The Hydrogen Halides and Hydrohalic Acids

Reactions of the Free Halogens 24-6 The Hydrogen Halides and Hydrohalic Acids... 

The hydrogen halides form hydrohalic acid solutions when dissolved in water. These solutions have the characteristic properties of acids, such as reactions with active metals to produce hydrogen gas. aco (Section 4.4) Hydrofluoric acid also reacts readily with silica (Si02) and with silicates to form hexafluorosilicic acid (H2Sip5) ... 

The hydrogen halides, HF, HCI, HBr, and HI, are all polar covalent gases. In the gas phase the interactions among the widely separated molecules are not very strong, so solute-solute attractions are minimal. Due to the polarity of the hydrogen halides and the hydration of their ions, the dissolution processes in water are very exothermic. The resulting solutions, called hydrohalic acids, contain predominandy ionized HX (X = Cl, Br, I). The ionization myo[ es protonation of a water molecule hy HX to form a hydrated hydrogen ion, H30, and halide ion X (also hydrated). 

It is common practice to refer to the molecular species HX and also the pure (anhydrous) compounds as hydrogen halides, and to call their aqueous solutions hydrohalic acids. Both the anhydrous compounds and their aqueous solutions will be considered in this section. HCl and hydrochloric acid are major industrial chemicals and there is also a substantial production of HF and hydrofluoric acid. HBr and hydrobromic acid are made on a much smaller scale and there seems to be little industrial demand for HI and hydriodic acid. It will be convenient to discuss first the preparation and industrial uses of the compounds and then to consider their molecular and bulk physical properties. The chemical reactivity of the anhydrous compounds and their acidic aqueous solutions will then be reviewed, and the section concludes with a discussion of the anhydrous compounds as nonaqueous solvents. 

Small quantities of the hydrogen halides (their solutions are called hydrohalic acids) and other volatile acids are usually prepared by adding concentrated nonvolatile acids to the appropriate salts. (Sulfuric and phosphoric acids are classified as nonvolatile acids because they have much higher boiling points than other common acids.) The reactions of concentrated sulfuric acid with solid sodium fluoride and sodium chloride produce gaseous hydrogen fluoride and hydrogen chloride, respectively. 

When this reaction involves fluorine, it is explosive, but it becomes less and less violent as we substitute chlorine, bromine, and iodine. The hydrogen halides dissolve in water to form hydrohalic acids. Hydrofluoric acid (HF) is a weak acid (that is, it is a... 

In practical terms, hydrochloric acid is the most important of the hydrohalic acids, the aqueous solutions of the hydrogen halides. About 3 million tons of hydrochloric acid are produced annually for use in cleaning steel before galvanizing and in the manufacture of many other chemicals. 

The name hydrogen halides, HX, is used for the anhydrous compounds, whereas their aqueous solutions are called hydrohalic acids. In their anhydrous condition, these are molecular compounds, whereas solutions in water bring about ionic dissociation to give the protonated solvent and the halide anion (equation 84). 

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