Physical properties of the hydrogen halides

HF is a colourless volatile liquid and an oligomeric H-bonded gas (HF), whereas the heavier HX are colourless diatomic gases at room temperature. Some molecular and bulk physical properties are summarized in Table 17.10. The influence of H bonding on the (low) vapour pressure, (long) liquid range and (high) dielectric constant of HF have already been discussed 

These results, together with infrared and electron diffraction studies, indicate that gaseous HF comprises an equilibrium mixture of monomers and cyclic hexamers, though chain dimers may also occur under some conditions of temperature and pressure  

The crystal structure of HF shows it to consist of planar zigzag chain polymers with an F-H F distance of 249 pm and an angle at F of 120.1°. 

The other HX are not associated in the gaseous or liquid phases but the low-temperature forms of crystalline HCI and HBr both feature weakly 

H-bonded zigzag chains similar to those in solid HF. At higher temperatures substantial disorder sets in. 

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TABLE 19.8 Some Physical Properties of the Hydrogen Halides... 

There are two monohydrido complexes which have been prepared by the oxidative addition of hydrogen halides to [Rh P(OMe)3 5][BPh4] (equation 223). Two other monohydrido complexes having hydrogen-bonded anions have been prepared by simultaneous substitution of and oxidative addition to dinuclear rhodium(I) complexes (equation 224). The anions give resonances at very low fields in the H NMR spectra of the complexes.1050 The physical properties of the monohydrido complexes are listed in Table 77. 

All neutral halogens are diatomic and readily reduced to halide ions. All combine with hydrogen to form gases which, except for HF, are strong acids in aqueous solution. Some physical properties of the halogens are summarized in Table 8-14. 

The physical properties of the acids HF, HCl, HBr, and HI do not form regular sequences, HF differing from the others in much the same way as H2O from H2S, H2Se, and H2Te. The abnormal properties of HF are discussed in Chapter 8 with other aspects of hydrogen bonding, as also are the structures of the crystalline hydrogen halides. 

Barium is a member of the alkaline-earth group of elements in Group 2 (IIA) of the period table. Calcium [7440-70-2], Ca, strontium [7440-24-6], Sr, and barium form a closely allied series in which the chemical and physical properties of the elements and their compounds vary systematically with increasing size, the ionic and electropositive nature being greatest for barium (see Calcium and calcium alloys Calcium compounds Strontium and STRONTIUM COMPOUNDS). As size increases, hydration tendencies of the crystalline salts increase solubilities of sulfates, nitrates, chlorides, etc, decrease (except fluorides) solubilities of halides in ethanol decrease thermal stabilities of carbonates, nitrates, and peroxides increase and the rates of reaction of the metals with hydrogen increase. 

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