The importance of the Third Law is that it allows us to determine the absolute Entropy of substances. D S o , because the Entropy scale is absolute. Entropy values are measured by observing the Temperature rise induced by the slow addition of heat to the system. Compare the Entropy curve , shown below, to the heating curve that have looked at in previous lectures. This curve is generated calorimetrically.
Heat is added slowly to the system over a very small Temperature range.
This heat divided by Temperature is the Entropy for that small jump in Temperature. The value of S at any given Temperature is the sum of these heat increments up to the Temperature of interest. With simple monatomic substances, we expect the S to be small, incoming heat is used almost exclusively to raise the T of the system, since there are no vibrational or rotational modes to excite. With more complex molecules, vibrational modes can also contribute to the increase in Entropy. The more complex the molecule, the higher the Entropy.