3rd law of thermodynamic

1. Aug 2, 2006

ArielGenesis

well at my chemistry class i was taught that according to the 3rd law of thermodynamic, zero entrophy exist at absolute zero for perfect crystal structure. But somewhere outside the class i also notice about einstein-bohr condensation that says that at zero kelvin, all particles would come togather at one point and overlap one another and thus, by no means a perfect crystal would exist in zero kelvin. so is it simply saying that zero etrophy never exist even at absolute zero?

ps: so far according to my knowledge, absolute zero is impossible and thus, the question is neglecting this fact.

2. Aug 2, 2006

lalbatros

A few days ago, there was another thread here about "negative absolute temperatures".

Apparently, these are sometimes possible: when a system has less possible configurations at higher energies than at lower energies. Like in your question, the relation between the configuration number and the internal energy is the key point.

I don't understand the 3rd law completely. But I think it is reasonnable, in most cases, to assume that zero absolute temperature corresponds to only one possible configuration (up to the degeneracy).

Often, but not always, I quess, this would be a crystal. Indeed, wide-spread wave functions for each atom would mean higher momentum and energy: so minimum energy could often mean localisation in a crystal structure. But would expect that to be the general rule.

Michel

PS: 3rd law (from Ishan Barin, Thermochemical data of pure substances)

Lewis anti Randall: "If the entropy of each element in some crystalline state be taken as zero at T= 0 K, every substance has a finite positive entropy; but at T= 0 K the entropy may become zero, and does so become in the case of perfect crystalline substances."

Planck's formulation: The entropy of pure phases in internal equilibrium approaches a constant value (independent of pressure, phase state and crystal structure) as the temperature approaches
zero.

Nernst: All state changes (such as reactions and transformations between phases) take place in the vicinity of the absolute zero (T= 0 K) without a change in entropy, DS -> 0 as T-> 0 .
It is impossible to cool a substance to the absolute zero by means of a process that passes through a finite number of steps.

The entropy values based on the condition S(0 K) = 0 , are known as "Third Law entropies" or "absolute entropies".

Last edited: Aug 2, 2006