字幕列表 影片播放 列印英文字幕 Hi. It's Mr. Andersen and this is chemistry essentials video 59. It's on using Gibbs Free Energy which is essentially energy that's available to do work. So if I were to push this sphere right here, it's going to move down and it's going to move back. Where's the energy coming from? Well that would be gravitational potential energy being converted to kinetic energy. Eventually it comes to rest at the bottom. But if we think of this as an analogy it really explains what's going on in a chemical or physical process. What we have is reactants and we have products. Those reactants are moving towards products, spontaneously some are moving back. And eventually it reaches equilibrium. What's happened to our Gibbs Free Energy? It's gone to zero. And so we look at Gibbs Free Energy, it's a really good indicator of if we have a spontaneous reaction or a non spontaneous reaction. So if Gibbs Free Energy is ever negative or less than 0, then we know that we have a spontaneous reaction. We have a scenario that looks like that. Where we have energy and that energy can be released. If we have a delta G greater than 0 then we have a scenarios that looks something like this. We have an uphill reaction. It's not going to occur spontaneously. In other words we're going to have to put a little bit of energy in for it to work. And then finally we can have equilibrium in the middle. Now in the last video we learned that the two things that really contribute to Gibbs Free Energy is enthalpy and entropy. In other words those two things together can help us determine if it's a spontaneous reaction or a non spontaneous reaction. But it doesn't answer every question. In this video I'm going to try to answer every question. What's missing is our temperature. In other words T is going to be incredibly important. And so if we're trying to figure out if it's a spontaneous process or not, one of the biggest things is enthalpy or the amount of internal energy. And so if we have a delta H that's negative, that's a good indication that we're going to have a spontaneous reaction. So in a thermite reaction our reactants have more energy than our products. It's a downhill reaction. And so we'd expect this to occur spontaneously. Likewise if we were to rust iron, same thing, we're going to have more energy before then we do after. And energy is going to be released. That energy is going to be released into the surroundings. But sometimes you'll have exceptions to that rule. So like a cold pack, if you think about that it occurs spontaneously but in fact it's actually consuming energy. And so we're going to have a delta H that's going to be a positive value. And so we can't just stick with enthalpy by itself. We have to add entropy to that. So if I had these two spheres and I had gas on the left side and I just open it up, we're going to see that gas move from left to the right. So we're going to have this irreversible process. And so what's going on there, we're not changing the energy. What's really going on is entropy. And so these two things together, enthalpy and entropy are very important in helping up figure if this is a spontaneous process or not. So we could put it on a grid like this. And so if we ever have a decrease in enthalpy, so that's going to be an exothermic reaction, or one that increases entropy, we know immediately that's going to be a spontaneous reaction. Likewise if we have the opposite of that, if we have an increase in enthalpy and a decrease in entropy we know that's going to be non spontaneous. Now the nice thing about that is the reverse of that we can automatically figure out is going to be spontaneous. But what's going to be in these other two spots. What's going to happen, for example, when we have an endothermic process but we're actually increasing entropy. Or vice versa? And so a good place to look at that is just the movement of ice to water. So if we're looking at ice and water, if we look at the molecules of ice there's going to be a huge amount of order there. And a lot of that has to do with these hydrogen bonds. If we look in liquid water it's going to be moving around. And so if we were to move from ice to water what's that process? That's simply melting. That's going to be an increase delta H. What does that mean? That's going to be an endothermic process. It's taking energy from its surroundings. So we're going to have a delta H that's positive. What's happening to our entropy? Our entropy is increasing. In other words our matter is becoming more dispersed. So where would that be over here? We have an increase in enthalpy and we have also an increase in entropy. And so let's go in the opposite. Let's say that we're actually freezing that water. What's going on there? We have an exothermic reaction. So we have a decrease in enthalpy and we also have a decrease in entropy. And so this is going to this block right here. And so in other words if we could figure out this one slide it would help us unlock what's in these other two grids right here. And so what do you know? Well you know that if we take ice and we have it in an area where our temperature is greater than 0 degrees celsius, this is going to be a spontaneous reaction. In other words if you ever take ice and put it in an area where it's warmer than 0 degrees celsius, we know that ice is spontaneously going to melt. Likewise if we take that water and put it in an area where it's less than 0 degrees celsius, then it's going to freeze. And we also know that if it's exactly at 0 we're not going to see any change. It's going to be non spontaneous in either direction. So we've really answered this question right here. So if we were to look at melting, if we ever have an increase in enthalpy, so this is an endothermic reaction and an increase in entropy, that will be spontaneous as long as we have a high enough temperature. Likewise if we have a decrease in enthalpy, so it's an exothermic reaction and a decrease in entropy, then that's going to be spontaneous but only at low temperatures. And so now we can finally come to Gibbs Free Energy, that equation. And hopefully it makes sense at this point. So if we were to look at Josiah Willard Gibbs' equation, this is going to be delta G on the left side. Remember if it's ever less than 0 we know this is a spontaneous reaction or process. If it's ever greater than 0 we know that it's non spontaneous. And so it totally makes sense, this grid right here. If you have a decrease in this number, that's a negative value. And if you have an increase in entropy, so our delta S is going up, since we're subtracting it, that's going to give us a negative value. Likewise if we go down to this non spontaneous right here, what do we have? We have a delta H which is going to be a positive value. That's going to make this value go up. And then our delta S, since it's negative, we're subtracting the negatives, so that's going to be a positive. And so hopefully these two make sense. But now this should unlock this grid right up here. So what happens if we increase our enthalpy? Well if we're increasing our enthalpy you would start to think well this is going to increase our delta G. But if we have a high temperature that's going to make our entropy more important since we're subtracting that value. And so we can get away with a low enthalpy or positive enthalpy if we have a really high temperature and an increase in entropy. Likewise the same thing down here. In this case if we can have a decrease in this enthalpy, but our entropy even though it is going down, we have a really low temperature, it's not going to swing as much. Now this also unlocks that whole idea of a cold pack. What's going on there? Well we have an endothermic reaction. Again it's taking energy from its surroundings. Our delta H is a positive value. And so why are we getting a spontaneous reaction, since we have a delta H that's going to be a positive value? Well, we're moving from ammonium nitrate into ions of ammonium and ions of nitrate. And since we're doing that we're increasing our entropy. And so since our delta H value is going to be so big, then we're going to have a spontaneous reaction. We're going to have an overall delta G that's going to be a negative value. And so again, Gibbs Free Energy and that equation is really powerful because it tells us exactly what's going to happen in that process. If it's less than 0 it's going to be spontaneous. If it's greater than 0 it's going to be non spontaneous. If it's equal to 0 we're at equilibrium. And I hope that was helpful.