Lesson Video: Measuring Enthalpy Changes | Nagwa Lesson Video: Measuring Enthalpy Changes | Nagwa

Lesson Video: Measuring Enthalpy Changes Chemistry • First Year of Secondary School

In this video we will learn how to measure changes in energy by performing calorimetry experiments. We’ll learn how to set up these experiments and how to use the results to calculate the enthalpy change for a chemical reaction.

15:24

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.

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