 Okay, in the previous video we talked about a way of putting different chemical reactions into different categories. There are many other ways of categorizing chemical equations, I'm going to show you a slightly different way. It is based on whether a chemical reaction absorbs heat, and I'm using heat and energy interchangeably, or whether the chemical reaction releases heat. And so here comes the explanation. Don't worry too much about the details about this graph. All I want you to think about is we have some chemical reaction, chemical A, being turned into chemical B. And depending on where you are on the Y axis, if you're up high, your molecule has a lot of energy, if you're down low, your molecule has less energy. So we're going to follow this curve, don't worry so much about the hill either. However, chemical A is relatively up high on the Y axis, chemical B is down low. What that means is A molecule A, whatever it is, has more energy than B. And so the question is what happens when A gets converted into B? Where did the extra energy that A had? A had a lot of energy, B doesn't have as much, where did it go? What happens is you have to account for the energy. The energy doesn't just sort of disappear. A got turned into a molecule called B that has less energy, and so some energy or heat was released to the outside world. So you can imagine that you've got a test tube filled with a bunch of chemical A, and that over a certain amount of time, it gets converted into B. But it gets converted into B plus some heat, or plus some additional energy. So what happens is maybe this was 10 degrees Celsius, maybe this is 20 degrees Celsius because heat was released to the outside world and you would be able to feel that, or a thermometer would be able to feel that as the temperature of this material going up. This is a type of chemical reaction. Instead of just saying A gets turned into B, to be a little bit more proper we would say A gets turned into B, plus it makes or releases some heat or energy. Another way of saying that is heat or energy is on the right side of the arrow, or on the right side of the chemical equation. This type of reaction when you have heat or energy released or put on the right side of the equation has a special name. It's called an exothermic reaction. Thermic basically means heat, exo means going outside. So in an exothermic reaction, heat or energy is released. It's released to the outside world because A used to have a lot of energy, got turned into something that had less energy, and the extra energy just got released to the rest of the world. That's called an exothermic reaction. Another way of saying that is energy or heat is a product. That just means it's on the right side of the arrow in the equation. Heat is released. If you do this type of reaction in a test tube, you might feel the test tube get warmer because heat is being released. Another way of saying that again is that the products, in this case B, they have less energy than the reactants, which was A. And so that's all sort of shown on this little chart over here. An exothermic reaction has heat or energy on the right side of the equation. If there's an exothermic reaction that releases heat, there's got to be another type of reaction that uses up heat. And before we get there, this is an example of an exothermic reaction because here's our arrow, and here's energy on the right side. I just showed you in an earlier video that I said this is called respiration. And I said respiration is a combustion reaction. And we do respiration. This is the formula for sugar. We eat sugar. We breathe in oxygen. We breathe out carbon dioxide. We also make some water in the process, and we get energy from the sugar. Respiration is also an exothermic reaction. In other words, respiration releases heat or energy. Now if there are reactions that release heat, there are also reactions that use heat. And here is sort of a graph showing that type of reaction. So in this case, pretend we have some chemical X being turned into some chemical Y. X has relatively low energy. Y has more energy. But the only way that we can take a low energy molecule and turn it into a higher energy molecule is we have to pump energy in. We have to add heat or energy to our X molecule to transform it into Y. And so if we were going to write a chemical equation, we would not just write X turns into Y. We would say heat or energy plus X turns into Y. You can see that heat or energy now is now on the left side of the arrow. So another way of saying that is heat disappears. So if I had a test tube, and it had a bunch of X molecules, and it was a certain, the test tube had a certain amount of warmth, it had a certain amount of temperature, I could use some of that warmth to turn the X molecules into Y molecules. And because of that, some of the heat would disappear. So see how there are only two heats here and there were four over on the left. Some of the heat went away, so this test tube got cooler when the reaction took place. And this type of reaction where heat disappears is called an endothermic reaction. Again, thermic means heat, endo means going inside of. So some of the heat went into the X molecule and turned it into the Y molecule. Endothermic reactions, heat or energy is a reactant. In other words, heat or energy is on the left side of the arrow. Heat is absorbed by the molecules. If you do this in a test tube, you might feel the test tube get colder. And another way of saying that is the products have more energy than the reactants. So if you look at this equation, it's endothermic. Why? Because energy is on the left side of the arrow. If you look at this equation carefully, you will see that it is the reverse of the respiration equation that I showed you on the previous slide. This also has a fancier name that you may have heard of. It's called photosynthesis, or this is the simplified equation for photosynthesis. Plants actually use carbon dioxide. You can think of them as breathing in carbon dioxide. They get water either from their roots or the rain. They get energy from the sun, so let me make a beautiful sun. And they use carbon dioxide water and sunlight to make sugars. And they also release oxygen which we breathe. So photosynthesis is an example of an endothermic reaction. I want you to know the difference between endothermic and exothermic reactions and be able to recognize them if I show you the equations. And last but not least, I want to talk about what a catalyst is. Please do not get caught up in this graph. I think this graph is kind of silly and it gets taught even though I think it's partly nonsense. All I want you to know is what a catalyst is. Should catalysts deal with chemical reactions? Imagine that you have some chemical A and it can get converted into chemical B. Now for some chemical reactions, maybe whatever your chemical is, A gets turned into B really, really fast. It's almost instantly if you have an A, it just gets turned over into a B. Some other reactions, maybe C getting turned into a D, might be slow. Might take days or years if you have a bunch of C molecules to turn into D. Catalysts are specific materials that can make a chemical reaction go faster. So if I have chemical C being turned into D and it's slow, if I had some special material that I could mix in with C and speed up the conversion, speed up its conversion into D, that would be called a catalyst. So catalyst, let me just kind of write my definition down, any material which speeds up a chemical reaction. That's all I want you to know about a catalyst. There are many different types of catalysts. We might talk about a few much later in the course, but that's it for now. So that is it for this unit. Good luck. Okay. Bye-bye. You're a brand new forward. But you said I wasn't a catalyst. You had $10 dinner. And you said thanks for the snack, but just a shot.