Endothermic vs Exothermic Phase Changes

[Jessica Lee 3D]

Could someone conceptually help me understand why vaporization, melting/fusion, and sublimation are endothermic reactions (and the reverse reaction of them are exothermic)? I could just memorize which process is which, but I think understanding why would help me apply this knowledge to more complex situations ;)

[Chem_Mod]

Hi Jessica,

Let's start by dissecting the terms endothermic and exothermic. Endothermic refers to processes that require an input of heat (adding heat). The Greek prefix "endo" means inside, and "thermic" refers to heat, so you can think of it as putting in heat. Exothermic refers to processes that release heat (losing heat). The Greek prefix "exo" means outside, so you can think of it as heat leaving the reaction.

Vaporization refers to liquid becoming gas. Melting/fusion refers to solids becoming liquids. Sublimation refers to solids becoming gases. All these processes are endothermic as you are adding heat to go from an ordered state to a very disordered state with more atomic movement (molecules can move more freely).

The reverse processes (condensation, freezing, deposition) are all exothermic because as you transition from a state of high energy (e.g., gas) to a state of low energy (e.g., liquid), you lose heat to the surroundings.

You can use these concepts to identify if other processes are endothermic or exothermic (consider if you are adding heat or losing heat). A quick and dirty way of identifying processes as endothermic or exothermic would be to check if the temperature of the substance/surroundings increase/decrease (e.g., an ice pack undergoes an endothermic reaction such that it absorbs heat from the swollen area so the temperature of the surroundings decrease while the temperature of the ice pack increases).

Hope that helps!

[Catherine Grigorian 3L]

The difference between endothermic and exothermic is that exothermic releases heat, so you can remember it as EXothermic, the heat EXits. For endothermic, it requires heat, so the heat goes in. Vaporization, melting, and sublimation, all share in common that the molecules inside go from a state of being more constricted, where it can not move as much, to a state of where the molecules move more. In order for this to happen, heat is needed to allow the higher energy, to allow for the molecules to move more. To provide more detail, lets look at melting more closely. In solid ice, the molecules cannot move, but in liquid they can move so you put heat in the system to get the molecules to move more, becoming liquid. For exothermic which include condensation, freezing, and deposition, in all of these processes, the substance goes from a state of more molecule movement to a state with less molecule movement. Therefore, to cause less movement, there needs to be less energy, so heat must be removed. The heat then leaves (EXit) the system and enters the surroundings/environment. For example, in freezing, the liquid water has more movement in its molecules and ice has less movement, therefore you must release heat/energy in order for freezing to occur.

[Ani Eulmessekian 3L]

Vaporization, melting/fusion, and sublimation are all endothermic processes! This is because each of these phase changes require the breaking of bonds. (E.g. Melting = Solid to liquid requires breaking of bonds, Vaporization = Going from liquid to gas requires breaking bonds, Sublimation = going from solid to gas requires bond breaking). These phase changes require a higher energy input in order to break their bonds, and thus need energy supplies to them (endothermic). The opposite can be stated for their reverse reactions. Exothermic reactions release energy when forming bonds. The "reverse" reactions for the phase changes listed above are going from a higher to lower energy state when creating new bonds, thus they release energy.

[marika roberts 3l]

Hello! So you are correct in saying that vaporization, melting/fusion, and sublimation are all endothermic reactions. To explain it simply, this is because in order to undergo these processes, the system requires energy in the form of heat. It helps me to picture at a molecular level. If you think of a solid becoming a liquid, the essentially static molecules are now moving much faster. Such a change even visually looks like an increase in energy. If you think of the endo in endothermic as "in", these types of changes need energy to go "in"to them to get things moving. On the flip side, the reverse change from liquid to solid would look like a slowing down of the molecules. We know about the law of conservation of energy and that energy can't just disappear so it has to somewhere. I think of it at "ex"iting the system so it is "ex"othermic.