Question: State third law of thermodynamics....

Question

State third law of thermodynamics.

Answer 1

Solution

The third law of thermodynamics relates the entropy of the crystalline solid to the temperature. The temperature is taken as absolute temperature.

Complete step by step answer:
It is a well-known observation that the entropy of pure substances increases with an increase in temperature because molecular motion (i.e., translational, vibrational, and rotational) increases with an increase of temperature. Conversely, entropy decreases with a decrease in temperature. In 1906, Nernst studied about the entropies of perfectly crystalline substances at absolute zero and put forward his following generalization that is known as the "third law of thermodynamics":
The entropy of perfectly crystalline solid approaches zeroes as the temperature approaches absolute zero.
In other words, we can say that the entropy of all 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:
The absolute entropy is taken as zero, because at absolute zero, a perfectly crystalline solid has a perfect order of its constituent particles, i.e., there is no disorder at all.
The entropy per mole of the substance under standard conditions at the specified temperature is called standard molar entropy ( $S_{m}^{\circ }$ ) or absolute entropy.
The absolute entropy of solids like carbon in graphite is 5.69, the carbon in diamond is 2.4, aluminium is 96.2, $CaO$ is 39.8, etc. The absolute entropy of liquids like water is 69.9, ethanol is 160.7, benzene is 159.8, etc. the absolute entropy of gases like ${{H}_{2}}$ is 130.6, $N{{H}_{3}}$ is 192.5, $C{{O}_{2}}$ is 213.6, etc.

Note: The most important application of the third law of thermodynamics is the calculation of the absolute entropies of the substances at room temperature. The absolute entropy can be calculated by the formula: $S=2.303\text{ }{{\text{C}}_{p}}\log T$ , where ${{C}_{p}}$ is the heat capacity at constant pressure and T is the temperature.