We are ready to begin our next unit and it's on thermochemistry. In thermochemistry we're going to be studying the connections between chemical reactions and heat transfer. This heat is a transfer of thermal energy in or out of the reaction. Now, some reactions when they take place will give off heat, and we'll say oh, that is giving off heat, it's getting hot. Some reactions absorb heat, so we might recognize that it is, oh, it feels cold to the touch. But it might have to absorb so much heat that we actually have to supply a lot energy, so we might put it in the oven. In the case of baking a bread, or we might put a buttson burner under it in a laboratory as a heating source to make it go. Some reactions give off heat. Some will absorb heat. So let’s begin with our first learning objective. In this learning objective, we’ve got a lot of terminology we’ve got to understand. We’re going to learn a language of studying thermal chemistry. So we're going to work through and learn various terms. Make sure before you go on to the next learning objectives that you think about these terms and become familiar with them, because I will use them throughout this unit, and they may not be things that you're familiar with. So the first term we have is energy. Energy is simply d, defined as the capacity to do work. Doesn't mean it necessarily is doing work, but has the capacity. Well, what is work? Work is defined as the directed energy change resulting from a process. So maybe you're going to push a book across a desk. Maybe you're going to run an engine for a train. So that it pulls the freight down the track. Maybe you're going to compress a gas within a combustion engine and squeeze on it. And that'll do something. So it is a directed energy change resulting from a process. We won't talk a lot about work right away, but we will eventually discuss work a little bit more in detail in a future learning objective. But let's focus on energy for now. Energy can be divided into two forms, it is either kinetic energy or it's potential energy. Kinetic energy is the energy produced by a moving object. So, as a book flies through the air, maybe it's going to land on a bug and squish a bug. There's the work. Maybe it's going to hit somebody. That wouldn't be good. So, let's not throw a, a book. But as an object moves, let's say water in a river, has the certain capacity to do work, it can push a boat down the river, it could, if there was enough of it, it could wipe out a village, we wouldn't want that to happen. If there was a big flood, so it is a moving object. But you could have a moving object down on the molecular level. Our molecules, even in a stationary object, are actually moving, so everything has some kinetic energy associated with it. Potential energy is energy available by virtue of an object's position. We often think of this as stored energy. So if you have a body of water being held back by a dam, a very large dam, that water is not flowing, but it has the potential to do work, if you were to open up the, the dam or is it, if it were to spring a leak. If I held a book high over my head, it's stationary. It is not in motion as a book, but it has a capacity to do work. I could drop it from a high position and squish that bug that's laying there on the floor. Okay? So that sort of thing is a potential energy. Now even down in the molecular level we have stored energy by virtue of the bonds that hold the atoms together. Continuing with the various forms of energy, we can also categorize energy according to its source. So chemical energy would be energy stored, this is a potential energy, stored within the bonds, the structural units of that chemical substance. You could have thermal energy, and we'll talk a lot about these two energies. But thermal energy is the energy associated with the motion of those atoms and molecules. So a object that has a lot of thermal energy we associate with a high temperature, and the movement of those molecules are greater. Now thermal energy is a type of kinetic energy, and in thermal chemistry we are really looking at these inner conversions between thermal energy and potential, I mean thermal energy and chemical energy. There might be solar or radiant energy. Radiant energy would be the energy from the sun as it warms our planet. So it gets converted to thermal energy. It goes through the, the system to get to our planet. And it starts moving our molecules faster, gets converted to thermal energy. So we can easily have a inner conversion between these. These are not the only ones. We could have a mechanical energy, electrical energy, there are different forms that energy takes, but no matter what form they take, they could fit into the category of kinetic or potential. So there is a law that governs energy which is a law of conservation of energy. And this law states that the total quantity of energy in the universe is assumed to be constant. So this is, this is actually the first law of thermodynamics but we'll just call it the law of conservation of energy at this point and what is the net result of that. Well, if you're not creating or destroying it, it is, however, being converted from one form to another. So it might seem like when you put a piece of wood in the fire and you burn it, and you're feeling that thermal heat warming up your room, you may be thinking that you're creating that thermal heat. You're crea, well, you are creating thermal energy, but you aren't creating energy. It is being transferred from the energy that was stored within the bonds that made up the wood and the oxygen that was in the air. So, it's never being just created or destroyed, it's just being converted. Now, let's define the term heat, since thermal chemistry is about heat, let's define that word. It is the transfer of that thermal energy between two bodies that are at different temperatures. So if you were to place your hand n an ice cube it feels cold to you, the reason it feels cold to you is there is a transfer of thermal energy from your hand which is hot into the ice cube if you were to put your hand on a warm roll that you are getting ready to eat and it feels warm to you that warm roll is, warm roll is transferring thermal energy from the roll into your hand. So that is a motion of that, those molecules are moving faster or slower, depending upon how that thermal energy is being transferred. So, a lot of times we'll talk about heat transfer but you, heat is a transfer. So it's a little bit redundant when I, when we do that, but I'll still make that, l'll still do that from time to time. Talking about heat transfer, but really what's being transferred is thermal energy and that's the definition of heat. Thermochemistry is a study of heat changes in chemical reactions. So when a chemical reaction takes place, it can transfer thermal energy into the surroundings or absorb thermal energy from the surroundings. We can have this transfer that takes place. Now when it's transferred, we say it's transferred between the system which is the part of the universe of interest to us, and in chemistry, that is most usually a chemical reaction. And the surroundings is every other part of the universe. So you have a little reaction taking place in a reaction chamber. And let's make it a square box and the reaction is in here, okay? And let's say that this reaction gives off heat. Well, it gives off heat to the surroundings, and that's the rest of the universe. Now really, we don't have to worry about the moon. Okay? Or further out, the planet Jupiter. Okay? We don't have to worry about what's happening out there, because it's the immediate surroundings that's going to be affected. But, the definition of surroundings is every other part of the universe. Because we want the system Plus the surroundings to equal the universe. Okay. So, we don't have to study Pluto and the moon. We just have to study the immediate surroundings, because that effect is drops off very rapidly as you move away from the reaction. Now let's focus then on the system. There are three ways to categorize systems. A system can be an open system. In an open system, both matter can enter and exit the system, and heat can enter and exit the system. So let's imagine for a moment, that is a, some water. And maybe it is hot. Okay? It's hot compared the surroundings. So we put some hot water in there. Well, if it's hot compared to the surroundings, we're going to have a net transfer of heat out, as these try to get to the same temperature. We Can also have the evaporation. Of water out of the flask. So both heat can enter an exit in an open system. And matter can enter and exit. Now if we take that same hot water and we put a cork in the top so that it can't evaporate away, then this would be a closed system. He can still enter and exit that flask, but you can't have evaporation, or anything added to it. The last one is an isolated system. Now, an isolated system is insulated somehow, so that heat can't enter or exit. So you're, you're keeping matter in, or not allowing it out. You're keeping heat in, or not allowing it out, in an isolated system. Now to see if you've got the right connections between those three, let's have you answer this question. Well, if you said it's a closed system, then you understand. You've got. The can is closed. You can't put more green beans in or take them out. It is a closed system. But you could cool it off. Stick it in the refrigerator and the green beans would get colder, or warm it on the stove, though I don't recommend doing that when the lid is still on. But you could put heat into the system. Now I have got, two separate things here. I've got what we see on the left which represents the reaction of methane gas. This is a combustion reaction. And we have one over here where ion taking some mercury oxide and converting it to mercury in oxygen as separate elements. There are two terms that we are representing here. One is an exothermic process, and in an exothermic process, thermal energy is transferred to the surroundings when that reaction takes place, so the reaction is the system. As a reaction takes place, this is a combustion reaction here, so I'm going to write combustion. [SOUND] When that reaction takes place, heat is given off, so we see the arrow pointing out this way. Why is it given off? Well, there is a certain amount of energy stored within the bonds of the methane in the oxygen. That stored energy as the bonds break and new bonds are formed ends up making these products and it doesn't have as much energy stored within it. So, where does that extra energy go? Well, it is given off to the surroundings. Okay? In the second, and we use the word exo, exits, it leaves, exothermic, for that process. The one on the right is an endothermic process. Now, mercury oxide, this is mercury two oxide, will be a very content compound, staying as mercury two oxide, unless you supply it with heat. And you're actually going to need to put a flame under it. Okay? So you have a flame under there that is converting that mercury two oxide to mercury and oxygen. So you're putting in heat. And so our arrow is going this way. We're putting end heat into that, [INAUDIBLE] into the system in order to break those bonds and form new ones. So there is less, sorry there's more energy stored within the Hg and the O2 separate, than there was in the mercury two oxide as the reactant. So, think about this process. You're going to have an ice cube. You will place it on a table, and it begins to melt. Is that going to be an endothermic process or an exothermic process? Well, if you said endothermic. Then you are correct. Okay, so we've been given a number of terms, system, surrounding, heat, energy, work, endothermic, exothermic. Make sure you study these terms. Get familiar with them. And then move on to learning objective number two.
