hydrogen chloride

Other Definitions
hydrogen chloride (dict)

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.

bgcolor="#ffddaa" colspan="2" \|
align="center" colspan="2" \|
bgcolor="#ffddaa" colspan="2" \| General
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
bgcolor="#ffddaa" colspan="2" \| Properties
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 constant	K_a = 10⁴, pK_a = -4
bgcolor="#ffddaa" colspan="2" \| Phase behavior
Triple point	160 K (-114 C), 13.8 kPa
Critical point	325 K (52 C), 8.3 MPa
bgcolor="#ffddaa" colspan="2" \| Gas properties
Standard enthalpy of formation Δ_fH⁰_gas	-92.31 kJ/mol
Standard molar entropy S⁰_gas	186.90 J/mol·K
Heat capacity C_p	29.14 J/mol·K (25 C)
bgcolor="#ffddaa" colspan="2" \| Hazards
Inhalation	sore throat, coughing, delayed lung edema
Skin	severe burns, ulceration
Eyes	burns, irreversible eye injury
bgcolor="#ffddaa" colspan="2" \| Handling
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), water, carbonates, metals
Storage	resistant compressed gas cylinders
bgcolor="#ffddaa" colspan="2" \| More information
Chemical data	NIST WebBook
MSDS safety data	External MSDS Wikisource MSDS
bgcolor="#ffddaa" colspan="2" align="center" \|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.

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 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, 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 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.

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.

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.

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 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.

Synthesis

One way hydrogen chloride gas can be formed is by adding water (or acid) to certain reactive chloride compounds such as phosphorus chloride, thionyl chloride (SOCl₂), or acyl halides. Another way 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.