Chapter 11. Microscale Gas Chemistry:

A. Appearance
     Hydrogen chloride is a colorless gas with a disagreeable, pungent odor.  When exposed to moist air, it forms fumes which are an aerosol of  HCl(aq).

B. Physical Properties of HCl

Molecular mass:  36.46 g/mol
melting point   -114.8 ^oC
boiling point   -84.9 ^oC

C. History
     The first recorded preparation of hydrogen chloride dates back 1100 years ago when dilute HCl was prepared by the Arabian alchemist Rhazes.  Methods for producing more concentrated forms of the acid were developed over the next three centuries and by 1200 AD, aqua regia, a mixture of concentrated HCl and HNO3, was being used to dissolve gold.  Concentrated HCl was first prepared by J. L. Glauber in 1648 by heating ZnCl2 and sand in a retort.

D. Natural Abundance
     Because of its reactivity, HCl as a compound does not usually occur in nature.
 

E. Industrial Production
     Hydrogen chloride is one of the largest volume chemicals produced in modern society.  Production follows a variety of pathways that depend on the resources available and the intended use of the product.  Where NaCl deposits are available, HCl can be produced with concentrated H2SO4 by the Leblanc process developed in late 1700s:

The Hargreaves process, developed a century later, is similar to the Lablanc process, however it utilizes SO2(g), H2O and air instead of H2SO4.  Because these reagents produce H2SO4 with an enormous amount of heat release, the overall reaction with NaCl is exothermic:

 Ultrapure HCl is produced by the direct combination of H2 and Cl2:

Hydrogen readily burns in an atmosphere of chlorine.  The HCl produced in this way can be used in the food industry.
 
 

F. Industrial Uses
     In terms of sheer volume, the largest industrial use of aqueous HCl is the pickling of steel.  'Pickling' means the removal of oxide coating by dipping the metal in HCl(aq).  Hydrogen chloride and hydrochloric acid have literally thousands of uses that cover every sector of the chemical and pharmaceutical manufacturing industry.  For example, the gelatin industry uses large amounts of HCl to decompose bones used as raw materials.  HCl catalyzes the conversion of starch into glucose.  Corn starch is converted into artificial maple syrup in this manner.

G. Gas Density of HCl
     The density of HCl(g) is 1.490 g/L at 25 ^oC and 1 atm which is 26% greater than that of air.

H. Gas Solubility of HCl
     Hydrogen chloride is extremely soluble in water and forms the familiar hydrochloric acid, HCl(aq).  At 0 ^oC, 823 g HCl dissolve per L H2O; this corresponds to 506 volumes HCl per 1 volume water.  Concentrated hydrochloric acid is 12 M and is one of only a few common strong acids that dissociate 100% in water: