the key points of the video. Reaction profiles give the amount
of energy in the system at various steps in the reaction. Those steps are the beginning when
there are reactants, the middle when there’s a transition state, and the end when
there are products. In a reaction-profile diagram, the
𝑥-axis gives you the progress of the reaction and the 𝑦-axis gives you the energy
level at that step of the reaction. The enthalpy change of the reaction
or △𝐻 is the difference in total energy between the products and the reactants.
The activation energy or 𝐸 a is
the energy required to add to the reactants to reach the high-energy transition
state. Energy and stability are inversely
related, so a high-energy particle has low stability and a low-energy particle has
high stability. And the activation energy can be
lowered with the help of a catalyst.
