Melting-Why do intermolecular bonds weaken?

[FireBones]

When describing change of phase, it is very commonly said that molecules in hotter materials vibrate more, weakening the intermolecular bonds within them.

My question is "How can increased vibrational velocity weaken these bonds?"

It certainly makes sense that greater vibration increases intermolecular spacing, and the strength of intermolecular forces decreases with spacing, but when you get around to analyzing the situation, the numbers are not compelling.

Take ice, for example. Average inter-molecular spacing is proportional to the inverse cube root of density. Heating ice from -10C to 0C decreases its density by about 1/10 of one percent. This means the average inter-molecular spacing goes down by about 1/30th of one percent.

I believe Hydrogen bonding decreases in strength as the inverse cube, but even if we were using weaker forces, with an inverse 6th, inverse 7th, or even inverse 12th power law, an increase of 1/30 of 1 percent in spacing leads to a decrease of less than 1 percent in the inter-molecular forces...hardly anything to hang a phase change on...


With that obvious answer out of the way, can someone explain how exactly a small increase in the average vibrational speed decreases the intermolecular forces significantly? [For example, the increase in average molecular velocity between -10C and 0C on average is less than 2%]

Note that nothing changes much even if I had picked -45C instead of -10C. The difference in densities is about 1/3 of one percent between ice at -45C and ice at 0, meaning the average spacing has only changed by a factor of 1.001 or so.

 

[Borek]

I don't think it is correct to say they weaken. I would say they are about the same, just kinetic energy of molecules becomes high enough to break them.

 

[FireBones]

Lindemann's theory of melting does indeed suggest the molecular forces are not significantly different, but Max Born's model was completely dependent on that idea.

Neither of those models have stood up to 70 years of scientific testing, but one would hope that neither is completely wrong.

Furthermore, there must be something to this because amorphous solids soften significantly with applied temperatures, as do metals. For example, iron will lose 2/3 of its strength when heated from about 300C to 650C, but it's average atomic separation only increased by 0.3 %

Anyway, many standard texts claim that the intermolecular forces are weakened, though it must be admitted that I am having trouble finding standard "intro to college chemistry" text that says that. I've found it in more multi-disciplinary texts, though.

I'd be interested in others' thoughts/insights or if people could take a quick look at their own chemistry texts to see what they say.

 

[Mapes]

I've also heard only what Borek's heard: not that increased vibrations weaken the bonds, but that the bonds are more frequently broken at higher temperatures.

 

[robert80]

I believe that the spacing is not the main issue. Issue when you rise the temperature or add energy is simply that the quantum statistical sum of all states is bigger. So you get states, which do not even exists at lower temperature OR lower energy. So to explain this with spacing only or the model with lower energy is wrong.

 

[Borek]

States DO exist, they can be not populated.

 

[robert80]

Yes Borek, you are absolutelly right. Sorry.

 

[robert80]

Or better Thay exist, they are populated, but with lower energy, the probability for to find much higher molecular quantum state could be seen as 0. so population on the states over the limit could be so small that we can throw it out from the equation. But they are populated in minor part. let's imagine tunel quantum effect. Higher the potential difference, more particles have to hit the potential difference in order to come to another side.