Question

Question #14f56

Answer 1

Simply put it states that the entropy of a perfect crystal at absolute zero will be zero.

Explanation:

In thermodynamics, there is no concept of absolute entropy . One can only define entropy differences between two states. Which is defined by
`\Delta S= int \frac{\deltaQ_{rev}}{T}`

This law in a way connects the two ways in which entropy is viewed. In statistical mechanics, entropy is defined through the boltzmann's law which is

`S=klnW`, where W is the multiplicity, or in other words, the number of states that have identical macroscopic properties. According to the Boltzmann's law a state which has a multiplicity of 1 will have zero entropy. So that means one could define a absolute entropy with respect to this "standard" of a zero entropy system.

Now, many systems could have a multiplicity of 1 for e.g. just one atom. A perfect crystal also has a multiplicity of one, and at absolute zero when there is no thermal motion at all, all atoms become equivalent, hence every configuration is equivalent and therefore a multiplicity of 1.

But then why a perfect crystal? working with a atom is not practical at all, as far as experiments are concerned. Which is why the definition has a perfect crystal, near perfect crystals can be obtained with controlled growth. Hence practically near perfect crystal at low temperatures are very good approximations to the zero entropy state and these are very useful for defining absolute entropy of elements or compounds.

PS: Today we know that quantum mechanically, even a perfect crystal will not have "zero entropy" because of what is called a Zero point energy (check it out).