Does Heat Really Weaken Intermolecular Forces During Melting and Boiling?

[PFuser1232]

Is it a misconception that adding heat into a system during melting or boiling weakens the intermolecular forces? Isn't it more appropriate to say that it separates the particles against their intermolecular forces, increasing their potential energy?

 

[dipole]

You're using the word heat and temperature interchangeably, and this is very very incorrect. Temperature refers to the way a system of particles are distributed among some set of energy levels or states, heat is a form of energy.

As far as your actual question, I don't see how the two statements really differ.

 

[BruceW]

I think Mohammed is saying that the phrase 'weakens the intermolecular forces' implies some kind of shielding of the electromagnetic field. This is not true (as I'm sure he knows). What's really going on, is that adding heat to the system will cause the temperature to increase. And the probability to find a molecule with a certain energy will be something like:
$$\exp(-E/T)$$
so, at higher temperatures, it is not uncommon to find the molecules with high energy. At very low temperatures, you will very rarely find a molecule with high energy.

 

[abitslow]

It is not a misconception that increasing temperature increases the equilibrium (average) distance in almost all (atomic/molecular) systems. It is a generalization. It has exceptions.
Bruce! Temperature remains constant at primary phase transition!
Heat is a term which is not uniquely defined. It is the (net) flow of energy as used most often, although it also commonly refers to certain types of energy, such as 'that which is not work', or 'that which is available to do work'.
The relationship between distance and force between two unbonded atoms is a curve which is extremely repulsive (call that positive) at very close distances, decreases towards zero and then increases again to a secondary (local) maxima before declining monotonically as distance increases to ∞. The local minima is the equilibrium state (more or less). MOST materials (but not all) expand when heated. If I add heat to an explosive, do I increase its potential energy?
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I don't think many would prefer your phrase " [heat] separates the particles against their intermolecular forces, increasing their potential energy". It doesn't capture the meaning very well at all. Heat is not a force, so this sentence suggesting that energy acts "against" a force is awkward.
Energy input into a system (via heating) by definition either increases kinetic energy or increases potential energy (or both, in some systems the separation of energy into P.E. and K.E. is impossible) So, of course heat added to a system increases potential energy in the system (keeping velocities constant). It is the rest of the statement that is a problem. If you are familiar with a harmonic oscillator then you should understand that force is instantaneous, and that increasing the period increases the energy (and so the rms force) so while the force at certain instants is actually HIGHER than at lower frequency, the distance is also increased on average.

 

[BruceW]

oh yeah. Thanks, slow. For some reason, I answered the question for what happens when not at a phase transition.