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 VII 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).
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They are acids because of their ability to release hydronium ions (H3O+) in aqueous solution.
Acid strength increases going down the group on the periodic table. In other words, acid strengths can be ranked: HI > HBr > HCl > HF. This is because the bond energies for the hydrogen halides further down the group are lower, so they dissociate more easily.
A common measure of acid strength is the pKa, which is based on the equilibrium between the dissociated and bonded forms of the acid in water. Acids with low pKa's are stronger than those with higher pKa values. The pKa's of the hydrogen halides can be ranked: HI < HBr < HCl < HF.
The hydrogen halides, with the exception of Hydrogen Fluoride, show an increase in boiling points and melting points down the series. 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) + nH2O(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)