Endothermic and Exothermic Reactions

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SPEAKER: Let's take a look at exothermic and endothermic reactions. Heat flow in reactions is important because heat flow is an important indicator that helps us understand whether or not a reaction will happen at all. A good simple example of heat flow in reactions is a match. Let's look at the heat flow in this reaction. In this reaction, the chemicals on the match head constituted what we call the system. That's where all the action was the system was releasing heat into the surroundings. That was the air surrounding the match. My hand is nearby. My body is nearby. The heat goes into the room. That's the surroundings. And the system and the surroundings constitute everything that we care about. That's the universe. Enthalpy is the thermodynamic term that we use to keep track of heat flow in reactions. In exothermic reactions, the value is going to be negative or less than zero in exothermic reactions. In endothermic reactions, heat is entering the system from the surroundings. Again, the system and the surroundings constitute the universe. The enthalpy, the thermodynamic term here is going to be positive. It's going to be greater than zero. So in the case of the match head, that was an exothermic reaction. And an example of an endothermic reaction would be melting ice, putting heat into a system. So now that we've covered exothermic and endothermic reactions, let's look at a chemical reaction and see if we can figure out the heat flow in this process. In this reaction, we're going to add a small crystal of sodium acetate to a solution that is supersaturated with sodium acetate. And you'll see that the temperature on the thermometer is 23, 24 degrees in this room. Let's go ahead and add that crystal. Take the lid off here just so you can see that it's liquid. Things are about to change. Let's take a small crystal and put that in there. And what you see is things are precipitating to the point where I can even flip this upside down. And it's solid. Now the real question is, what happened with the heat flow? Let's pull our thermometer out. Let's stick it in there and see what our thermometer reads for us. What do you see? So now, given what you know, is this an exothermic or endothermic reaction? So we can see that the temperature has increased. Heat in the chemical system was leaving. We've generated heat here. This is an exothermic reaction. And the enthalpy for the reaction will be a negative value, less than zero. And let's take a look at another chemical reaction and see if we can also follow the heat flow there. In this flask, there is barium hydroxide. And in this little tray here, we have ammonium nitrate. And we're going to mix these two solids together and stir them up. And I'm gonna try to figure out the heat flow in this process. Right here I've got a board. And the board, we've left wrapped in a wet towel. And I'm putting this flask on there. And I'm gonna stir up these chemical reactants. Remember that the chemical reactions constitute the system. The system is this white stuff that's gonna turn into a slush. We're gonna have to stir here for a few minutes. So we've been stirring for a few minutes here. And our reactants have turned into a slush here. The reaction has occurred. And really what we're dealing with here is products. The question is, was this an exothermic or endothermic reaction? The answer to this question can be tough to figure out sometimes for new students. When they're thinking about the heat flow, it looks like the heat has left. It's gone away. And so sometimes people would think that this was an exothermic reaction. What's actually happened is that heat was used to make and break bonds in the system. Those chemicals reacted. And they needed the heat in the surroundings to form the products. So this is an endothermic reaction. Let's look at the chemical equation. The white powders were barium hydroxide, octahydrate, and ammonium nitrate. And they reacted to form barium nitrate and ammonia. And we would say that the delta H for this reaction is greater than zero. It is an endothermic reaction. Heat was entering the system to form the products. Enthalpy is an extensive property. An extensive property means that the amount of heat evolved in this case for our reaction is dependent on the amount of the materials. So if we wanted to make this reaction even colder, we could increase the amounts that we used. So for example, with water, if you have water and you put heat into the water to turn it into steam, it requires some energy. The enthalpy is 44 kilojoules per mole. So what would you predict the energy or the heat required to heat 2 moles of water and convert it into steam? This question may seem like a simple one. But it checks our understanding that enthalpy is an extensive property. The solution to this problem is embedded in the enthalpy units. So we can think of the enthalpy value, 44 kilojoules per mole, as being related to the chemical equation as it's written. In the equation, 1 mole of water is being converted to 1 mole of steam, or mathematically, 1 mole times 44 kilojoules per mole equals 44 kilojoules of energy. So for 2 moles of water, 2 moles times 44 kilojoules per mole is 88 kilojoules per mole. So the point here is that the heat energy or enthalpy, the energy required to make this reaction go, is dependent on how much of the material we have.