Learn about Advanced Placement Chemistry, Thermodynamics 3, in this comprehensive video by bannanaiscool.
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Rob Lederer: Remember all those little half trues that we always have to correct somewhere along in the way. When you think you'd be done with that by now, but no, am sorry. You know when we were doing some energy questions before and we would calculate the Delta H, and then we would say well, how much ice you can melt with the heat provided from this reaction or something like that. And we always try to calculate by taking the amount of heat released in the Delta H, how much ice we can melt on things like that. Well, you know what? The amount of heat that comes off of an exothermic reaction, not all of that heat is going to actually be able to do useful work for us. Always, some heat in a reaction must be donated in order to increase the entropy of the molecules and increase the entropy of the universe. So, really you cannot get all the energy off of a reaction and directly apply it to a job. We are always losing the energy somewhere. that's why you can never make a perpetual motion machine, it can't happen. Entropy must actually take some of that energy and so, you know, the first laws of thermodynamics says, you know, you can't come ahead in terms of energy, you can only break even. The second law of thermodynamics says not, you can't even break even, sorry. So, look Gibbs comes up with his calculation to determine how much free energy can come off with the system. If you take the Delta H of a reaction, and then take away certain amount of energy that's going to be applied to entropy, then you'll get an amount of energy that can be used for useful work. that's called Gibbs free energy. So his free energy give out that free, freed up to do useful work that's his calculation and why this is going to be useful because, in this equation whenever you get a negative value for Delta G, a reaction is spontaneous, negative Delta G spontaneous reaction. So, if we know Delta H for reaction and we know the Delta S and scientists have actually given us charts. You probably have a data table in the back of your text board that has Delta S value, by the way when you don't write down anything after Delta S is system. So, you look out the system values for something, for a molecule, and then you actually arrive at whether -- Delta S for molecule order equation. You can arrive that whether a reaction is actually spontaneous or not. Let's look at one. So, here it is. You've got water solid turning into water liquid. You want to know if that is spontaneous at a temperature of minus ten degree celsius. You know that already, of course now water doesn't melt at minus ten; no, no. So here is the thing, we can actually prove that with some numbers and a formula too. Now, the Delta H for this reaction is 6.03 kilo joules per mole of water solid turning into water liquid. Also, I'm going to tell you that the Delta S for this is not something you can necessarily just look up, but you can see here, the Delta S is 22.1 joules per kelvin mole. Look at that. Entropy actually has a value, as a unit its joules per Kelvin. Delta H divided by T joules per kelvin per mole. So, I give you those values and say is it spontaneous at minus ten, plug it into the formula, Gibbs free energy formula and this is what you get. Delta G equals the Delta H here, but you got to turn it into joules, which is what I did in order for it to be able to add or subtracted from or subtracted from it, a joules a unit as well. So, make sure you do that. Be very careful of that. All the units are the same. Then the Delta S value goes into here, but the temperature has to be converted to Kelvins because, this unit is in Kelvins and has to be in Kelvins and you know what? It just makes perfectly good sense, if you had zero degrees Celsius that zero here, if you put it in for temperature times to the Delta S, which is blown away and give you nothing, but that's not good. You can't have that. The only time that you have absolutely no entropy in a system is at 0K, zero Kelvins, which is