CID 313
HCl
Hydrogen chloride
- H 1
- Cl 1
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- IUPAC name
- chlorane
- Molar mass
- 36.458(10) g/mol
- Atoms
- 2
- Polarity
- polar
- SMILES
- Cl
- InChIKey
- VEXZGXHMUGYJMC-UHFFFAOYSA-N
- PubChem CID
- 313
Dissolved in water, hydrogen chloride is both the muriatic acid sold as a scale remover and the acid your stomach makes. On its own, at room temperature, it is a gas, like the other three hydrogen halides. NIOSH, the US National Institute for Occupational Safety and Health, calls it colourless to slightly yellow with a pungent, irritating odour. Its card notes that the gas ships liquefied under pressure.
Each molecule pairs one hydrogen atom with one chlorine atom, for a molar mass of 36.46 g/mol. The IUPAC name is chlorane. OpenStax gives the gas a standard enthalpy of formation of −92.3 kJ/mol, so forming it from its two elements gives out heat. In moist air the gas fumes strongly and throws off corrosive vapours of hydrochloric acid.
Structure and bonding
Two atoms and one single bond make up the whole model. The large green sphere is chlorine and the small white one hydrogen, and the Labels button prints Cl and H on them. Ball-and-stick shows the stick joining them, while Stick reduces the pair to one rod. Space-filling merges them into a lopsided outline, a big chlorine sphere wearing a small hydrogen cap, 1.306 Å between centres in this computed model.
With no central atom there is no bond angle, and the viewer cannot draw where the shared electrons spend their time. OpenStax places them nearer the chlorine atom, the more electronegative of the pair, electronegativity being the pull an atom exerts on the electrons in a bond it shares. Chlorine carries a partial negative charge and hydrogen a partial positive one. One bond means that bond’s dipole, the separation of partial charges along it, sets the molecule’s polarity, so hydrogen chloride is polar.
The drawing matches the gas, not the solution. Dissolved in water, the molecule ionises completely. Hydrochloric acid therefore holds hydronium ions, H3O+, and chloride ions, Cl−, rather than the pair drawn here.
Where it turns up
Parietal cells in your stomach lining secrete hydrochloric acid, a component of stomach acid. The same acid is sold as muriatic acid for household and building work, such as removing scale, and the acid is also used to clean swimming pools. The gas escapes from volcanoes and forms when many plastics burn.
Chipmakers etch semiconductor crystals with the acid and purify silicon by way of trichlorosilane. Oil and gas wells take it too: after water it is the largest single liquid in a fracturing fluid, where it dissolves minerals and starts cracks in the rock. With concentrated nitric acid, concentrated hydrochloric acid forms aqua regia, “royal water”, a mixture that dissolves gold and platinum. Pickling tanks in steelworks hold it as well, stripping the oxide coat from steel before galvanising.
Why it matters
Dipole–dipole attraction explains why the gas liquefies far more readily than fluorine. The positive end of one molecule pulls on the negative end of the next. OpenStax links this to a boiling point of about −85 °C, against about −188 °C for F2.
In water it is a strong acid. OpenStax ranks binary acids by two trends, valid where the solvent does not level them. Down group 17 the bond to hydrogen weakens, so acidity rises from HF to HCl, HBr and HI. Across a row the bond grows more polar, so HCl is stronger than H2S.
The US Environmental Protection Agency (EPA) puts the aggregated US product volume for 2023 at 8.5 to under 10 billion pounds. OpenStax describes one route, sulfuric acid acting on a chloride salt, usually sodium chloride because it is the cheapest. PubChem’s Hazardous Substances Data Bank (HSDB) adds direct reaction of the elements and recovery from other processes. About 90 per cent of the acid comes that last way, as a by-product of making chlorinated solvents, fluorocarbons, isocyanates, magnesium and vinyl chloride, among others.
Properties
| Property | Value | Unit | Source |
|---|---|---|---|
| Molecular formula (Hill order) | ClH | ||
| Molar mass | 36.458(10) g/mol | g/mol | IUPAC/CIAAW 2024 |
| Hydrogen atoms | 1 | ||
| Chlorine atoms | 1 | ||
| Bonds | 1 | ||
| PubChem molecular weight | 36.46 g/mol | g/mol | PubChem |
Safety
Hydrogen chloride is not harmless in any everyday form: the gas, the concentrated acid and household muriatic acid are all corrosive, and PubChem’s classification carries the signal word Danger. It causes severe skin burns and eye damage, is toxic if inhaled, and with water attacks many metals, releasing flammable hydrogen. The PubChem Laboratory Chemical Safety Summary and the NIOSH card on hydrogen chloride are the references for anyone handling it.
Questions
Is HCl polar or nonpolar?
Polar. In Cl2 and H2 the two atoms are identical, so they share their electrons equally, which OpenStax calls a pure covalent bond. In HCl the shared electrons spend more time near the chlorine atom, so the bond is polar. OpenStax adds that when a molecule has only one bond, that bond decides the polarity of the whole molecule.
Is hydrogen chloride the same as hydrochloric acid?
Not quite. Hydrogen chloride is the pure compound, a gas at room temperature made of HCl molecules. Hydrochloric acid is its solution in water, and the US Agency for Toxic Substances and Disease Registry notes that the gas forms it on contact with water. In solution the molecules ionise completely into H3O+ and Cl− ions. The commercial fuming acid holds 38 per cent hydrogen chloride.
Why is HCl a stronger acid than HF?
OpenStax ranks the hydrogen halides in order of rising acid strength as HF, HCl, HBr and HI. Going down group 17, the bond between hydrogen and the halogen weakens, and the acid strength of these binary compounds rises as that bond gets weaker. In water, any acid stronger than the hydronium ion reacts completely to form hydronium ions, so HCl ionises completely. The solvent matters too: in ethanol, HCl, HBr and HI all act as weak acids.
Why does the stomach make hydrochloric acid?
Parietal cells in the stomach lining secrete it, keeping the stomach contents at a pH of 1.5 to 3.5. That acidity converts pepsinogen into pepsin, the enzyme that digests protein, helps denature the proteins in food and kills much of the bacteria swallowed with it. When the pH falls too low, stomach cells suspend the secretion and make more mucus. Histamine is one signal that stimulates the parietal cells to release it.
Appears in these articles
- An acid gives a proton and a base takes it: Arrhenius, Brønsted–Lowry and Lewis Hydrogen chloride gives its proton to water effectively completely; acetic acid gives only a small fraction.
- What holds atoms together: ionic, covalent and metallic bonds Table salt has no molecules in it; hydrogen chloride gas is nothing but molecules.
- A polar bond is not a polar molecule: carbon dioxide against water Carbon dioxide's two bonds are polar, yet the molecule is not.
Sources
- PubChem CID 313, hydrogen chloride
- IUPAC/CIAAW, Abridged Standard Atomic Weights 2024
- PubChem Laboratory Chemical Safety Summary for hydrogen chloride (CID 313)
- NIOSH Pocket Guide to Chemical Hazards, Hydrogen chloride
- OpenStax, Chemistry 2e, §7.2 Covalent Bonding
- OpenStax, Chemistry 2e, §7.6 Molecular Structure and Polarity
- OpenStax, Chemistry 2e, §10.1 Intermolecular Forces
- OpenStax, Chemistry 2e, §14.3 Relative Strengths of Acids and Bases
- OpenStax, Chemistry 2e, §18.5 Occurrence, Preparation, and Compounds of Hydrogen
- OpenStax, Chemistry 2e, §18.9 Occurrence, Preparation, and Compounds of Oxygen
- OpenStax, Chemistry 2e, §18.11 Occurrence, Preparation, and Properties of Halogens
- OpenStax, Chemistry 2e, §5.3 Enthalpy
- OpenStax, Anatomy and Physiology 2e, §23.4 The Stomach