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  • 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.

Hi. It's Mr. Andersen and this is chemistry essentials video 59. It's on using Gibbs Free

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B1 US spontaneous entropy delta reaction energy decrease

Using Gibbs Free Energy

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    李育真 posted on 2015/12/29
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