4 - The Second and Third Laws of Thermodynamics: Entropy

Abstract

This chapter describes the second and the third laws of thermodynamics. The Kelvin statement of the second law of thermodynamics involves cyclic processes, which are processes in which the final state of the system is the same as its initial state, and states that it is impossible for a system in a cyclic process to turn heat completely into work done on the surroundings. The mathematical statement of the second law establishes a new state function, the entropy. The mathematical statement provides a means of calculating the entropy change of any process that begins and ends at equilibrium states. The second law of thermodynamics governs whether any macroscopic process can occur spontaneously. The criterion is that the entropy of the universe cannot decrease. The third law of thermodynamics allows the entropy of a pure crystalline substance to be set equal to zero at absolute zero of temperature. Entropies relative to the entropy at zero temperature are called absolute entropies. A consequence of the second and third laws of thermodynamics is that no finite number of operations, either reversible or reversible, can bring an object to absolute zero temperature.