Hydrated acids and acid salts

Special interest attaches to the hydrates of acids and acid salts because of the possibility that some or all of the protons may be associated with the H2O molecules to form ions H30 or more complex species. The acids HX form hydrates with the following numbers of molecules of water of crystallization  

HF behaves quite differently from the other halogen acids, and nothing is yet known of the structures of its hydrates. The hydrates of HCl melt at progressively lower temperatures (-154°, -174°, and -24-9°C) and the crystal structures of all three are known. The i.r. spectrum of the monohydrate (at —195°C) indicates the presence of pyramidal H30 ions structurally similar to the isoelectronic NH3 molecule. This has been confirmed by the determination of the crystal structure. In corrugated 3-connected layers of the simplest possible type (Fig. 15.25(a)) each 0 or Cl atom has three pyramidal neighbours, so that the units are Cl and H30 ions, (i). The distance 0—H--C1 is 2-95 A and the interbond angle 0—H—0 is 117°. 

The angle subtended at 0 by two Cl is 110°, the H atoms lying about 0-1 A off the 0-H—Cl lines. (The shortest distance between 0 and Cl of adjacent layers is 343 A.) 

The dihydrate of HCl contains H5O2 units, (ii), in which the central O-H-0 bond is extremely short, and the 0-H-Cl bonds have a mean length of 3-07 A (range 3 04-3-10 A), appreciably longer than in (H3O) Cr. The same units are found in the trihydrate, here hydrogen-bonded to 2 H2O (O-H-0,2-70 A) and to 

The system H2SO4-H2O is complex, freezing-point curves indicating the existence of six hydrates, with 1, 2, 3, 4, 6-5, and 8 H2O. In H2SO4. H2O the number of H atoms is sufficient for four hydrogen bonds per unit of formula. The 

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The crystal structures of hydrated acids and acid salts are of interest because in them some of the hydrogen ions may be associated with water molecules to form hydroxonium ions H30+. The structure of hydrogen chloride monohydrate, HC1.H20, is formed by the superposition of a series of puckered sheets, of the type shown in fig. 12.14, in which each chlorine and each oxygen atom is co-ordinated by three atoms of the other kind arranged at the corners of a shallow trigonal... 

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