Video Transcript
Energy plays a very important role
in chemistry. We know that things have energy,
but how do we go about measuring it? In this video, we’ll learn how to
measure changes in energy by performing calorimetry experiments. We’ll learn how to set these
experiments up and how to use the results to calculate the enthalpy change for a
chemical reaction.
Most chemical reactions either give
off or take in energy over the course of the reaction. And energy is released when a
chemical bond is made and absorbed when a chemical bond is broken. At outside of chemistry class, by
knowing the amount of energy that we get from burning fuel, we can figure out how
far we can go in a car. And the amount of energy that’s in
the foods we eat is printed on the nutrition labels. In all these cases, we can use the
results from calorimetry experiments to measure energy.
To be more precise, we can’t
actually measure the amount of energy that something has, but we can measure the
change in energy as a result of a process, which we can usually accomplish by
determining the amount of heat that flows into or out of a system. So, say that we wanna know how much
energy our body would gain by eating a plate of fries. There’s no way for us to hook up a
probe to the fries to figure out how much energy we’d get. But what we can do is determine how
much energy the fries give off as the result of a process that consumes them
completely.
So, what we could do is set our
fries on fire and let them burn. Letting the fries burn would give
out a certain amount of heat to the surroundings, and we can measure this amount of
heat. And then, once we know the amount
of heat, we can figure out the amount of energy that must have been in our
fries.
Now, to measure changes in energy
like this, we’re going to need to establish a couple things. First, we’re gonna need some way to
precisely control the surroundings. After all, we’re scientists, and we
want our results to be accurate. If we just burn the fries in an
open room, we’ll never be sure exactly how much heat was released into the
surroundings. Secondly, it’s easiest for us to
measure quantities like an object’s mass, its temperature, its volume, its pressure,
or other similar properties. So, we need a way to determine heat
that’s in terms of properties that are easy to measure. Let’s address this second concern
first.
From experience, we probably know
that heat is related to changes in temperature. For instance, when we remove heat
from something, the temperature decreases. And when we add heat to something,
the temperature increases. But heat also seems to be related
to the kind of material that we’re talking about. A pool on a hot day can be in
extremely comfortable temperature, but a metal bench can be too hot to sit on. This is because different objects
have different specific heat capacities, which is the amount of heat that’s needed
to change the temperature of one gram of a substance by one degree Celsius.
Here are the specific heat
capacities for a few common substances. Looking at our list, we’ll see that
the metals on this list, steel and copper, have a relatively low specific heat
capacity, while water is on the other end of the spectrum with a high specific heat
capacity. This explains why water can be a
comfortable temperature on a hot day, while the metal bench isn’t. It takes more energy to change the
temperature of water than it does for a metal.
Looking at this formula for
specific heat capacity, we’ll notice that we can easily rearrange it to find the
amount of heat by multiplying both sides by the mass and the change in
temperature. So, this is the formula we’ll use
in calorimetry experiments to calculate the amount of heat that was transferred to
the surroundings as a result of our calorimetry experiment. This equation tells us that if we
measure the change in temperature as a result of our process and we measured the
mass before the experiment, as long as we know its specific heat capacity which we
can easily look up, we’ll be able to determine the amount of heat that was given off
as a result of that process.
Here, positive values of heat
correspond to heat being gained, and negative values of heat correspond to heat
being lost. So now, we have this equation that
tells us we can calculate the amount of heat that’s absorbed or released as a result
of a process by measuring the change in temperature. So now, we just need an
experimental setup that will allow us to do this. There are two main setups for
performing calorimetry experiments. Both are performed in pieces of
equipment that we call calorimeters. One is performed under constant
pressure conditions and the other is performed under constant volume conditions.
Let’s focus on the
constant-pressure calorimeters first, which are also usually called simple
calorimeters. The first thing that we’re going to
need is some kind of vessel that’s a good insulator, since we don’t want most of the
heat to escape into the environment. We want most of it to stay trapped
within the vessel that the reaction is occurring in. Further much fancier choices of
insulating vessels, a styrofoam cup gets the job done for this purpose. It’s very insulating, and it’s easy
to use. And we can improve the insulating
qualities of the styrofoam cup by stacking two of them together.
Since this experiment can be
performed in a styrofoam cup, you’ll often see it referred to as a coffee cup
calorimeter. The vessel is then filled with
water and covered with an insulating lid that has a hole for both a thermometer and
a stirrer to go through the lid. To perform the experiment, we place
the sample inside the vessel and stir the stirrer so that the heat will be evenly
distributed throughout the water. Then, we measure the change in
temperature.
Since the sample goes into water in
this kind of calorimetry experiment, this kind of setup is particularly good for
measuring the energy change that’s associated with an aqueous reaction or the energy
that’s involved in dissolving a salt or another substance. We could also use it to determine
the specific heat capacity of a material by first heating the material up, placing
it in the vessel, and calculating the change in temperature as it cools down.
We are, of course, making a couple
assumptions when we perform this experiment. We’re assuming that the maximum
temperature that we read accurately reflects the amount of heat that’s given off by
the process. We’re also assuming that no heat is
escaping the calorimeter. We’re also assuming that no heat is
absorbed by the calorimeter itself. But of course, the thermometer, the
stirrer, the lid, and the vessel itself will absorb some amount of heat. All of these assumptions together
mean that the energy that we calculate as a result of a calorimetry experiment will
be less than the real energy that’s given off by a process.
Now, let’s move on to the
constant-volume or combustion calorimeter. Like the name suggests, we’re
setting things on fire in this kind of calorimetry experiment. So, this kind of calorimeter will
be good for combustion reactions where we burn things like fuel. And it would also be the suitable
choice of calorimeter for performing the experiment that we discussed at the
beginning of the video with the fries.
In this kind of calorimeter, we’ll
again have some kind of insulating container full of water that’s fitted with a lid
that has a thermometer and a stirrer. But this time, since we’re burning
our sample, we don’t want to put the sample inside the water. So, we’ll have a sample chamber
that contains the sample plus oxygen gas since oxygen gas is needed in combustion
reactions. Since the sample will be ignited
inside the sample chamber, the chamber’s often referred to as a bomb or a bomb cell,
and this entire calorimeter is often just referred to as a bomb calorimeter.
Finally, we need some way to ignite
our sample, which is usually done electrically using an ignition box that remotely
plies an electrical charge to that bomb cell. To perform this experiment, we’ll
ignite the sample using the ignition box, and the resulting combustion reaction will
cause heat to enter the water. We’ll, of course, want to stir the
stirrer throughout the experiment to make sure this heat is being transferred evenly
throughout the water, and then we’ll measure the change in temperature.
Again, we’re making some
assumptions here that the max temperature reflects the amount of heat that’s given
off and that no heat is escaping from the calorimeter into the surroundings. Unlike a constant-pressure
calorimeter though, in constant-volume calorimetry experiments, we generally know
the specific heat capacity of our calorimeter. And we can include the amount of
heat that the calorimeter absorbs into our calculations.
Now that we’ve learned how to set
up and perform calorimetry experiments, let’s finally figure out how to calculate
the change in energy from a chemical reaction using the results of a calorimetry
experiment. The first thing we’ll do is use the
change in temperature that we recorded in the experiment and the mass of the water
that was inside our calorimeter to calculate the amount of heat that was absorbed by
the water. Now, this amount of heat that the
water absorbed will be equal to the amount of heat that the reaction gave off but
opposite in sign since if the water absorbed a certain amount of heat, that means
the system lost that amount of heat. And vice versa, if the system
gained a certain amount of heat, that means the water will have lost that same
amount of heat.
Now, keeping track of the sign here
is not necessarily so important if we’re only interested in calculating the heat
that the reaction gave off, as we can usually just report the magnitude of heat. But if we’re interested in energy,
we will want to keep track of the sign, since a negative change in enthalpy
corresponds to an exothermic reaction and a positive change in enthalpy corresponds
to an endothermic reaction, and we wouldn’t wanna switch the signs up.
In a constant-pressure calorimetry
experiment, the change in enthalpy as a result of a reaction will be equal to the
heat that it gives off. But calculating the energy change
for a constant-volume calorimetry experiment requires further knowledge of the laws
of thermodynamics, since the heat given off in a constant-volume process is not
equal to a change in enthalpy but rather a change in internal energy, which is
simply a different way of expressing the energy that a system has. But luckily in chemistry, we’re
most interested in the amount of heat that’s given off in things like aqueous
reactions. So, we’ll be primarily focused
around the results of constant-pressure calorimetry experiments where the amount of
heat given off is equal to the enthalpy change.
Now, let’s get some practice using
the results of calorimetry experiments to perform calculations.
In an experiment, it was found that
a reaction resulted in 80 grams of water changing temperature by 15 degrees
Celsius. What is the value in joules for the
heat energy transferred in this reaction? Use a value of 4.2 joules per gram
per degree Celsius for the specific heat capacity of water.
The experiment that this question
is talking about is most likely a calorimetry experiment. Calorimetry experiments are
performed in devices called calorimeters, and their goal is often to find the change
in energy that’s associated with the process. To perform a calorimetry
experiment, we place our sample, in this case our reactants, inside the calorimeter,
which will give off heat that the water will absorb. We can then measure the change in
temperature of the water to calculate the amount of heat that the reaction gave off,
which is what we’re being asked to calculate in this question.
We can calculate the amount of heat
that’s transferred by using the results of a calorimetry experiment through this
formula that tells us that the heat is equal to mass times the specific heat
capacity times the change in temperature. We can calculate the amount of heat
that’s transferred by a reaction using a calorimeter by using the temperature change
of the water because the amount of heat that the reaction gives off will be equal to
the amount of heat that the water absorbs.
So, the problem tells us that we
have 80 grams of water, and the specific heat capacity is given to a value of 4.2
joules per gram per degree Celsius. And the problem tells us the change
in temperature is 15 degrees Celsius. We’ll notice that our units cancel,
leaving us in units of joules, which is what the problem asked for. And multiplying everything through,
we’ll find that 5040 joules of heat were transferred as a result of this
reaction.
Usually, the goal of calorimetry
experiments is to calculate the change in energy as a result of a reaction. Under constant-pressure conditions,
the change in energy will be equal to the heat that we calculated. But this problem didn’t quite give
us enough information to determine that, since we’re not told if the change in
temperature was an increase or a decrease. So, the change in energy could be
5040 joules or it could be negative 5040 joules. We just wanted to be sure, given
this amount of information. But this question didn’t ask us for
the change in enthalpy; it just asked us to determine the amount of heat energy that
was transferred in this reaction, which is 5040 joules.
So, now, we’ve learned all about
calorimetry experiments and how to use them to measure changes in energy. So, let’s summarize with the key
points. Calorimetry experiments can be used
to measure changes in energy. There are two kinds of
calorimeters: constant-pressure or simple calorimeters and constant-volume or
combustion calorimeters. In both calorimetry setups, the
change in energy of our sample or reaction will cause heat to be transferred into
the water of the calorimetry vessel, which will cause an increase in the temperature
which we can measure using a thermometer. And we can use this formula to
relate that change in temperature to a quantity of heat.
And for constant-pressure
calorimetry experiments, we can relate a change in enthalpy to the quantity of heat
that’s transferred. However, the energy that we measure
in these experiments will always be less than the true energy change of a reaction
since some heat will be lost to the surroundings, although we can minimize this by
improving the insulating qualities of our calorimeters.
