Hydrogen Chloride

Author: Hans Lohninger

Hydrogen chloride (HCl) is a colorless gas which forms white fumes of hydrochloric acid when brought into contact with atmospheric humidity. Inhalation of the gas can cause severe burns of the nose, throat, and upper respiratory tract (which may lead to death in severe cases). Hydrogen chloride may also result in severe burns of the eyes.

The hydrogen and the chlorine atom are connected by a covalent single bond, which is highly polar, since the chlorine atom is much more electronegative than the hydrogen atom. Thus the molecule shows a large dipole moment with the negative charge at the chlorine atom.

When dissolved in water, the HCl gas dissociates and forms hydronium and chloride ions:

HCl + H₂O H₃O⁺ + Cl^-

Production

Hydrogen chloride can be produced on an industrial level by oxidizing hydrogen with chlorine (this process is performed in an "HCl burner", since the reaction is exothermic and yields a flame):

H₂ + Cl₂ 2 HCl

In the laboratory HCl gas may be generated by the reaction of sodium chloride with concentrated sulfuric acid⁽²⁾:

NaCl + H₂SO₄ NaHSO₄ + HCl (at room temperature)
NaCl + NaHSO₄ Na₂SO₄ + HCl (at 200°C)

Usage

Most of the industrially generated HCl gas is dissolved in water and converted to hydrochloric acid. HCl gas is also used in the production of vinyl chloride and alkyl chlorides:

C₂H₂ + HCl CH₂=CHCl (vinyl chloride)
CH₂=CHCl + HCl CH₃CHCl₂ (1,1-dichloro ethane)

(1)	In the absence of water, hydrogen chloride can still act as an acid if it is solved in a polar solvent, such as methanol.
(2)	A historic remark: this reaction - which is the first step of the Leblanc process for producing sodium carbonate - has caused severe air polution in the early days of the industrial revolution, since the HCl gas has been vented to the ambient air. Later on the producers were forced by law to dissolve the HCl gas in water.