Signification of Thermodynamics Third Law

Signification of Thermodynamics Third Law

The third law of Thermodynamics stated that the entropies of substances at 0 k can be assigned the value of zero is the third law of thermodynamics.

The results suggest that all substances have the same entropy at absolute zero. (We will come, shortly; to a more careful statement to this effect generalization is related to another, quite different, observation.

Absolute zero is unattainable: a variety of chemical and physical phenomenon can best be studied at very low temperatures. As a result, techniques have been developed to produce these temperatures. Liquid nitrogen, which boils at 77 k at 1 bar pressure, is now a common industrial substance. Liquid nitrogen can be used to cool helium which can be compressed, cooled and expanded to yield liquid helium is 4 k. temperatures somewhat below 1 K can be obtained by boiling off some of the helium at reduced pressures. Still lower temperatures can be reached by magnetization demagnetization procedures, with liquid helium drawing off the released thermal energy. In this way temperatures down to about 10-4 K have been reached.

From such low temperatures studies comes the realization that any method that can be used to lower the temperature “peters out” as the temperature approaches absolute zero. A summation of the experiences of those who attempt to reach lower and lower temperatures is that the absolute zero of temperature is unattainable.

This generalization is related to the indication that the entropy changes for all reactions would be zero if the reactions occurred at absolute zero. Suppose product substances C and D had, together, greater entropy than reactant substances A and B. suppose this entropy difference persisted through low temperature and on down to absolute zero. The A + B = C + D reaction could then be imagined to occur reversibly at some very low temperature and in doing so to reduce the entropy, and the energy, of the thermal surroundings. If substances had different entropies at absolute zero, temperatures could be reduced to, and below, absolute zero.

Thus there is an equilivalence between the idea that absolute zero cannot be reached and the reorganization that the entropy changes for all reactions would be zero at absolute zero. This conclusion must be restricted to materials that are in the thermodynamically most stable state for this temperature range. (One finds, for example, that many materials are frozen into a metalstable glassy state as the temperature is reduced, and this state may be different, in fact the absolute zero is approached due to the slowness with which the crystalline form is produced. Since the metalstable state cannot be converted directly to the stable state by a reversible process. The entropy of the glassy state could be different, in fact higher, than that of the crystal at absolute zero. Since the metalstable state cannot be converted directly to the stable state by a reversible process, this entropy difference could not be used in attempts to reach absolute zero.)

We come to the chemically useful statement of the third law of thermodynamics, quoted from the classic thermodynamics text by Lewis and Randall. If the entropy of each element in some crystalline state be taken as zero at the absolute zero of temperature the substance has finite positive entropy; but at the absolute zero of temperature the entropy may become zero, and does so become in the case of perfect crystalline substances.

The third law makes it possible to assign entropy values, described as absolute entropies, to chemical compounds. Absolute entropies, most of which were evaluated from calorimetric results and the third law, are included in appendix B.

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