Heat Change During Phase Changes

[Karrin Evans 1C]

Hi! I have another conceptual question about phase changes. We know that, in order for bonds to break during melting/boiling, heat has to be applied to the system. We also know that this heat is applied directly to those bonds, so the heat of the sample itself doesn't change. I'm confused about why that's the case because it doesn't seem to match my experience with hot liquids. For example, when I stick my finger in shimmering oil, it hurts less than sticking my finger in oil at its smoke point. If the temperature of the oil isn't changing as it goes from liquid to gas, then what is?

[SanthoshNandakumar3L]

^^^ Hi, I am also confused on the difference between "Heat" and "Temperature." Because temperature can remain constant even though Heat is being added. Can someone please clarify this concept. Thank you.

[Madeline Redmond 2A]

The temperature of a sample doesn't change when the phase is shifting from solid to liquid or liquid to vapor because the energy of the heat that is being added to the system is going to break the bonds. The reaction of vaporization or melting are endothermic, meaning that they absorb heat when the reaction takes place. This means that when heat is added to the system, the energy that the heat provides is used to break the bonds of the solid or liquid sample and turn it into liquid or vapor respectively while the temperature remains the same.

[Phoebe 1H]

So the system provides just enough heat in order to break bonds and not change the temperature every single time? How is that possible? Or is there a quantifiable number that is associated with the amount of heat/energy that is used to break bonds? and doesn't it depend on how many bonds and what type of bonds there are?

[Katherine Phan 1J]

So the system provides just enough heat in order to break bonds and not change the temperature every single time? How is that possible? Or is there a quantifiable number that is associated with the amount of heat/energy that is used to break bonds? and doesn't it depend on how many bonds and what type of bonds there are?

The quantity that gives the amount of heat/energy used to break bonds is called the enthalpy of vaporization (or fusion or sublimation). Usually, it is given in units of kJ/mol, so it depends on the moles of the substance. It does not depend on the number or type of bonds because phase changes are physical changes, so you are not breaking the intramolecular bonds that hold atoms together within a molecule. Instead, the energy is used to break the intermolecular forces between the molecules themselves.