Hydrogen halide

Hydrogen halides (or hydrohalic acids) are acids resulting from the chemical reaction of hydrogen with one of the halogen elements (fluorine, chlorine, bromine, iodine), which are found in Group 7 of the periodic table. Astatine is not included in the list because it is very rare, unstable and not found as the acid in substantial quantities. Hydrogen halides can be abbreviated as HX where H represents a hydrogen atom and X represents a halogen (fluorine, chlorine, bromine or iodine).

They are acids because of their ability to release hydronium ions (H₃O⁺) in aqueous solution. With the exception of HF, the halogen halides are strong acids, with acid strength increasing down the group. This results from strong hydrogen bonding in HF, which must be overcome before dissociation is possible, and the very small F⁻ ions which impose order on nearby water molecules and hydronium ions increasing free energy (making dissociation unfavorable). Below HCl however, there is no hydrogen bonding, the anions get larger, and bond enthalpy decreases meaning dissociation happens with increasing ease.

A common measure of acid strength is the p"K"_a, which is based on the equilibrium between the dissociated and bonded forms of the acid in water. Acids with high p"K"_a's are weaker than those with lower p"K"_a values. The p"K"_a's of the hydrogen halides can be ranked: HI < HBr < HCl < HF.

Alone of the hydrogen halides, hydrogen fluoride exhibts hydrogen bonding between molecules, and therefore has the highest melting and boiling points of the HX series. From HCl to HI the boiling point rises. This is due to the increasing numbers of electrons in the molecules that in turn leads to an increase in the frequency and strength of Van der Waals forces.

The anhydrous hydrogen halides are all colourless gases at STP except for hydrogen fluoride, which boils at 19 °C due to strong hydrogen bonding. The gaseous hydrogen halides condense "fog" out of water vapor in the air. Thus, concentrated hydrohalic acid solutions produce visible white fumes. This occurs because the hydrohalic acids react with water vapor to form tiny droplets of their concentrated aqueous solutions and dissociate, as follows:

:HX(g) + ("n"+1)H₂O(g) → H₃O⁺(aq) + X⁻(aq) + "n"H₂O(l)

Whether "n" is large or small depends on the humidity, with "n" approaching infinity as the humidity approaches 100%.

This phenomenon should not be confused with the dense white fumes formed when the hydrogen halides react with concentrated ammonia, forming ammonium halides:

:HX(g) + NH₃(g) → NH₄X(s)

References

*http://www.chemguide.co.uk/inorganic/group7/acidityhx.html#top