Third Law of Thermodynamics

It is a well known observation that the entropy of a pure substance increases with increase of temp because molecular motion (i.e., translational, vibrational and rotational) increases with increase temperature. Conversely, entropy decreases with decrease of temperature. Nernst, in 1906, made an important observation about the entropies of perfectly crystalline substances at absolute zero and put forward the following generalization known as the 'third law of thermodynamics':

The entropy of a perfectly crystalline solid approaches zero as the temperature approaches absolute zero.

In other words,

The entropy of all perfectly crystalline solids may be taken as zero at the absolute zero of temperature.

Since entropy is a measure of disorder, the above definition may be given molecular interpretation as follows :-

At absolute zero, a perfectly crystalline solid has a perfect order of its constituent particles, i.e., there is no disorder at all. Hence, the absolute entropy is taken as zero.

The entropy per mole of the substance under standard conditions at the specified temperature is called standard molar entropy (S_m°) or absolute entropy.

Problems:

1. The entropy of crystalline substances at absolute zero going by the third law of thermodynamics should be taken as

A. 100

B. 50

C. Zero

D. Different for different substances

Solution:

The entropy of crystalline substances at absolute zero going by the third law of thermodynamics should be taken Zero