let's start combining what we're learning about now entropy and enthalpy which we know is going to have a big play in determining whether or not something is spontaneous all right so the question here would be why is water freezing below zero degrees Celsius vontavious even though entropy is decreasing all right so let's unpack this question water freezing what's that mean chemically what's going on with the change the physical channel solid liquid to solid right that's what's going on so water is going from liquid to solid alright and so the spontaneous or the entropy is decreasing it's going from liquid to solid right so liquid was increasing so liquid to solid is decreasing so our Delta s would be what positive or negative negative and it's negative but that happens right at zero degrees south on you've put liquid water in a freezer you come back at some point in time it will be frozen it'll be solid okay unless you're like ten years old and you're trying to make popsicles I don't like kool-aid it never freezes it takes forever to freeze you keep on looking it's still liquid chemistry breaks down sometimes okay but no so it's definitely going to freeze below zero degree Celsius bye and naba and the entropy is decreasing but last time we met about that last time we met last time we met we said that the second there's a second law of thermodynamics which says states that the entropy of the universe has to increase but now our entropy is decreasing well the good thing well the reason why is that this system water and liquid is not the universe that's just a physical process so if we're really going to talk about we have to put things into perspective or that so the Delta asked for the universe including Pluto Pluto still in the universe all right so the Delta ask the universe obviously the universe is big all right so we usually have to break things up and think about this a little bit more strategically so we could say the universe is our Delta s of our system so this water freezing is our system and then plus everything else what do we usually call everything else the surroundings yep so this is the reaction will be our system and so the change in entropy for the universe just consists pretty much just of the system and the surroundings we know that the system is negative Delta s for a system but for this to be spontaneous and it is the Delta s well let me put this up here Delta s in the universe has to be positive we have to follow the law so so far we've got a negative number this is negative I don't know what it is we can calculate it later but right now we just know it's negative so turns out this entropy of the surroundings has to be positive the entropy of the surroundings has to be positive for this to be spontaneous okay so how is that going to work well it turns out that's when we start to incorporate the other factor or our aunt will be change and it turns out the aunt will be change for the system is also negative alright okay so the Delta H the enthalpy change is negative yeah I just deal that put it so what's that mean if enthalpy is negative well well just what's that mean about the enthalpy change what we call a reaction with that's will be change of this negative x to a thermic so what's that mean yeah losing heat or transferring heat to the surroundings so the entropy is going down for this reaction but it's also transferring heat to the surroundings so let's draw that so here we have our reaction or system it's transferring heat to the surroundings what is that going to do to the surroundings increase the increase the enthalpy for surroundings so transferring Heat what happens when you give something heat increases the temperature the surroundings temperatures gonna increase right we're transferring heat to it temperature is going to increase so the temperature is gonna go up of the surroundings delta T would be positive yes what's going to happen to the entropy of the surroundings if our temperature of the surroundings is increasing it's going to increase yep remember that we saw that on this lot as we increase the temperature entropy goes up so the entropy of the surroundings is going up so what's my sign for change in entropy of the surroundings positive so that's where the positive sign comes in for our equation here and obviously since this is spontaneous that means this positive is going to be bigger than this negative so we're gonna get a positive answer for Delta s of the universe so that's how our enthalpy impacts entropy if the reaction is exothermic is going to cause the temperature of the surroundings to go up increasing the entropy of the surroundings what were to happen if the reaction is endothermic it's gonna absorb heat from the surroundings the surroundings is going to transfer heat to the system to the reaction what's going to happen to the temperature temperature of the surroundings it's going to go down so the entropy of the surroundings is going to go down so that's how Ansel P is going to impact the entropy it's going to impact the interview of the surroundings increasing it if it's exothermic decreasing it if it's endothermic