hi there everybody and welcome back to AP Chemistry my name is Jeremy kug and in this video we're continuing Unit Nine this time with topic six which is about the free energy of dissolution if you're new to my channel take a look around the AP Chemistry complete course playlist and uh all the other videos I have to get you ready for the AP exam and don't forget about my ultimate review packet which is found at ultimat review packet.com which is a complete review and practice program to get you ready to score five on that exam now let's jump into the free energy of dissolution let's imagine that we have two substances right here we have potassium chloride and we have silver chromate now I have the dissociation equations for both of these substances so let's take a look at the Gibbs free energy changes for these two processes now the first one here KCl potassium chloride its Delta G value is negative 5.3 K per Mo of reaction now the fact that that number is a negative value tells me that this process is thermodynamically favored and so that means that we'd say that potassium chloride is going to be soluble in distilled water at standard conditions which are you know 25° C one atmosphere pressure and so forth now notice that the other process here silver chromate has a posit positive value for Delta G the fact that that Delta G value is positive tells me that this process is not thermodynamically favored so that means that we would in all likelihood say that silver chromate is not going to be soluble this is in fact an insoluble substance now let's think about why this is the case it's one thing to calculate the Delta G values for these substances but it's another thing to talk about why the Delta G is you know positive or negative for these two substances well let's take a look at the pottassium chloride first of all now as we've already learned in unit 9 there are two driving forces that tell us if a process is going to be thermodynamically favored or not and those two forces are enthalpy and entropy let's take a look at the Delta H and Delta s values for this process now for this process Delta h has a value of positive 17.1 K per mole now that's endothermic isn't it now we've already learned in this unit that the universe doesn't really like endothermic processes does it and so the fact that this Delta H is endothermic is not favorable that does not contribute to the solubility of this substance but how about the entropy change well the Delta s for this process is a positive 76.4 JW per Mo per Kelvin and we know that the Universe likes it when the entropy is increasing that is favorable that would contribute to the favorability of this process and so what we can say is even though this is an endothermic process the fact is that the change in entropy for this process is so favorable that it can actually counteract the endothermicity of this process okay the fact that it's endothermic uh is okay because the entropy is increasing by so much and so yes we can say that KCl is going to be soluble because of that now let's go to the other example that we had on here the silver chromate now once again we're going to look at those two driving forces the enthalpy which is Delta H and the entropy which is Delta s now the enthalpy change for this process is positive 61.7 KJ per mole and we know that that's endothermic isn't it and the universe really doesn't like endothermic processes this is not favorable toward the thermodynamic favorability of this process well how about entropy well we can crunch the numbers here and see that the entropy change for this process is -22 Jew per Mo per Kelvin and we know that the universe doesn't like entropy going down either so that's that's also not favorable so since both of the driving forces are saying not favorable it is safe to say that silver chromate is not really going to dissolve very well under any circumstances that's why we can say this Delta G is absolutely a positive value now before we go on here there's something I need to to point out that I know some students will take a look at this and they'll question this we just said that the Delta s is a negative value here and this might be the opposite of what you might be thinking because we know that a solid turning into aquous ions that's normally considered an increase in entropy isn't it well it's not always the case it's not always the case the fact is that some dissolution processes actually cause a decrease in entropy so that rule that we learned you know early on in this unit is not a hard and fast rule the fact that we have I surrounding these water molecules in solution may actually be a more orderly State than having that original crystal structure in some cases that actually works out that way and so it is possible for the entropy to go down like we see here now let's take a look at another process here let's take calcium chloride now calcium chloride is a substance that as we know is quite soluble if you've ever had a chance to work with that in the laboratory and here's the equation for its dissolution and once again we're going to take a look at the two driving forces enthalpy Delta H and entropy Delta s so the enthalpy for this process is a 81.4 k per mole now what does the universe think about this well guess what the universe likes exothermic processes doesn't it so that is favorable so you know that's something going in favor of calcium chloride being soluble now how about the change in entropy well that's a negative 45.1 Jew per Mo per Kelvin here's another one of those cases where you know we're dissolving this in water and it actually causes the the entropy to go down well guess what the universe doesn't like that does it that would be an unfavorable thing well guess what we know that if we crunch the numbers here we can find that the Delta G is actually a negative value this is a favorable process and the reason for that is even though the entropy is going down the change in enthalpy the fact that this is an exothermic reaction is so favorable that it can actually counteract that decrease in entropy of this process and so that's why we say that calcium chloride is soluble in fact it's very soluble in water as we can see from that Delta G value right there being a negative number now let's work a couple of examples here a couple of problems as we uh wrap up uh this part of unit 9 let's consider the equation for the dissociation of barium sulfate so here we have the equation and we're given it looks like a Delta G value and the Delta G is positive 56.8 K per mole so the question is is barium sulfate soluble in water and explain your answer well it goes back to the Delta G value since this is a positive number that means that this reaction this process here is not thermodynamically favored and so because that Delta G is a positive number it's greater than zero the process is not thermodynamically favored so we can say that barium sulfate is not going to be soluble in water at least not at standard conditions let's try one more example this time we're going to consider the equation for the dissociation of iron 3 chloride and there we have the equation and this time we're given a Delta s value it says that it is determined iron 3 chloride is significantly soluble in water is the dissociation equation above exothermic or endothermic and explain your answer well if the iron 3 chloride is soluble in water that means that the Delta G must be a negative value doesn't it so it's thermodynamically favorable and we know that the fact that entropy is decreasing this Delta s is a negative number that's not favored is it that's not something that's favorable toward this being thermodynamically favored so guess what the only way this process can be thermodynamically favored is if the driving force is the enthalpy this has to be an exothermic process as we see here and if you're not sure exactly why that's the case think about it in this way imagine if this were an endothermic process okay the universe doesn't like thermic processes and it doesn't like the fact that entropy is decreasing so if that were the case this process would not be thermodynamically favored at all but you know that's not the case it is thermodynamically favored so it has to be exothermic I know this is kind of an abstract concept I hope you learned something from this video hope you enjoyed it if you did please consider giving me a thumbs up I would really appreciate it and don't forget that you can continue checking out my videos here over unit 9 and if you're looking for a wonderful way to get ready for the AP exam here coming up in May then check out my ultimate review packet it's a great price it's very reasonable and you'll find all kinds of practice problems hundreds of practice problems that I have prepared to get you ready to get that five on the AP exam got full unit summary videos study guides practice to get you ready so thanks for watching this video and hope to see you next time God