Video Transcript
In this video, we will explore the
meanings of exothermic and endothermic as they apply to reactions and examine the
energy transfers involved.
Let’s consider the simplest
possible reaction, the formation of a single bond between two hydrogen atoms. Typically, a hydrogen atom has a
single proton and no neutrons as its nucleus with one electron in the electron
cloud. When the hydrogen atoms approach
one another, the electron of one hydrogen atom is attracted to the nucleus of the
other. When they get close enough, a
covalent bond is formed; the two electrons are shared between the two nuclei.
However, energy cannot be created
or destroyed. So when the two atoms bond
together, all the energy must still be there. All the chemical potential they had
when they were separated becomes kinetic energy as they rotate or vibrate as a
molecule. When this high-energy excited
hydrogen molecule impacts on other particles, some of this energy is lost and the
hydrogen molecule becomes more stable. So overall, chemical potential
energy in our reactants becomes kinetic energy, which is lost to the surroundings,
likely as heat. Taken in isolation, the hydrogen
molecule is more stable than the separated atoms. So broadly speaking, we can say
that bond formation releases energy.
So, what about the reverse? The reverse of bond formation is
bond dissociation, otherwise known as bond breaking. Let’s take our stable hydrogen
molecule from the last section. We can send a particle at it with a
lot of chemical energy, exciting the molecule. If the conditions are right, we can
break apart the hydrogen molecule, turning kinetic energy into chemical potential
energy. This means that if we’re just
speaking about the molecule and the atoms, bond dissociation requires external
energy, so we often describe it as absorbing energy. This energy will often be delivered
from the surroundings. The words we’re going to use from
here on are exothermic and endothermic. But what exactly do they mean?
The two words both contain
-thermic. This is because, most of the time,
most of the energy transferred in reactions is thermal energy, what we commonly call
heat. A hot object has more thermal
energy than a cold object. This energy, naturally, will flow
from the hot object to the cold object, so it’s quite natural to think about the
energy entering or leaving a system as thermal energy. The exo- in exothermic means
outside or simply out. The endo- in endothermic means
within or simply in. So it would make perfect sense if
exothermic meant thermal energy out and endothermic meant thermal energy in.
But chemists recognize that there
are many forms of energy like light and sound that are involved in reactions. So, in fact, exothermic means
energy of any kind out and endothermic means energy of any kind in. You’ll just have to remember that
the thermic bit doesn’t really mean what you expect it to mean. Going back to what we were talking
about in terms of bonds, during bond formation, chemical potential energy is
converted to other forms of energy that are released, so bond formation is
exothermic. Meanwhile, bond dissociation, the
opposite, is endothermic. So we’ve looked at exothermic and
endothermic as they apply to single bonds. But what about more complicated
reactions?
Typically, when chemical reactions
happen, bonds in the reactants break and subsequently bonds in the products are
formed. We call any substances that exist
along the way intermediates although the name doesn’t matter that much. We can label the total energy
needed to break the old bonds energy in and the total energy we get out when the new
bonds form energy out. If the energy out is more than the
energy in, then, overall, energy has been released and the reaction is
exothermic. However, if the energy in is
greater than the energy out, then, overall, energy has bean absorbed and the
reaction is endothermic.
Combustion reactions, like the
combustion of methane, are good examples of exothermic reactions. In the products, the bonds are
stronger than in the reactants, so overall we see a release of energy into the
surroundings. We also see releases of energy
during state changes; for instance, when a liquid turns into a solid, when water
freezes, a small amount of energy is released to the surroundings. A good example of an endothermic
reaction is the decomposition of calcium carbonate, a vital process in the
production of cement. This requires a significant amount
of energy. On the other hand, ice melting is
much more mild and doesn’t require anywhere near as much energy as the decomposition
of calcium carbonate, but it is an endothermic process.
When talking about reactions, we
often only care about the reactants and products and nothing else. We call the reactants and products
the system. Anything else, we simply call the
surroundings. Quite often, when we’re talking
about energy, we’re only really talking about the energy of the system. Energy in increases the energy of
the system, and energy out decreases it. So, for energy in, the change in
energy is positive. And for energy out, the change in
energy is negative. So overall, if energy is released,
the energy of the system goes down, the change in energy is negative, and the
reaction is exothermic. On the other hand, overall, if
energy is absorbed, then the energy of the system goes up, the change in energy is
positive, and the reaction is endothermic.
We can also talk about the energy
of the system in terms of its enthalpy. It’s beyond the scope of this video
but exactly the same principle applies when we’re talking about exothermic and
endothermic reactions. So for an exothermic reaction, the
change in enthalpy will be negative; the enthalpy of the system will have decreased
because energy has been released to the surroundings, while in an endothermic
reaction, the change in enthalpy is positive; the enthalpy of the system will have
increased because energy will have been absorbed from the surroundings.
So, we’ve already discussed that
energy doesn’t need to be thermal energy to be exchanged between the system and the
surroundings. However, we can generally still use
energy changes to predict whether the surroundings will get hotter or colder. If we see the surroundings get
hotter because of a reaction, then it’s pretty certain that the reaction is
exothermic and vice versa. But if the surroundings get colder,
then the reaction is endothermic. Now that we know so much about
endothermic and exothermic processes, let’s have some practice.
Consider the chemical reaction
equation shown. H plus H react to form H single
bond H. Is this reaction endothermic or exothermic?
The reaction equation shows that
we’re forming a bond between two hydrogen atoms, so this is an example of bond
formation. Generally, energy is released to
the surroundings when bonds are formed. In an endothermic reaction, energy
is absorbed. And in exothermic reaction, energy
is released. Therefore, in this reaction, since
we are only forming a single bond and we are breaking no bonds at all, this reaction
is exothermic.
So, that one was relatively
straightforward. Let’s do a tougher one.
A chemical reaction has a reaction
enthalpy of positive 430 kilojoules per mole. Is it endothermic or
exothermic?
So what we have here is an unknown
chemical reaction where reactants are transforming to products and the change in
enthalpy is positive. What this means is that because of
the reaction, more energy must have entered the system than has left it. An endothermic reaction is a
reaction where the energy in exceeds the energy out. This is usually accompanied by a
decrease in the temperature of the surroundings. Meanwhile, an exothermic reaction
is one where the energy released is greater than the energy absorbed. This is usually accompanied by an
increase in the temperature of the surroundings.
Since the enthalpy change is
positive, the enthalpy after the reaction is greater than the enthalpy before the
reaction. The reaction is, by definition,
endothermic even though we don’t know what the reactants and products are.
Let’s finish up with the key
points. In exothermic reactions, more
energy is released than it’s absorbed. In exothermic reactions, the total
energy of the system goes down. So the change in energy is
negative, the change in enthalpy is negative, and the surroundings generally heat
up. The formation of bonds is an
example of an exothermic process. On the other hand, in endothermic
reactions, more energy is absorbed than it’s released. In endothermic processes, the total
energy of the system goes up. So the change in energy of the
system is positive, the change in enthalpy is positive, and the surroundings
generally cool down. And finally, bond dissociation or
bond breaking is an example of an endothermic process.
