Question
Define third law of thermodynamics. Give its molecular interpretation. What is the most important application of this law?
Solution
Third law of thermodynamics states: “The entropy of all perfectly crystalline solids may be taken as zero at absolute zero.”
Molecular interpretation: Entropy is a measure of disorder. Thus, at absolute zero a perfectly crystalline solid has a perfect order of its constituent particles i.e. there is no disorder at all. Hence absolute entropy is taken as zero.
Application. It helps in the calculation of the absolute entropies of the substances at room temperature (or at any temperature T). A simplified expression for the absolute entropy of solids at temperature T is given by,
where Cp is the heat capacity of the substance at constant pressure.
