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Hydrogen Chloride

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-

This solution is called hydrochloric acid(1), which is a strong acid with a high acid dissociation constant.

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

This reaction runs smoothly at room temperature in the dark, but becomes explosive when exposed to ultraviolet radiation (due to the increased formation of chlorine radicals by ultraviolet light - see also the chlorination of alkanes).

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)

In order to evolve HCl as a gas, the sulfuric acid must be (almost) free of water since HCl readily dissolves in water but has only low solubility in concentrated sulfuric acid (96%).

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)

References

(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.

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