Phase Changes: Exothermic or Endothermic?

I'm wearing a hot pink shirt because we're going to talk 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 are phase changes. They are

endothermic

, so in this diagram I have some phase changes that are very familiar to you in everyday life. I have some solid ice, a big chunk of solid ice that melts to become liquid water and then I have this liquid water boiling to become vapor, which is a gas, so these fish changes are

endothermic

or

exothermic

, let's think about it. what we have to do with the heat to make them happen, so I have my piece of solid.

I have to heat it more to get liquid water and then when I get liquid water. I have to heat it more and more to end up with steam. Now, when we want to determine if something is exothermic or endothermic, sometimes it is not so useful to think about whether we need to heat it up more. or if we need to make something colder instead, it's best to think about the direction in which the heat moves. Let me show you what I mean, let's use this red arrow here to show the direction that the heat is moving.

Well, here I have this solid. ice and when I want to turn it into water I have to put heat into it, so here these arrows show the heat coming in, maybe this heat is coming from a stove, maybe the heat is coming from the sun, maybe the heat is coming from a fire or something like that . like that, but whatever the source, the heat moves to the solid ice and melts it and then once I have liquid water, I have to heat it to get steam again, maybe the heat comes from the sun or the stove or something like that. like that but it is moving inward, so we can say that heat is moving from the environment towards the ice, heat is moving from the environment towards the water, so heat is moving from the environment towards the water here or if we wanted to sound more scientific about it, we would say let's say the heat in this case moves from the surroundings a fancy word for the environment into the system the system is simply what we want to focus on here the system is the ice here the system is liquid water but in all these cases when heat is moving from the environment into a system, we are talking about an endothermic process, you can remember this because n sounds like n and that is exactly what H, what is happening, the Heat is moving into the system, it's moving into the water, it's moving into the endothermic ice.

Now, sometimes people get confused and think wait, wait, wait. I thought an endothermic process was one that felt cold and here we are talking about having to heat things up and somehow that is an endothermic process, well sometimes things that are endothermic feel cold but sometimes they don't. and instead of thinking if something feels cold or hot, it is better to always think about the direction in which heat moves and you will never be wrong, and if heat moves from the outside towards something, it is an example of a process endothermic, let's look at some exothermic processes.

Well, what I have here is some vapor that condenses to form liquid water. You may not be familiar with that word, but it's just what we call it when steam turns into water that condenses and now liquid water. is freezing to make solid ice to make each 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 is my article, Let's think about the direction in which heat moves. If we want to go from this really hot gas to this cooler liquid water, we're going to want to get the heat out of the vapor, okay, and here are the red arrows showing the movement of that heat, maybe the heat is coming out of the vapor and going. in a freezer or refrigerator or something like that and similarly, to go from liquid water and freeze it to make solid ice, we have to take heat out of it to cool it, so that the heat moves away from the liquid water in this case we are talking about. about an example where heat leaves a system to the surroundings and when heat leaves vapor or liquid water we call it an exothermic process just for review, when melting and boiling we have to put heat into the system and that means it is an endothermic process, on the other hand, for condensation, which is from gas to liquid or freezing from liquid to solid, the heat has to leave the system towards the surroundings in order to cool it we have to remove that heat and that is an example of an exothermic process.

Now in the examples I did above I was talking about water, okay, but this is no different with any other substance, whether it's ethanol or methane or anything else. anything you can think of to melt and boil we have to put heat in, they are endothermic, they condense and freeze, we have to remove the heat, it's exothermic water, anything else, it doesn't matter, so don't think it's just water, now finally we can. Describe these endothermic and exothermic phase changes using some numbers, as we have said earlier, an endothermic process is one in which the enthalpy change, Delta H, is a positive number and exothermic processes are those in which Delta H is a negative number, okay, so check.

This here is the chemical equation for melting. I go from solid H2O to liquid H2O and the Delta H for the reaction, a positive number. Check that, so it has to be endothermic and the same for boiling, which is liquid H2O to gas liquid H2O to vaporize the Delta H is a positive number again, so it is an endothermic process, the condensation when going from gas to liquid It's a negative Delta H, so it's exothermic, and similarly, freezing when going from liquid to solid is also a negative Delta H, so, uh, that's how you can tell if a phase change is a process. endothermic or exothermic.

If you are putting heat around the system, it is endothermic. If heat leaves the system to the surroundings to cool something, it is an exothermic process.