Introduction to Thermochemistry and Enthaply

The other day we talked about different types of energy and how we can convert between them. For example, there is potential energy, which is a type of energy and some is based on its position. A car at the top of a hill, for example, has a lot of energy. potential energy, we can release the brake on that car and push it rolling down the hill and eventually those cars get more and more kinetic energy, which is the type of energy something has if it's moving. There is also a type of energy called chemical energy that is stored in the bonds between atoms and molecules, gasoline, for example, has a lot of chemical energy and if we dropped a lit match into a large tank of gasoline, we would not want to be near any of it. nearby place, but what we do If we convert a lot of that chemical energy from gasoline into thermal energy, now it's particularly important that you understand the energy and how we can convert it, so if you're a little on the fence about this topic, I suggest you come back and you look at yesterday's lesson or the last lesson we looked at, which was called conservation of energy and conversions, take a look at it and once you feel really comfortable with it, go ahead and come back to this, but if you feel pretty clear about the energy and how. we can go from one form to another let's go ahead and start we talked about thermal energy for a second, that will be the main topic of our evaru of our lesson today thermal energy when it flows from one object to another we call it heat let's imagine that we have wrapped our cold hands around a cup of hot chocolate a cup of hot chocolate let's think about the way heat will flow heat will flow from the cup of hot chocolate to our cold hands and that's the way heat always flows heat flows from a hot object to a cold object and the heat flow stops when the two objects are the same temperature, so after the hot chocolate has cooled enough and I have used my hands and they are the same temperature, there is no heat.

The flow occurs because it always flows from hot to cold. We would never see a situation like this. Imagine that you wrap your cold hands around a steaming cup of coffee and find them getting colder because he is being absorbed by the coffee. I would never have done it. Because it never flows from a cold thing to a hot thing, it always flows from a higher temperature area to a lower temperature. Now thermal energy is particularly important in chemistry because, after all, what would chemistry be if it weren't for big explosions and big fires? all as a chemist I do things almost every day that would make my mother cringe, so don't show her these videos.

She came here, to school, took me out of the room and gave me a stern talking to in the hallway. she'll probably let me probably make me quit my job, so make sure she doesn't see this, but joking aside, safety is important to keep in mind. I have a couple of demonstrations planned here that will show us how thermal energy is involved. in some reactions and these are going to be very interesting, but I want you to know that I have taken a lot of safety precautions. These are not things they should do at home, for example, I will wear protective glasses that will protect my eyes in case something goes wrong and I also know what I am doing.

I have taken a lot of time to plan these demonstrations to ensure that they will run safely and I have also taken time to prepare the chemicals and such so that nothing goes wrong. Also, I'm going to be handling some acid in this first demonstration, someone is going to be wearing gloves, and also, I'm going to do a full demonstration in what's called a fume hood or behind this piece of plexiglass so that if something goes wrong (it which I don't expect), the glass will protect me in this first demonstration, which is one of my favorites to do.

I mixed a chemical called potassium chlorate together with a common table boat, basically we are going to see how much thermal energy a small amount of sugar. I mix these two compounds together, they are both grayish white powders in this dish here. and I'm just going to pour it over this on this plate here, spread it out so the more it's exposed to the air, it looks pretty good now like this, there's no reaction, but if I put just a drop of sulfuric acid here we'll get the reaction started, let's put a little sulfuric acid here and let's take a look at what's going to happen, that's cool, isn't it?

As you can see when I added that tremendous amount of sulfuric acid. of heat was released from this reaction, this is what we call an exothermic reaction EXO is a Greek prefix meaning out or out of and that's why I added a little bit of sulfuric acid to the potassium chloride in the sugar, a huge amount of energy. came out of the reaction, so an exothermic reaction is one that releases energy to the environment. Now let's look at another type of experiment that also deals with thermal energy, but in a slightly different way. We have just seen an example of an exothermic reaction that released an enormous amount of heat into the environment.

Let's take a look at another type of experiment. What I'm going to do here is add two compounds to this flask. What I have here is some ammonium chloride. I poured it in and I'm going to mix it. with a little bit of this barium hydroxide that I just weighed out, well, let's put that in here, it should be enough and what I'm going to do is start mixing this and stirring it. It's hard for you to see them together now, but this is getting more and more liquid as I stir it. Another change is taking place.

This is getting very, very cold in my hand. Let me show you what I can do. I'm going to put just a drop of water on this board here and stick this bottle right on the board like this. I'm going to leave it there for now, we really don't have enough time to make this work so I didn't want to anymore. In advance, like on a cooking show, let me show you what eventually happens. So, as you can see, the board is completely stuck to this flask because the solution here has cooled so much that it was able to freeze the water and freeze the board. for this flask, this is what we call an endothermic reaction, it is the opposite of an exothermic range in an endothermic reaction, the reaction that occurs in this flask is absorbing heat from the environment, endo is a Greek prefix meaning within or within, so It makes sense that an endothermic reaction is one that absorbs heat from the environment into the reaction mixture, which is why an endothermic reaction often feels cold because it is absorbing energy, for example, from our hands.

Now let's go back to the classroom, review these concepts and see. how we can use some mathematical and graphical techniques to understand these ideas in a little more depth to be able to observe exothermic and endothermic reactions and a little more depth, we have to define some new terms, the first of them is the idea of ​​the system and the environment are not particularly complicated concepts, but they come up almost every time we want to talk about thermal energy and transfer of thermal energy between different things. A system is anything I choose to focus my attention on, for example if there is a reaction. what interests me in this beaker I can consider vigor to be the system and everything around it, the rest of the universe is considered these rounds if I am interested in a particular cow or a field, I can consider that cow to be ​​my system and everything else, that universe is an environment, well, how does it relate to the reactions that we saw in an example of an exothermic reaction for you?

I said it created a large amount of heat that it let out into the environment. This is true, but a more precise way to define an exothermic reaction would have been to say that an exothermic reaction releases heat from a system to the surroundings and these red arrows here show the direction of heat transfer in an exothermic reaction in showing that The system was this little pile of chemicals that I added on a plate and the surroundings were where everything else was, but the most notable thing was the air nearby because I could actually feel the air getting warmer due to the heat being transferred to it from the system?

On the other hand, an endothermic reaction is one in which thermal energy is transferred from the environment to the system. The second demonstration I did, the flask was able to freeze the water it was placed in because it was extracting thermal energy from it. water because water is part of the environment and the reaction that was happening in the flask, now let's take a look at how we can use some graphical techniques to better understand the ideas of exothermic and endothermic reactions, but before we do that there is another The order of the vocabulary we have to define this is the term enthalpy.

Enthalpy is one of those terms that in life, like dating or going out, is much easier to understand than defining it in a textbook. Let's say that enthalpy has to do with heat content. of a particular system, so enthalpy is less about whether something is hot or not or what its temperature is, although that's kind of important, it's more about its potential to create heat, it's about a system , it is about the amount of energy there is in a particular. system, for example, a log has much more enthalpy because it is capable of producing more than, for example, ashes at the same time.

A heat pack we haven't used yet has more potential to create heat than that heat. package after you've released your heating after you're done with a reaction, so let's look at an enthalpy graph and see how it changes during a particular reaction. Let's first look at an exothermic reaction. Here's my graph and I'm going to put F of P on my axis and here we often use a term H, the letter H to denote enthalpy and enthalpy increases as we go up the y axis, so burning a log would be a very good example of an exothermic reaction.

We said that relatively speaking, a The log because it has a high potential to generate heat has a fairly high enthalpy, so let's put it here after burning it, we get a variety of products and we get carbon dioxide, water and ash because the combustion of a log is Rarely completed, that's why we get the ashes. These don't have as much potential to generate heat as the trunk, so they have a lower enthalpy, so I'm going to mark them here. Here we have water with CO2 and ashes. In science we are often concerned with the change that has taken place during a process the difference between the beginning and the end and scientists have a very fancy term for this they like to use the Greek letter Delta to talk about the change in this particular reaction I'm interested in Delta H or the enthalpy change that takes place from the beginning to the end of the reaction.

Now the reactants had a fairly high enthalpy, the products have a fairly low P angle, so let's see how this changes the enthalpy in this case it doesn't go up but down, so I can say that it has a negative Delta H for an exothermic reaction because emits heat to the environment. If this is a complicated concept for you, let's use a money analogy, imagine that we are very rich, so we have a high initial value for the amount of money we have and then we spend a lot of money putting that energy into our environment. the same way that an exothermic reaction releases heat in it. their environment so we go when we spend money on everything we see we buy sunglasses on the street we buy Louis Vuitton bags we buy newspapers we give money to everyone who asks for change because we are giving all this money what is happening? is the amount of money we end up with is less than the amount we started with, so we could say that our money Delta, the change in the amount of money we have from the beginning to the end of the process is a negative value which The same thing happens to you is a negative change in enthalpy value because all that heat was released into the environment now let's look at a graph of an endothermic reaction we'll set this up the same way put in my enthalpy value Right there, an endothermic reaction doesn't exists in many ways, the opposite of an exothermic reaction.

A good analogy for an endothermic reaction would be someone who goes and tries to steal as much money as they can to get money out of the environment and they hold on to it, they keep it for themselves, so someone who starts off without a lot of money but then steal it or take it from your environment you end up with more energy, sorry, more money in the end than At the beginning they had an example of that chemically, it could be, for example, a cold compress. A cold compress feels cold because a reaction occurs in it that draws energy from the environment.

That cold compress is placed on the skin. It feels cold. Because it draws heat from the skin, a cold pack starts with a relatively low enthalpy, so we'll put it here, but during the process of endothermic reactions we said that thermal energy is drawn from the environment and then a cold pack. They pack the chemicals in that cold compress, they retain that thermal energy that they attracted, so they increase the amount of enthalpy they have because they are attracting it from the environment, so at the end of the reaction a cold occurs. The package has more thermal energy inside than whenstarted, so we'll put the used cold pack back in here.

We are interested in the change in enthalpy as Delta H of this reaction goes up or down in this case, since we can see that the products we end up with have more enthalpy than our reactants, so the initial enthalpy change So the Delta H of this reaction has a positive value. Endothermic reactions have a positive Delta H. and exothermic reactions have a negative Delta H. Make sure you understand this and understand the graphics and terms before continuing. This is a really important thing and we'll keep coming back to it over the next one. a week or so, if you're a little shaky on what you've seen so far, go ahead and rewind this and watch it again if you're ready, let's finish the last thing we're going to talk about. about today