Hydrogen chloride

From Academic Kids

This article is about the gas hydrogen chloride. For a solution of hydrogen chloride in water see hydrochloric acid. For hydrogen chloride salts see hydrochloride.
Hydrogen chloride
Molecular model of hydrogen chloride.
Systematic name Hydrogen chloride
Other names
Molecular formula HCl
Molar mass 36.46 g/mol
Appearance colorless gas,
fumes in moist air
Odor acrid, pungent
CAS number 7647-01-0
Hazards corrosive
R/S statement R: 34-37 S: 26-36-45
Density and phase 1.64 g/l, gas (0 °C)
Solubility 72 g/100 ml in water (20 °C)
Melting point -114.2 °C
Boiling point -85.1 °C
Acid dissociation
Ka = 104, pKa = -4
Phase behavior
Triple point 160 K (-114 °C), 13.8 kPa
Critical point 325 K (52 °C), 8.3 MPa
Gas properties
Standard enthalpy
of formation
-92.31 kJ/mol
Standard molar entropy
186.90 J/mol·K
Heat capacity Cp 29.14 J/mol·K (25 °C)
Inhalation sore throat, coughing,
delayed lung edema
Skin severe burns, ulceration
Eyes burns, irreversible eye injury
Personal protection use only in a
well-ventilated area
or use a gas mask
OSHA PEL limit 5 ppm
NIOSH IDLH limit 50 ppm
Reacts with bases (violently),
carbonates, metals
Storage resistant compressed
gas cylinders
More information
Chemical data NIST WebBook (http://webbook.nist.gov/cgi/cbook.cgi?Name=hydrogen+chloride&Units=SI)
MSDS safety data External MSDS (http://avogadro.chem.iastate.edu/MSDS/HCl.htm)
Wikisource MSDS
Except where noted otherwise, data relate
to standard temperature and pressure.

Hydrogen chloride, also known under the name HCl, is a highly corrosive and toxic colorless gas that forms white fumes on contact with humidity. These fumes consist of hydrochloric acid which forms when hydrogen chloride dissolves in water. Hydrogen chloride gas as well as hydrochloric acid are important chemicals in chemistry, science, technology, and industry. The name HCl often refers somewhat misleadingly to hydrochloric acid instead of the gaseous hydrogen chloride.



The hydrogen chloride molecule HCl is a simple diatomic molecule consisting of a hydrogen atom H and a chlorine atom Cl connected with a covalent single bond. Since the chlorine atom is much more electronegative than the hydrogen atom, the covalent bond between the atoms is quite polar. Since the overall molecule has a large dipole moment with a negative partial charge δ- at the chlorine atom and a positive partial charge δ+ at the hydrogen atom, diatomic hydrogen chloride has a highly polar molecule. Therefore, it is is very soluble in water (and in other polar solvents).

Upon contact with water, it immediately ionizes, forming hydronium cations H3O+ and chloride anions Cl- through a reversible chemical reaction with the water:

HCl + H2O → H3O+ + Cl-

The resulting solution is called hydrochloric acid and is a strong acid. The acid dissociation or ionization constant, Ka, is very large, which means HCl dissociates or ionizes practically completely in water.

Even in the absence of water, hydrogen chloride can still act as an acid. For example, hydrogen chloride can dissolve in certain other solvents such as methanol, and protonate molecules or ions and act as an acid-catalyst for chemical reactions where anhydrous (water-free) conditions are desired.

HCl + CH3OH → CH3O+H2 + Cl-

HCl protonating a methanol (CH3OH) molecule

Because of its acidic nature, hydrogen chloride is a corrosive gas, particularly in the presence of any moisture.

Missing image
Hydrochloric acid fumes turning pH paper red showing that the fumes are acidic

Health effects

Hydrogen chloride forms corrosive hydrochloric acid on contact with body tissue. Inhalation of can cause coughing, choking, inflammation of the nose, throat, and upper respiratory tract, and in severe cases, pulmonary edema, circulatory system failure, and death. Skin contact can cause redness, pain, and severe skin burns. Hydrogen chloride may cause severe burns of the eye and permanent eye damage.


Alchemists recognized since the Middle Ages that hydrochloric acid (then known as spirit of salt or acidum salis) gave off hydrogen chloride as a vapor which was called marine acid air.

In the 17th century Johann Rudolf Glauber used salt (sodium chloride) and sulfuric acid for the preparation of sodium sulfate, releasing hydrogen chloride gas.

2 NaCl + H2SO4 → 2 HCl + Na2SO4

In 1772, Carl Wilhelm Scheele also ran this reaction and is sometimes credited with its discovery. Joseph Priestley prepared pure hydrogen chloride in 1772, and in 1818 Humphry Davy proved it was chemically composed of hydrogen and chlorine.

During the Industrial Revolution, demand for alkaline substances such as soda ash increased, and Nicolas Leblanc developed a new industrial-scale process for producing the soda ash. In the Leblanc process, salt was converted to soda ash, using sulfuric acid, lime stone, and coal, giving hydrogen chloride as by-product. Initially, this gas was vented to air, but the Alkali Act of 1863 prohibited such release, so then soda ash producers absorbed the HCl waste gas in water, producing hydrochloric acid on an industrial scale. Later, the Hargreaves process was developed, which is similar to the Leblanc process except sulfur dioxide, water, and air are used instead of sulfuric acid in a reaction which is exothermic overall. In the early 20th century the Leblanc process was effectively replaced by the Solvay process, which did not produce HCl. However, hydrogen chloride production continued as a step in hydrochloric acid production.

Historical uses of hydrogen chloride in the 20th century include hydrochlorinations of alkynes in producing the chlorinated monomers chloroprene and vinyl chloride, which are subsequently polymerized to make polychloroprene (Neoprene) and polyvinyl chloride (PVC), respectively. In the production of vinyl chloride, acetylene (C2H2) is hydrochlorinated by adding the HCl across the triple bond of the C2H2 molecule, turning the triple into a double bond, yielding vinyl chloride.

The "acetylene process", used until the 1960s for making chloroprene, starts out by joining two acetylene molecules, and then adds HCl to the joined intermediate across the triple bond to convert it to chloroprene as shown here:

Missing image

This "acetylene process" has been replaced by a process which adds Cl2 to one of the double bonds in 1,3-butadiene instead, and subsequent elimination produces HCl instead, as well as chloroprene.


Most hydrogen chloride produced on an industrial scale is used for hydrochloric acid production. The method of hydrogen chloride production from sodium chloride and sulfuric acid is also used on an industrial scale.

A common way to produce hydrogen chloride in the chlor-alkali industry, is using an HCl oven, where hydrogen and chlorine gases combine in an exothermic reaction to give hydrogen chloride gas.

Cl2 + H2 → 2 HCl

This reaction can be used to give a very pure product for the food industry.

Hydrogen chloride can also be produced from chlorine and other hydrogen-containing compounds, such as hydrocarbons. Chlorination of organic compounds may lead to HCl production as follows:

R-H + Cl2 → R-Cl + HCl

Further reaction of the chlorinated compounds with hydrogen fluoride can give fluorinated compounds and HCl as a product.

R-Cl + HF → R-F + HCl

When the reactions are run under anhydrous conditions, HCl gas is produced. The organic end products include halogenated polymers and chlorofluorocarbons.


One way hydrogen chloride gas can be formed is by gradually adding water (or acid) to an excess of certain reactive chloride compounds such as phosphorus chlorides, thionyl chloride (SOCl2), or acyl chlorides. Adding more water would absorb the HCl gas forming hydrochloric acid. For example, cold water can be gradually dripped onto phosphorus pentachloride (PCl5) to give HCl in this reaction:[4]

PCl5 + H2O → POCl3 + 2 HCl

Hot water could liberate more HCl by hydrolyzing PCl5 all the way to ortho-phosphoric acid.[4] Reaction of water with phosphorus trichloride (PCl3) also yields HCl.[4]

Reaction of thionyl chloride with water would give sulfur dioxide (SO2) gas as well as HCl. For the reactions of thionyl chloride or acyl chlorides with water, see thionyl chloride or acyl halide.

Another way to synthesize HCl gas is to use the following HCl generator.

HCl generator

Small amounts of HCl gas for laboratory use can be generated in a HCl generator using different methods:

  • liberation from concentrated hydrochloric acid using concentrated sulfuric acid.
  • liberation from concentrated hydrochloric acid using dry calcium chloride.
  • by reaction of sulfuric acid with sodium chloride: NaCl + H2SO4 → NaHSO4 + HCl↑

Typically, hydrochloric acid or sulfuric acid is added dropwise from a dropping funnel into a flask containing the reagent or drying agent under magnetic stirring. The HCl gas can by dried by bubbling through concentrated sulfuric acid. Washing bottles are needed to prevent the suction of solutions into the apparatus.


These are some of the uses for hydrogen chloride gas:

  • Most hydrogen chloride is used in the production of hydrochloric acid.
  • hydrochlorination of rubber
  • production of vinyl and alkyl chlorides
  • chemical intermediate in other chemical production
  • use as babbitting flux
  • treatment of cotton
    • delinting
    • separation from wool
  • used in semiconductor industry (in pure grade)
    • etching semiconductor crystals
    • converting silicon to SiHCl3 for purification of silicon

Hydrogen chloride usually comes in compressed gas cylinders that are either red and brown or grey with a yellow band.

See also

Reference External Links

  1. Chemical and laboratory data (http://www.spezialgase.de/specialgasescatalog/gase/chlorwasserstoff/)
  2. MSDS safety data (http://www.jtbaker.com/msds/englishhtml/H3880.htm)
  3. Chemical data (http://www.c-f-c.com/gaslink/pure/hydrogen-chloride.htm)
  4. Hydrogen Chloride (http://www.ucc.ie/ucc/depts/chem/dolchem/html/comp/hcl.html)

de:Chlorwasserstoff sv:Väteklorid


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