In this module we're going to do a brief review of thermochemistry because it's important in our understanding of thermodynamics. Our goal is to make sure that students understand the meaning of enthalpy, system, surrounding, and state function and they apply to thermo chemical equations; so that we can take this information and use it we're looking at additional thermodynamic calculations. When dealing with thermochemistry we have to define the system and the surroundings. The system can be defined by the user so this can vary. Typically we're looking at a chemical reaction, is the system and everything around it is the surroundings. In looking at thermo dynamics we need to go back to the definition of what a state function is. Note, that a state function depends only on the initial and final state it is completely independent a path. It only matters where we began and where we ended. Some examples of state functions include enthalpy. which would represent as Delta H. Delta T, or change in temperature and delta V or change in volume. The Delta sign is what indicates that we are dealing with a state function.
and delta V or change in volume. The Delta sign is what indicates that we are dealing with a state function. We can talk about the heat associated with any process this is typically in units at the joules or kilojoules. When we know the he'd measured at a constant pressure we can further qualify that by calling it the enthaly and frequently what we're looking at is the change in enthalpy. The Delta H, the difference between in the enthalpy of the reactants and the enthalpy of the products. Generally this is reported in units at kilojoules per mole of substance. A couple reminders of how we deal with Delta H values. Remember that if I have a reaction A plus B yields C and the Delta H for that reaction equals 100 kilojoules. Then the enthalpy of the reverse reaction or C going to A plus B Delta H which will be equal to -100 kilojoules and this is because it's a state function and only depends on the initial and final state. We will be able to take advantage of this again as we look at some thermodynamic values later in this unit. We can also look at what happens we multiply the reaction by some factor. If I have 2A place 2B yields 2C then the Delta H will be two times 100 kilojoules or 200 kilojoules. One of the main purpose of exploring thermo dynamics is to understand will a reaction happen spontaneously or not. We can look at enthalpy and they are all exothermic reaction spontaneous? Are all endothermic reactions non spontaneous? Remember exothermic means heat is being released by the system and Delta H is less than 0. For endothermic heat is gained by the system and has a Delta H value greater than 0. So this is just one of the few questions we're going to be asking as we explore thermodynamics. This is a demonstration of an endothermic reaction. And an endothermic reaction we need to supply heat in order to make the reaction progress. Sometimes, you actually have to add a heat source but this reaction can take enough heat from the surrounding to run and so you feel it getting cold. We are going to attempt to freeze this flask to the board with a couple drop of water because it gets that cold. These are two dry chemicals that i will mix together. I have barium hydroxide and ammonium chloride. that i will mix together. I have barium hydroxide and ammonium chloride. It is kind of unusual to have 2 dry It is kind of unusual to have 2 dry ingredients that will actually react well. When this reaction proceeds it produces some ammonium gas, which I will be able smell. It also releases some water, that was hydrated in the barium hydroxide. and it will start getting a little slushy. When it get slushy I am going to add the drops of the water to the board. and set the flask down and then we will wait a while. I am going to add a couple drops of water. This flask has an indention on the bottom so I am going to try to freeze it on either side of the flask. And now we will wait. So if all works well, this flask, when I pick it up will be frozen to the board. this flask, when I pick it up will be frozen to the board. Alright so we have our endothermic reaction. The demonstration you are about to see will be taking place out side and you will see why here in a little bit. but it involves two substances. One, is iron oxide which is basically rust in a powered form. And the other one is powered aluminum . When these two substance are mixed together we won't have anything happen until we light them, And we don't want to light them with a match or something we have to approach And we don't want to light them with a match or something we have to approach closely to. So we have a secondary reaction. And this reaction will have glycerin and some Potassium permanganate. As those substances are added together it will give a delay which will allow Dr. Salt to back away from the reaction. Then smoke will back away from the reaction. Then smoke will billow out, and the reaction will take place. Then smoke will billow out, and the reaction will take place. And we will talk more about the reaction after you get a chance to see it. So the reaction you just saw is a very spectacular demonstration of an exothermic reaction. It produces so much heat that the iron which is a product of the reaction is in the molten state and pours out of the bottom of the flower pot. Not only are when demonstrating an exothermic reaction meaning giving off heat. We are also demonstrating the reaction being a spontaneous process. When you add the glycerin and you add the Potassium permanganate together it spontaneously generates enough heat to initiate the other reaction. In this unit of thermo dynamics you will be learning about entropy and free energy which are used to predict if a reaction will take place spontaneously.
