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Current time:0:00Total duration:14:00

Boiling point elevation and freezing point depression

Video transcript

let's think about what might happen to the boiling point or the freezing point of any solution if we start adding particles where we start adding solute to it and for I guess just our visualization let's just think about water again doesn't have to be water it can be any solvent but let's just think about water in its liquid state the particles are reasonably disorganized but they're there because of their kinetic energy but they still have that hydrogen bonds that that wants to make them be near each other so this is in the liquid state and they have a reasonable amount of kinetic energy you know each of these particles is moving in some direction rubbing against each other bouncing off of each other now to move it into the solid state or to freeze it what has to happen the ice has to enter kind of a crystalline structure it has to get pretty organized so let's say it has to look something like this the water molecules are gonna have a regular structure where the hydrogen bonds dominate any kind of kinetic movement they want to do and all the kinetic movement they're just vibrating in place so you have to get a little bit orderly right there alright and then you know obviously this lattice structure goes on and on with a gazillion of of water molecules but the interesting thing is is that this somehow has to get organized so what happens if we start introducing molecules into this water let's say the example of sodium actually I won't do any example let's just say some arbitrary molecule if I were to introduce it there if I were to put something let me draw it again so now I'll just do that same I were to introduce some molecules and let's say they're pretty large so they bump they push all of these water molecules out of the way so the water molecules are now on the outside of that and let's say I have another one that's over here some relatively large molecules of solute relative to water that's because the water molecule really isn't that big now do you think it's going to be easier or harder to freeze this are you going to have to remove more or less energy to get to a frozen state well because these molecules they they're not going to be part of this lattice structure because frankly they wouldn't even fit into it they're actually gonna make it harder for these water molecules to get organized because these you know to get organized they have to get it you know the right distance for the hydrogen bonds to form but in this case even as you start removing heat from the system you know maybe the ones that aren't near that aren't near the that aren't near the the solute particles they'll start to organize with each other they'll start to organize with each other but then when you introduce a solid particle let's say a solid particle sitting right here it's going to be very hard for someone to organize with this guy to get near enough for the hydrogen bond to start taking hold so these would this distance would make it very difficult and so the way I think about it is that these solute particles make the structure irregular they add more disorder and we'll eventually talk about entropy and all of that but they they make it more irregular and it's making it harder to get into a regular form and so the the intuition is is that this should lower the boiling point or make it oh sorry lower the melting point so solute solute particles make it a make you have a lower lower boiling point or you have to go let's see if we're talking about water at standard temperature and pressure or at one atmosphere then instead of going to zero degrees you might have to go to negative 1 or negative 2 degrees and we're gonna talk a little bit about what that is now what's the intuition of what what this will do when you when you want to go into a gaseous state when you want to boil it so the you know my initial gut was hey I'm already in a disordered state which is closer to what a gas is so wouldn't that make it easier to boil but it turns out it also makes it harder to boil and this is how I think about it remember everything with boiling deals with what's happening at the surface and we talked about that when our vapor pressure so at the surface we said you know if I have a bunch of water molecules in the liquid state we knew that although the average temperature might not be high enough for for the water molecules to evaporate that there's a distribution of kinetic energies and some of these water molecules on the surface because the surface might be going fast enough to escape and when they escape into the into vapor then they create a vapor pressure above here and if that vapor pressure is high enough they can kind of you can almost view them as linemen blocking the way for more molecules to to kind of run behind them as they block all of the other ambient air pressure above them so if these molecules start if there's enough of them and they have enough energy they can start to push back or to push outward is the way I think about it so that more guys can come in behind them so like I hope that lineman analogy doesn't completely lose you now what happens if you were to introduce solute into it you know some of the solid particle might be down here probably doesn't have much of an effect down here but there's some of it's going to be bouncing on the surface so they're going to be taking up some of the surface area and because and this is at least how I think of it since we're going to taking up some of the surface area it's going to be you're going to have less surface area exposed to the solvent particle or to the solution or the stuff that'll actually vaporize so you're going to have a lower vapor pressure you're gonna have a lower vapor pressure and remember your boiling point is when the vapor pressure when you have enough particles with enough kinetic and energy out here to start to start pushing against the atmospheric pressure when the vapor pressure is equal to the atmospheric pressure you start boiling but because of these guys okay I have a lower vapor pressure so I'm gonna have to add even more kinetic energy more heat to the system in order to get enough vapor pressure up here to start pushing back the atmospheric pressure so solute also raises the boiling point raises boiling point so the way that you can think about it is solute when you add something to a solution it's gonna make it want to be in the liquid state more whether you lower the temperature it's gonna want to stay in liquid as opposed to ice and if you raise the temperature is gonna want to stay in liquid as opposed to gas and I found this neat hopefully it shows up well on this video have to give you credit this is from Kemper dude edu / g GC help / solutions / e boil that HTML but i thought it was a pretty neat graphic of or at least a visualization this is just the surface of water molecules and it gives you a sense of you know just how things vaporize as well as that there's are some things on the surface that just bounce off and here is an example where they visualize sodium chloride at the surface and because the sodium chloride is kind of bouncing around on the surface with the water molecules fewer fewer of those water molecules kind of have the the room to escape so the boiling point gets elevated now the question is by how much does it get elevated and this is one of the neat things in life is that the answer is actually quite simple the change in boiling or look or or freezing point so the change in change in temperature of us a temperature of vaporization is equal to some constant times the number of moles the number or the at least the mole concentration the molality times the molality of the solute that you're putting into into your solution so for example if I have let's say I have oh I don't know one one kilogram of so let's say my solvent is water I'll switch colors it's getting is I and I have one kilogram of water and let's say we're just at atmospheric pressure and let's say I have some sodium chloride na CL and let's say I have oh I don't know let's say I have two moles of NaCl so two I'll have two moles the question is how much will this raise raise the the boiling point of this water so first of all you just have to figure out the molality molality which is just equal to which is just equal to the number of moles of solute there's two moles divided by the number of the number of kilograms of solvent so let's say we have two kilograms of sorry one kilogram of solvent one kilogram this was of course moles so our molality is two moles per kilogram so we just have to figure out what this constant is and then we'll know the temperature elevation and actually that same Purdue site they gave a list of tables I haven't run the experiments myself they have some neat charts here but they say okay water normal boiling point is a hundred degrees Celsius at at standard atmospheric pressure and then they say that the constant is 0.51 to Celsius degrees per mole so let's just say 0.5 so it equals 0.5 so k is equal to 0.5 so they and I want to be very clear here because this is this is a very I won't say a subtle point but it's an interesting point so I said that there's two the molality of I just realized I made a mistake I said the molality of sodium chloride is 2 two moles per kilogram but that would be if sodium chloride stayed in it's in this in this molecular state if it stayed together right but what happens is that the sodium chloride actually disassociates and we learned all about in that previous video it each molecule each comp reach each sodium chloride pair disassociates into two molecules right into a sodium ion and a chloride chlorine anion and because of that because this is disassociates into two the molality is actually going to be two times the number of moles of sodium chloride I have so it's going to be two times this so my role ality will actually be four this is an interesting point if I was dealing with and I wrote it here so this right here is glucose and this is sodium chloride or at least slowly maclaurin is crystal form one one molecule I guess you can view it as or one salt of it I guess you could just view it as one of these little pairs right here but the interesting thing is is you could have the same number of moles of sodium chloride when you viewed as a compound and glucose but glucose when it goes into water it just stays is one molecule of glucose so a mole of glucose will disassociate into a mole of glucose in water I guess it won't disassociate it'll just stay as one mole while a mole of sodium chloride will turn into two moles because it's it disassociates it turns into two separate particles so in my example when I start with a mole of this I end up actually once I dissolve it in water and I ended up with two with a with a two with two moles per kilogram or sorry four moles per kilogram of molality because this turns into two particles so given that the molality is four moles right two moles of sodium two moles of chloride per kilogram so I just use that constant that I just got from Purdue and then I get the change in temperature is equal to that constant 0.5 0.5 times 4 which is equal to 2 degrees so my boiling point will be elevated by 2 degrees now if I had the same number of moles I've had two moles of glucose dissolved into my water I would only get half as much half of a much of an increase because the molality would be half as much because it doesn't turn into two particles in some textbooks you'll actually see it written like this you'll actually see the same formula written like change in boiling temperature or vapor per temperature or how would everyone think is equal to K times M times I where they'll say this is the molality of the of the compound you're talking about so in this case if this would this number would be two and then this is and then I is a number of of molecules or the number of things that it disassociates into so in this case this would have been two and that's where we would have gotten four times K which is 0.5 which is 2 in the case of water this would be I'm sorry in the case of the glucose this would still be 2 but it only turns into one particle when it goes into water so that would be 1 so you'd only have a one degree increase in the in the boiling point of water now freezing point is the same thing change in freezing point change in freezing point is also proportional to the molality and you can either say the molality of the original non in water compound times the number of compounds that disassociates into although these this K is going to be different for freezing as it than it is for for boiling and of course this KH ancient pressures and for different elements but the really big takeaway is is just to realize that even if you have a mole of this and a mole of that and they're going to be dissolved into the same amount of water because this dissociates into two particles and this dissociates into one for every or this is Associates into two moles for every mole of the crystal you have this only distillate this doesn't disassociate it just days as one this will have twice as large an effect on the freezing point change or on the boiling point elevation than the glucose will