Freezing process enthalpy change

The reverse processes, condensing and freezing, have enthalpy changes of the same magnitude but opposite sign ... 

Vaporization and condensation are opposite processes. Thus, the enthalpy changes for these processes have the same value but opposite signs. For example, 6.02 kJ is needed to vaporize one mole of water. Therefore, 6.02 kJ of energy is released when one mole of water freezes. 

Enthalpy changes accompany such processes as the dissolution of a solute, the formation of micelles, chemical reaction, adsorption onto solids, vaporisation of a solvent, hydration of a solute, neutralisation of acids and bases, and the melting or freezing of solutes. 

Let us first examine what thermodynamics predicts when this process occurs at the ordinary freezing point of water under atmospheric pressure, 273.15 K. The measured enthalpy change (the heat absorbed at constant pressure) is... 

The three states of water are so common because they all are stable under ordinary conditions. Carbon dioxide, on the other hand, is familiar as a gas and a solid (dry ice), but liquid CO2 occurs only at external pressures greater than 5 atm. At ordinary conditions, solid CO2 becomes a gas without first becoming a liquid. This process is called sublimation. Freeze-dried foods are prepared by sublimation. The opposite process, changing from a gas directly into a solid, is called deposition—you may have seen ice crystals form on a cold window from the deposition of water vapor. The heat of sublimation (Aff°ubi) is the enthalpy change when 1 mol of a substance sublimes. From Hess s law (Section 6.5), it equals the sum of the heats of fusion and vaporization ... 

Atrs H molar enthalpy of a transition between any two phases in general Molar enthalpies of vaporization, sublimation, and fusion are positive. The reverse processes of condensation (gas liquid), condensation or deposition (gas solid), and freezing (liquidssolid) have negative enthalpy changes. 

It was shown that the folded conformation is favored by ca. 3 kcal/mcd in enthalpy and the entropy change is 3—4 e.u., vdiich is unfavorable to the folded form. The entropy loss on going to the folded conformation is readfly understandable in terms of the freezing out of all motion of the aromatic side chain in the folded form. The intramolecular interaction is not the process accompanyit the release of bound or ordered solvent molecules (hydroj obic interaction). It seems likely that the folded conformation of the aromatic cyclic dipeptides is stabilized by interaction of amide dipole and aromatic induced-dipole, dispersion forces between polarizable rr systems of the amide group and the aromatic ring, and some sbortHranged, h ily directional effects. As a consequence, the intramolecular interaction is characterized by invariant AH and AS with solvent and it is not destroyed even in amide solvent. 

Most physical and chemical changes occur at nearly constant atmospheric pressure—a reaction in an open flask, the freezing of a lake, a biochemical process in an organism. In this section, we discuss enthalpy, a thermodynamic variable that relates directly to energy changes at constant pressure. 

Notice that the enthalpy and entropy changes are both positive for the heating process, but they are both negative for the freezing and cooling processes. 

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