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Endergonic, exergonic, exothermic, and endothermic

Endergonic, exergonic, exothermic, and endothermic reactions.

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Video transcript

- [Voiceover] So we have some words here that relate the different reactions and whether they absorb or release different types of energy. So the first word here, exothermic. Exothermic the root of the word is therm which relates to heat and these word indeed means a reaction that releases heat. Releases, it releases heat. And one way to think about it if you're thinking about constant pressures or change in enthalpy it can be viewed as your, how much heat you absorb or release. So a negative change in enthalpy means that you're releasing heat. One way to think of, if you view enthalpy as heat content you have less heat content after the reaction than before it was meant you release heat. Which means you're change in enthalpy is going to be less than zero so these all mean the same thing. Well, this is true. You're releasing heat. This is a same thing as releasing heat if you talk about constant pressure. Constant pressure which is a reasonable assumption if you're doing something in a beaker that's open to the air or if you're thinking about a lot of different biological systems. Now based on that logic what do you think this word means, endothermic. Well endothermic, therm same root and now your prefix is endo so this is a process that absorbs heat. Absorbs heat. Or if you're thinking of a constant pressure, you can say your enthalpy after the reaction is gonna be higher than the enthalpy before the reaction. So your delta H is going to be greater than zero. All right, fair enough. Now let's look at these two characters over here. Exergonic and endergonic so exergonic the root here is ergon and you might not be as familiar with that as you are with therm but you might have heard the word ergonomic. Say, hey that's a nice ergonomic desk. That means it's a desk that's good to do work at or it's a nice ergonomic chair. An ergon does indeed come from the Greek for work. And so exergonic is a reaction that releases work energy or at least that's what the word implies. Releases, let me do that in the same color. This is something that is going to release work energy. And endergonic, same logic, well that's gonna be something based on just the way the word is setup that absorbs work energy or uses work energy. Now one of our variables or properties that we can use to think about energy that can be used for work is Gibbs free energy and the formula for Gibbs free energy, if we're thinking about constant pressure and temperature, so let me write that down. So if we're talking about constant pressure and temperature then the formula for Gibbs free energy or you can even view this as a definition of Gibbs free energy. The change in Gibbs free energy, let me do this in another color. The change in Gibbs free energy is equal to our change in enthalpy minus, use in the different color. Minus our temperature times our change in entropy and if this looks completely foreign to you, I encourage you to watch the video on Gibbs free energy but the reason why this is related to energy for work is okay, look I have my, whether I'm absorbing or I'm releasing heat and I'm subtracting out entropy which is kind of the energy that is going to the disorder of the universe and what's left over is the energy that I can do for work. That's one way to think about it. So you can see that this relates work energy to change in enthalpy right over here. So exergonic, something that releases work energy could say that has less work energy after the reaction than before it, your delta G is going to be less than zero. So let me write that down. So here our delta G is going to be less than zero and these things, these are reactions that release work energy, we've seen it in the video on Gibss free energy. We consider this to be spontaneous. Spontaneous. These are going to move forward. So these over here, the ones that absorb work energy, well they're gonna have more work energy in the system than before is one way to think about it. So your delta G is going to be greater than zero and we say these are not spontaneous. So these are not spontaneous. Now that we have the definitions out of the way and we have a way to relate these variables, let's look at these different scenarios of things that are exothermic and exergonic or exothermic and endergonic and see why they make an intuitive sense. So in this first reaction it's exothermic, our delta H is less than zero. That means it has less enthalpy after the reaction than before which means it released heat and so you can see here, this heat is being released. And where did that energy come from? Well when it bonds in this new configurations on a net basis the electrons are able to go to lower energy states and release that energy. And heat, if you're thinking about in a microscopic scale it's something that's raising your temperature at least locally which it means just about transferring kinetic energy to these microscopic molecules. Remember when you're talking about heat or temperature you're thinking about these macro variables but on a microscopic variable, you're talking about kinetic energies and potential energies and things like that. So what's happening is these electrons or when they get into a new configuration and they're going to release energy and that can be transferred to the individual molecules. So you see here, we've released energy and we also have an increase in entropy. We have more entropy after the reaction than before the reaction. We have more objects right over here, there's more states in which they could actually be in and they're actually moving faster. So this one, we see if you just apply, if you apply the formula over here this is gonna be less than zero. This over here, delta S is going to be greater than zero. The temperature, that's gonna be absolute temperature in terms of Kelvin so it's always going to be positive and so this whole term is going to be positive so you're gonna have a negative, minus a positive it's going to be negative. So our delta G is going to be less than zero and we see that this is spontaneous. This is going to move forward and it makes sense, it releases energy, the electrons like it. It creates a more disordered state. Another way to think about it is think about trying to do the reaction the other way, you're gonna have to get some energy for those electrons to get into a higher energy state when they form these new bonds your gonna have to get these four constituents together in the exact right way. That seems a lot less likely to happen than going in from the left to the right. Now let's think about something that absorbs heat and this one's a little bit counter intuitive. It absorbs heat but it's still going to be spontaneous. It's still going to be exergonic. It's still going to happen. So delta H is greater than zero so it absorbs heat to happen. So I have these two molecules with these different constituents, they are about to collide and we're saying that the temperature is high. If the temperature is low, this might not be spontaneous but if the temperature is high enough it will be. So the temperature high in a microscopic basis, you're saying, okay these things just have them, really high kinetic energy, they're going to ram into each other really fast and they're gonna ram into each other so fast that they can form all these other constituents. So you have the net entropy, you have the net entropy has increased. Even though over here our electrons are in a higher energy state to form this configuration so it had to absorb heat, so it had to absorb heat energy. So we could say heat but heat on a microscopic level, we're just talking about kind of kinetic energy of these molecules. So it have to absorb it but where did that energy come from? Well, it came from the kinetic energy of the molecules. They might had a certain kinetic energy before but then some of that gets lost so when they all get banged up into their different configurations. If you're saying, well I still don't get this. Think about trying to do this reaction the other way. Try to get these four constituents in the right time, all together, even though if they're happening, if they're put together at the right way their electrons could configure in a way to release energy but this is super high temperature. This is a really, really chaotic system. It's not gonna go from right to left, it's gonna go from left to right. When it's really chaotic, things are banging each other really fast, you're more likely to go on a direction of higher entropy. So now let's look at, and so this is spontaneous even though it absorbs heat. If you're not draining the heat away locally, your temperature at least around these molecules it will go down. But as source we're assuming constant temperature for this so you can assume that in a macro level that temperature dissipates and gets absorbed outside of the system somehow. Now, let's look at this configuration. It's exothermic so delta H is less than zero, less enthalpy after the reaction than before so it's releasing heat but it's not spontaneous. And it's not spontaneous because it's reducing the entropy in the world. It's reducing the entropy in the world and the entropy matters because our temperature is high. One way to look at this equation is entropy doesn't matter when temperature is low. Temperature is really scaling your entropy but when temperature is high, entropy start to take over. This variable starts to matter a lot. So over here, because entropy is negative, this isn't gonna, this thing's not gonna actually happen. So if these things were coming together very slowly, their electrons could configure in just the right way so that they can get to a lower energy state and release energy. But they're buzzing pass each other so fast that they're not gonna have a chance to do that. If you think about it the other way, this reaction is much more likely to happen. If you have a bunch of this diatomic molecules running around, they're gonna bump into each other so fast that they're gonna knock the constituents off of these diatomic molecules or at least the way it's depicted, it kinda looks like a diatomic molecule. And they might absorb some of that kinetic energy in doing it, in order to go from right to left but that's more likely to happen. So from left to right not spontaneous because entropy really matters at this high temperature. Then finally, and this one's pretty intuitive something that needs heat, something that needs heat energy and has a reduction in entropy that's definitely not going to be spontaneous. So this is greater than zero, this is less than zero but then you're subtracting it so this whole thing is greater than zero, this delta G is going to be greater than zero. Delta, let me do that in a green color. This delta G is going to be greater than zero and it makes sense that you have these two molecules that you have to get together in just the right way. They need heat in order to proceed with this reaction to kind of excite, to excite the electrons to higher energy state to get into this, I guess you could say less stable bond. Why would they do that? The reaction is much more likely to go in this way or if you had a bunch of these molecules they're all knocking into each other, they get into a more stable configuration and there's more entropy when they split up than when they actually stay together. So delta G greater than zero. This is endergonic and endothermic and of course, this one was delta G greater than zero. Even though this would release energy that the things that's so chaotic they're not gonna have a chance to do that and you're much more likely to go in the direction of maximizing entropy and so this one also is not spontaneous.