Boiling Point Confusion

[ilovemedi]

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In TBR it says that when salt is added to the surface of an ice cube, water molecules attract to the salt, and can be removed from the lattice (melting the ice). Then it says "this is an exothermic process, so additonal energy is released to melt the ice further." Ummm, isn't it an ENDOthermic reaction since melting = adding heat inside the system? Also in an earlier chart it had "Melting: Delta H > 0 ~ endothermic).

 

[empo]

In TBR it says that when salt is added to the surface of an ice cube, water molecules attract to the salt, and can be removed from the lattice (melting the ice). Then it says "this is an exothermic process, so additonal energy is released to melt the ice further." Ummm, isn't it an ENDOthermic reaction since melting = adding heat inside the system? Also in an earlier chart it had "Melting: Delta H > 0 ~ endothermic).

Ice melting is endothermic. Salt dissociating in water is exothermic, so ice would melt faster when this process is added.

 

[ljc]

In TBR it says that when salt is added to the surface of an ice cube, water molecules attract to the salt, and can be removed from the lattice (melting the ice). Then it says "this is an exothermic process, so additonal energy is released to melt the ice further." Ummm, isn't it an ENDOthermic reaction since melting = adding heat inside the system? Also in an earlier chart it had "Melting: Delta H > 0 ~ endothermic).

Melting is definitely endothermic. I think what they're saying here is that when NaCl dissolves, it releases energy (energy of solution). So the energy needed to break apart Na(+) from Cl(-) is less than the energy released when those ions are bonded to water. This extra energy then goes into helping to melt the ice further.

 

[MedPR]

I assume the exothermic reaction they are talking about is the dissociation of the salt. Whatever energy that puts out will be used in the endothermic melting of the ice. How else would salt melt ice? The only other way that we melt ice (usually) is to add heat.

You aren't adding heat when you pour salt on your driveway, so why would it melt? Something exothermic must be happening.

 

[chiddler]

Not exactly relevant, but I was wondering why ice is less dense than liquid water.

http://www.edinformatics.com/interactive_molecules/ice.htm

this answered it well, for anybody else who is curious.

 

[MedPR]

Not exactly relevant, but I was wondering why ice is less dense than liquid water.

http://www.edinformatics.com/interactive_molecules/ice.htm

this answered it well, for anybody else who is curious.

I prefer this method 🙂

 

[chiddler]

but how does that address density?

 

[MedPR]

but how does that address density?

Draw a line straight up from the triple point. What do you see? As pressure increases, there is a phase change from solid to liquid. Increasing pressure = decreasing volume = increasing density.

It's not a molecular explanation like yours, but it's a useful application of phase diagrams.