Hydrogen chloride

Hydrogen chloride
General
Systematic name	Hydrogen chloride Chlorane
Other names	Chlorohydric acid hydrochloride hydrochloric acid
Molecular formula	HCl
Molar mass	36.46 g/mol
Appearance	Colorless gas, hydroscopic.
CAS number
Properties
Density and phase	1.477 g/l, gas (25°C)
Solubility in water	72 g/100 ml (20°C)
Melting point	-114.2°C (158.8 K)
Boiling point	-85.1°C (187.9 K)
Acidity (pK_a)	-4
Structure
Molecular shape	Linear
Dipole moment	1.05 D
Hazards
MSDS	External MSDS
Main hazards	Toxic, corrosive.
NFPA 704
Flash point	Non-flammable.
R/S statement	R: , , , . S: , , , , , , .
RTECS number	MW4025000
Supplementary data page
Structure and properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Other anions	Hydrogen fluoride Hydrogen bromide Hydrogen iodide
Other cations	?
Related compounds	Hydrochloric acid
Except where noted otherwise, data are given for materials in their standard state (at 25°C, 100 kPa) Chemical infobox

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.

Chemistry

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 is a highly polar molecule. Therefore, it 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, K_a, 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.

Health effects

Hydrogen chloride forms corrosive hydrochloric acid on contact with body tissue. Inhalation of the fumes 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 to the eye and permanent eye damage.

History

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.

2NaCl + H₂SO₄ → 2HCl + 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, limestone, 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:

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.

Production

Most hydrogen chloride produced on an industrial scale is used for hydrochloric acid production. The method of hydrogen chloride production from sodium chloride and sulphuric 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 → 2HCl

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.

Synthesis

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:^{^*}

PCl₅ + H₂O → POCl₃ + 2HCl

Hot water could liberate more HCl by hydrolyzing PCl₅ all the way to ortho-phosphoric acid.^{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↑
By the electrolysis of molten sodium chloride.

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.

Applications

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.

External links

Hydrogen compounds | Chlorides | Nonmetal halides

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