Biochemistry: The Chemical Reactions of Living Cells, Volume 1 - D. Metzler 1980

Energetics of Biochemical Reactions
Thermodynamics
Entropy and Heat Transfer

Why does heat always flow from a warmer body to a colder one, and Why is the reverse process never observed? We know that the direction of heat flow is governed by the Temperature difference, but this raises another question: what actually is temperature? Reflections on this issue, as well as on the mutual interconversion of work and heat, led to the Discovery of the second law of Thermodynamics and the Introduction of a new thermodynamic function: Entropy 5.

Let us consider a phase transition at constant temperature and pressure, such as the melting of ice. At temperatures just above 0 °С, ice melts completely, whereas at temperatures just below 0 °С, it does not melt at all. At 0 °С, equilibrium is established: in thermodynamic terms, the melting of ice at 0 °С represents a reversible process. Based on what criteria could one predict such behavior of Water? Many familiar processes, such as combustion, are characterized by the fact that spontaneous reaction is accompanied by the release of a large amount of heat, i.e., ∆H in this case is negative. However, during the melting of ice, heat is absorbed. At 0 °С, ∆H for the process is 6.008 кДж∙моль^-1; nearly the same value is obtained both just below 0 °С, when the ice does not melt at all, and just above 0 °С, when the ice melts completely. In the latter case, ice melting is a spontaneous process for which ∆H is positive. Taking these facts into account, it is clearly evident that the sign of the enthalpy change alone cannot serve as a criterion for reaction spontaneity.

The Nature of the ice–water transition was understood only after realizing the fact that when ice melts, In addition to an increase in H by 6.008 кДж∙моль^-1 (which increases the internal energy of the molecules—translational, vibrational, and rotational), There is a decrease in the system's order. Although historically entropy was introduced from other considerations, it is now understood as a measure of "microscopic disorder." When ice melts, the entropy S increases because the Structure becomes less ordered.