Structures of ice

Figure 2.6. The tetrahedral structures of ice (a), (fc) are planes through sheets of selected oxygen nuclei (open circles), hydrogen nuclei (shotm in the insert as solid circles) are not shown in the main drawing. The insert illustrates the overlap of oxygen line pairs and the hydrogen nuclei, thus forming the hydrogen bonds (dotted lines)...

The crystal structure of ice is hexagonal, with lattice constants of a = 0.452 nm and c = 0.736 nm. The inorganic compound silver iodide also has a hexagonal structure, with lattice constants (a = 0.458 nm, c = 0.749 nm) that are almost identical to those of ice. So if you put a crystal of silver iodide into supercooled water, it is almost as good as putting in a crystal of ice more ice can grow on it easily, at a low undercooling (Fig. 9.2). 

Complete and Incomplete Ionic Dissociation. Brownian Motion in Liquids. The Mechanism of Electrical Conduction. Electrolytic Conduction. The Structure of Ice and Water. The Mutual Potential Energy of Dipoles. Substitutional and Interstitial Solutions. Diffusion in Liquids. 

The Structure of Ice and Water. It has not yet been necessary to consider in detail the properties of particular solvents. In Table 1 we gave a list of values for the dielectric constants of various solvents but apart from this we have not yet paid attention to the observed properties of solvents or of the ions which are commonly dissolved in them. Before continuing the discussion which was in progress in Sec. 23, it will be useful to review in some detail the state of our knowledge of the liquids which are used as solvents, and of the species of ions which are most often studied in solution. Although non-aqueous solutions are of great interest for the sake of comparison, nevertheless aqueous solutions are still of paramount importance, and we shall pay most of our attention to H20 and D20 and to ions dissolved in these liquids. 

Fig. 20. The structure of ice Molecules numbered 8, 7, 6 are in contact with 5, while molecules 5, 4, 3, 2 arc in contact with 1. Molecules 2, 3, 4 are among the next-nearest neighbors of 5, while molecules 0, 7, 8 are among the next-nearest neighbors of 1. [Diagram taken from E. J. W. Verivey, Rec. trav. chim. 60, 893 (1941).]...

The theory of the structure of ice and water, proposed by Bernal and Fowler, has already been mentioned. They also discussed the solvation of atomic ions, comparing theoretical values of the heats of solvation with the observed values. As a result of these studies they came to the conclusion that at room temperature the situation of any alkali ion or any halide ion in water was very similar to that of a water molecule itself— namely, that the number of water molecules in contact with such an ion was usually four. At any rate the observed energies were consistent with this conclusion. This would mean that each atomic ion in solution occupies a position which, in pure water, would be occupied by a water moldfcule. In other words, each solute particle occupies a position normally occupied by a solvent particle as already mentioned, a solution of this kind is said to be formed by the process of one-for-one substitution (see also Sec. 39). 

The structure of ice. In ice, the water molecules are arrenged in an open pattern that gives ice its low density Each oxygen atom (red) is bonded covalently to two hydrogen atoms (gray) and forms hydrogen bonds with two other hydrogen atoms. 

Figure 4.9 Instantaneous crystal structure of ice. From General Chemistry, Pauling and Pauling 1970, 1953 and 1947 by W. H. Freeman. Reproduced with permission.

Self-Test 7.10B Suggest a reason why the entropy of ice is nonzero at T = 0 think about how the structure of ice is affected by the hydrogen bonds. 

FIGURE 8.5 The structure of ice notice how the hydrogen bonds hold the water molecules apart from one another in a hexagonal array. The two gray spheres between the oxygen atoms indicate the two possible locations of the hydrogen atom in that region of the structure. Only one of the positions is occupied. 

Let us now make the following assumptions (to be supported later by a discussion of the entropy) regarding the structure of ice. 

The contribution of this lack of regularity to the entropy of ice is thus R In 3/2 = 0.805 E. U. The observed entropy discrepancy of ice at low temperatures is 0.87 E. U., obtained by subtracting the entropy difference of ice at very low temperatures and water vapor at standard conditions, for which the value 44.23 E. U. has been calculated from thermal data by Giauque and Ashley,7 from the spectroscopic value 45.101 E. U. for the entropy of water vapor given by Gordon.8 The agreement in the experimental and theoretical entropy values provides strong support of the postulated structure of ice.9... 

The structure of ice is seen to be of a type intermediate between that of carbon monoxide and nitrous oxide, in which each molecule can assume either one of two orientations essentially independently of the orientations of the other molecules in the crystal, and that of a perfect molecular crystal, in which the position and orientation of each molecule are uniquely determined by the other molecules. In ice the orientation of a given molecule is dependent on the orientations of its four immediate neighbors, but not directly on the orientations of the more distant molecules. 

The structure of ice. (a) Each oxygen atom is at the center of a distorted tetrahedron of hydrogen atoms. The tetrahedron is composed of two short covalent O—H bonds and two long H—O hydrogen bonds, (b) Water molecules are held in a network of these tetrahedra. 

In this section, rather than give a detailed account of theories of the liquid state, a more qualitative approach is adopted. What follows includes first a description of the structure of ice then from that starting-point, ideas concerning the structure of liquid water are explained. 

There are various theories on the structure of these species and their size. Some authors have assumed the presence of monomers and oligomers up to pentamers, with the open structure of ice I, while others deny the presence of monomers. Other authors assume the presence of the structure of ice I with loosely arranged six-membered rings and of structures similar to that of ice III with tightly packed rings. Most often, it is assumed that the structure... 

The structure of ice is shown in the diagram. The crystal structure of ice is essentially tetrahedral. When water melts, the hydrogen bonds are progressively broken. The molecules pack closer together and so an initial reduction in volume of the liquid occurs before the usual expansion effect from raising the temperature is observed. Water, therefore, has its maximum density at 4°C. 

Water on Hallovsite. Central to the controversy is the observation that clay crystals present a planar array of oxygens (and hydroxyls in the case of kaolinite) which have hexagonal (or nearly) symmetry with a periodicity similar to that found in the crystal structure of ice. Because of this geometric similarity, it has frequently been assumed that water adsorbed on a clay surface will preferentially adopt an ice-like configuration. When looked at in detail, it is difficult to find unequivocal evidence to support this. 

To proceed further we require some information about the structures of ices Ih, II and III, and how these structures can be converted, one to the other. In the following we draw heavily upon an excellent discussion published by von Hippel and Farrell 74>. 

Fig. 44. Hexagon structure of Ice Ih (a) reference plane of puckered rings perpendicular to "c (b) hexagon channels viewed parallel to "e (c) fore-shortened channel in "a" direction (from Ref. 74>)...

At this stage in the literature, there is no method available by which one can directly determine the orientation of molecules of liquids at interfaces. Molecules are situated at interfaces (e.g., air-liquid, liquid-liquid, and solid-liquid) under asymmetric forces. Recent studies have been carried out to obtain information about molecular orientation from surface tension studies of fluids (Birdi, 1997). It has been concluded that interfacial water molecules, in the presence of charged amphiphiles, are in a tetrahedral arrangement similar to the structure of ice. Extensive studies of alkanes... 

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