Chemistry · Ch 5 — Thermodynamics
Absolute Entropy and Third Law of Thermodynamics
Absolute Entropy and Third Law of Thermodynamics
Absolute Entropy and Third Law of Thermodynamics
Molecules of a substance can move in a straight line in any direction (translational motion), spin like a top (rotational motion), and have bonds that stretch and compress (vibrational motion). When the temperature of the system rises, these motions become more vigorous, and entropy increases. When temperature is lowered, entropy decreases.
The third law of thermodynamics states:
Third Law of Thermodynamics: The entropy of any pure crystalline substance approaches zero as the temperature approaches absolute zero (0 K).
This is because there is perfect order in a crystal at absolute zero. The statement is confined to pure crystalline solids because theoretical arguments and experimental evidence have shown that the entropy of solutions and supercooled liquids is not zero at 0 K.
The importance of the third law lies in the fact that it permits the calculation of absolute values of entropy for pure substances from thermal data alone. For a pure substance, this can be done by summing in increments from 0 K to 298 K. …