Changes of state during a decrease in temperature

[_V1qY]

My science teacher is not very good at explaining things, and I need help.

He set us homework about changes in temperature, one question "Explain why the temperature of the wax remains constant during solidification".

In the lesson, he explained how as a solid is increasing in temperature, when it reaches a certain point (0 degrees Celsius for water) the energy being used to heat the solid stops being used to heat the solid, and instead, is being used to break down the intermolecular bonds to change the solid to a liquid.

This is fine, but what I don't understand, and I need to understand for this question, is: when a liquid, in this case molten wax is cooling, it has a loss in thermal energy. So where the fuck is it getting the energy to change state?

 

[_V1qY]

OK, so now a friend told me that's it's because it's an endothermic reaction, meaning that it absorbs heat from its surroundings. Does this mean a change of state can never occur in a vacuum as there is no energy to absorb?

 

[PityOnU]

My science teacher is not very good at explaining things, and I need help.

He set us homework about changes in temperature, one question "Explain why the temperature of the wax remains constant during solidification".

In the lesson, he explained how as a solid is increasing in temperature, when it reaches a certain point (0 degrees Celsius for water) the energy being used to heat the solid stops being used to heat the solid, and instead, is being used to break down the intermolecular bonds to change the solid to a liquid.

This is fine, but what I don't understand, and I need to understand for this question, is: when a liquid, in this case molten wax is cooling, it has a loss in thermal energy. So where the fuck is it getting the energy to change state?

I'm going to guess that the state change from solid to liquid is endothermic, while the state change from liquid to solid is exothermic.

It would also stand to reason that particles in a liquid have a lot more random energy than those constricted in solid form. That energy needs to be dispersed for it to become a solid from a liquid, in this case in the form of heat, which stalls the cooling process.

No expert, but it stands to reasons. Maybe some chem-heads here want to pitch in.

 

[_V1qY]

I'm going to guess that the state change from solid to liquid is endothermic, while the state change from liquid to solid is exothermic.

It would also stand to reason that particles in a liquid have a lot more random energy than those constricted in solid form. That energy needs to be dispersed for it to become a solid from a liquid, in this case in the form of heat, which stalls the cooling process.

No expert, but it stands to reasons. Maybe some chem-heads here want to pitch in.

Thanks, for confirming me there. But does anyone know the answer to my second post?

 

[JustChillin1414]

I'm currently a first year chemistry student so I hope I can explain this properly.

This is a diagram for water, as you can see the water requires energy to change states, but the temperature of the water itself does NOT change during the state change, this is because it is using all the energy to change the state, not to heat the water.

Now pretend this is wax, from the liquid to the solid state the wax is releasing energy (I'm assuming the solid state has less energy), but this release doesn't cause a temperature change because it is causing the state change. In this case it would be an exothermic reaction (releasing energy).

If you wanted to prevent the change from liquid to solid you would have to prevent an energy decrease (stop it from cooling), i.e. putting the molten wax in a temperature controlled room which is above the freezing point.

I hope this helps!

 

[PityOnU]

OK, so now a friend told me that's it's because it's an endothermic reaction, meaning that it absorbs heat from its surroundings. Does this mean a change of state can never occur in a vacuum as there is no energy to absorb?

If you're heating it, that's energy to absorb, so yes it could melt in a vacuum. Think icy meteor passing near the sun.

 

[AbyssalMonkey]

No, there is still radiant energy that dissipates through a vacuum (electromagnetic waves are a great example), therefore I believe that energy and by extension "heat" can be lost in a vacuum and as such, a state change could occur in a vacuum.

Seems reasonable to me, but I don't have a source to back it up.

EDIT: Ninja'd

 

[Lacius]

This is fine, but what I don't understand, and I need to understand for this question, is: when a liquid, in this case molten wax is cooling, it has a loss in thermal energy. So where the fuck is it getting the energy to change state?

When molten wax cools, it's forming chemical bonds and releasing energy (exothermic). It's only when you're breaking a chemical bond (e.g. when ice melts) that energy is required from somewhere in order to do it (endothermic).

 

[AbyssalMonkey]

When molten wax cools, it's forming chemical bonds and releasing energy (exothermic). It's only when you're breaking a chemical bond (e.g. when ice melts) that energy is required from somewhere in order to do it (endothermic).

While this is mostly true at a fundamental level, I think that the OP is misunderstanding what a change in state is.

The period of time during which the state is changing you have both of the incoming and outgoing states at the same temperature. An easy example would be changing water to ice by reducing the temperature (lets say in a freezer). When the water hits the 0 degrees Celsius mark you start to have the forming of ice. During this process it makes sense that you would have both ice and water at 0 degrees because ice just doesn't freeze instantly (unless you have it supercooled, but that's another topic all together), and it is during this period that you have both ice and water that the temperature does not change. The reason is simple, if the temperature kept dropping at a steady rate, you would end up with -1 degrees water, which does not happen under normal circumstances; the reverse also holds true, you can not have ice at 1 degrees.

 

[_V1qY]

I think the issue was that I was mixing up exothermic and endothermic, which just confused me so much I was dismissing simple principles that I actually already knew and understood.

Thanks GBATemp, my #1 source for homework help!

 

[_V1qY]

So, to word an answer to the question: "Explain why the temperature of the wax remains constant during solidification", something like this should suffice?

"Because as the molten wax is cooling, when it reaches solidification point it's energy is instead of being released exothermically like before, it is being used to create intermolecular bonds to form the solid. This is known as a change in state."

 

[aalokishere]

AFAIK it should be the energy given out is from bond making process and so no temperature change.
FYI bond breaking needs energy, bond making gives it.

 

[FAST6191]

11 replies and nobody used the terms latent heat or entropy?

Also for what it is worth I would be careful when using plastics as a reference point/example for such problems as you have such concerns as the glass temperature. Though similar mechanisms are present in other materials (ductile to brittle transformation in metals for example) in plastics the order it happens in and what goes can make for some odd looking graphs when you are just trying to grasp the basic principles.

 

[AbyssalMonkey]

11 replies and nobody used the terms latent heat or entropy?

Also for what it is worth I would be careful when using plastics as a reference point/example for such problems as you have such concerns as the glass temperature. Though similar mechanisms are present in other materials (ductile to brittle transformation in metals for example) in plastics the order it happens in and what goes can make for some odd looking graphs when you are just trying to grasp the basic principles.

I thought about it, then I realized that there is no point in over complicating things with entropy, gibbs free energy, and enthalpy of state change.