Phase Changes: Exothermic or Endothermic?

I'm wearing a hot pink shirt because we're going to her about heat in this lesson specifically we're going to talk about how heat moves during

phase

changes

and we're going to learn which

phase

changes

are

exothermic

and which

phase

changes

are

endothermic

so in this diagram here I have some

phase

changes

that are very familiar to you from daily life I have some solid ice a big hunk of solid ice melting to become liquid water and then I have this liquid water boiling to become

steam which is a gas so are these

phase

changes

endothermic

or

exothermic

let's think about what we have to do with heat to make them happen so I have my chunk of solid ice I've got to make it hotter in order to get liquid water and then when I get liquid water I've got to make that hotter and hotter and hotter in order to end up with steam now when we want to figure out with some of these extra thermic or

endothermic

sometimes it's not as useful to think about whether we need to

make it hotter or whether we need to make something colder instead it's best to think about the direction that heat is moving let me show you what I mean let's use this red arrow here to show the direction that heat moves okay so here I have the solid ice and when I want to turn it into water I've got to put heat into it okay so here these arrows show heat going and maybe this heat is from a stove maybe the heat is from the Sun maybe the heats from a fire or something like that but

whatever the source the heat is moving into the solid ice and melting it and then once I have liquid water I have to put heat into that in order to get steam again maybe the heats coming from the Sun or the stove or something like that but it's moving in so we can say that heat is moving from the environment into the ice heat is moving from the environment into the water so heat moves from environment into water here or if we wanted to sound more scientific about it we'd say that heat in

this case moves from the surroundings a fancy word for the environment into the system the system is just whatever want to focus on here the system is the ice here the system is a liquid water but in all of these cases when is moving from the surroundings into a system we are talking about an

endothermic

process you can remember this because n sounds kind of like in and that's just what happened what's happening heat is moving into the system it's moving into the water it's

moving into the ice

endothermic

now sometimes people get confused and I think we will wait I thought an

endothermic

process was one that like felt cold and here we're talking about having to make stuff hotter and somehow that's an

endothermic

process well sometimes things that are

endothermic

feel cold but sometimes they don't and instead of thinking about whether something feels cold or whether it feels hot it's best to always think about the direction that heat moves and

you'll never get this wrong and if heat is moving from the outside into something it is an example of an

endothermic

process let's look at some

exothermic

processes okay so what I have here is some steam condensing to form liquid water you may not be familiar that word but just a what we call it when steam becomes water condensing and now liquid water is freezing to make solid ice to do each one of these

phase

changes

we have to make it colder but instead of thinking about having to make

something hotter or something colder let's think about the direction here's my paper let's think about the direction that heat moves okay if we want to go from this really hot gas to this cooler liquid water we're going to want to move heat out of the steam okay and here are the red arrows showing the movement of that heat maybe the heat is leaving the steam and going into a freezer or refrigerator or something and similarly to go from liquid water and freeze it to make solid ice

we have to get heat out of it to cool it down so the heat is moving away from the liquid water in this case we're talking about an example where heat moves out of a system into the surroundings and when heat moves out the steam or the liquid water we call this an

exothermic

process ex meaning out of and that's the direction of heat movement in these processes okay so just to review in melting and boiling we have to put heat into the system and that means that it's an

endothermic

process on the other hand for condensing which is gas to liquid or freezing liquid to solid the heat has to come out of the system into the surroundings to cool it now we have to pull that heat out and that is an example of an

exothermic

process now in the examples that I did previously I was talking about water okay but this is no different with any other substance whether it's ethanol or methane or any anything you can think of melting and boiling we have to put heat in there

endothermic

condensing and freezing we have to take heat out it's

exothermic

water anything else it doesn't matter so don't think that it's just water now finally we can describe these

endothermic

and

exothermic

phase

changes

using some numbers as we've said previously an

endothermic

process is one where the change in enthalpy that Delta H is a positive number and the

exothermic

processes are ones where the Delta H is a negative number okay so check this out here is the chemical equation

for melting I'm going from solid h2o to liquid h2o and the Delta H for the reaction a positive number check that out so it has to be

endothermic

and the same thing for boiling which is liquid h2o to gas h2o liquid to steam the Delta H is a positive number again so it's an

endothermic

process condensing going from gas to liquid is a negative Delta H so it's

exothermic

and similarly freezing going from liquid to solid is a negative Delta H as well so that is how you can tell whether a

phase

change is an

endothermic

or an

exothermic

process if you're putting heat in from surrounding the system its

endothermic

if heat is going from system out into the surroundings to cool something down it is an

exothermic

process