CID 402
H2S
Hydrogen sulfide
- H 2
- S 1
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- IUPAC name
- sulfane
- Molar mass
- 34.076(20) g/mol
- Atoms
- 3
- Polarity
- polar
- SMILES
- S
- InChIKey
- RWSOTUBLDIXVET-UHFFFAOYSA-N
- PubChem CID
- 402
Rotten eggs, sewers and many hot springs share one smell, and it belongs to hydrogen sulfide: two hydrogen atoms bonded to a single sulfur atom. The name under the rules of IUPAC, the International Union of Pure and Applied Chemistry, is sulfane, and other names include sulfuretted hydrogen, sewer gas and stink damp. Its molar mass is 34.08 g/mol. As a gas it is about 1.19 times as dense as air, and that extra weight is why it tends to gather at the bottom of poorly ventilated spaces.
Dissolved in water, it forms hydrosulfuric acid, a weak diprotic acid: each molecule can release two hydrogen ions, one after the other. Because that acid is weak, solutions of soluble sulfides are basic. The sulfur in it is easy to oxidise, which makes hydrogen sulfide a good reducing agent, a substance that hands electrons to another. Lit in air, it burns with a flame of pale blue.
Structure and bonding
Hydrogen sulfide’s sulfur atom sits in the middle, with a hydrogen atom on each of its two single bonds. The readout counts three atoms and two single bonds. The two bonds do not line up. They meet at the sulfur at close to a right angle, so both hydrogens sit on the same side of it. OpenStax classes the molecule as nonlinear, and the Stick view, which keeps only the two rods, makes that angle easy to follow.
The Space-filling view shows the atoms at their full size, and the two small hydrogens sink into the side of the much larger sulfur sphere. The Labels button prints S on the central atom and H on the other two.
The shape explains the polarity. Each S–H bond has the same bond moment. Because the two point in different directions, they add up to a nonzero dipole, and OpenStax names H2S as a polar molecule. The drawing shows the neutral molecule. In water, part of it gives up a hydrogen ion to become HS−, and then S2−.
Where it turns up
Decaying organic matter releases hydrogen sulfide, and so do natural gas, sulfur hot springs, liquid manure and the sludge left from sewage. Many volcanic gases carry it too, where it probably forms as sulfide minerals react with water. Along coasts, sulfate-reducing bacteria, most of which live without oxygen, turn sulfate into hydrogen sulfide as they break down algae. In doing so they regenerate NAD+ from NADH; NAD+ is an electron carrier the cell must recycle to keep breaking down sugar. Other bacteria in your mouth and gut make it from material that contains vegetable or animal protein.
Human activity accounts for about 10 per cent of global emissions. Industry releases it as a byproduct of oil refineries, mines and tanneries, of rayon and wood pulp mills, of sugar beet processing and of laying hot asphalt. Well water can hold it naturally, and treatment with ozone or a manganese dioxide filter removes it.
Why it matters
OpenStax notes that large amounts of sulfur come from hydrogen sulfide recovered when natural gas is purified. The removal works by oxidation: with limited oxygen, two H2S molecules and one O2 give sulfur and water. Burned in enough oxygen, the same gas yields water and sulfur dioxide instead. Both reactions happen because the sulfur, at an oxidation state of −2, is not at its maximum.
Industry also uses the gas directly. Chemical plants turn it into elemental sulfur and sulfuric acid, and it is used to make the heavy water some nuclear reactors need. The Hazardous Substances Data Bank (HSDB), quoted in PubChem, names thiols as its most common industrial application: organic compounds in which an SH group sits on a hydrocarbon group. Some catalysts used in oil refining reach their full activity only when their metals are sulfides. In 2006 United States production was reported at one billion pounds or more.
Properties
| Property | Value | Unit | Source |
|---|---|---|---|
| Molecular formula (Hill order) | H2S | ||
| Molar mass | 34.076(20) g/mol | g/mol | IUPAC/CIAAW 2024 |
| Hydrogen atoms | 2 | ||
| Sulfur atoms | 1 | ||
| Bonds | 2 | ||
| PubChem molecular weight | 34.08 g/mol | g/mol | PubChem |
Safety
Hydrogen sulfide is a toxic and extremely flammable gas. Every Globally Harmonized System (GHS) classification in its PubChem record carries the signal word Danger, and the European Union’s harmonised classification adds “Fatal if inhaled”. OpenStax rates it as toxic as hydrogen cyanide. NIOSH, the US National Institute for Occupational Safety and Health, notes that the sense of smell tires quickly and cannot be trusted to warn of it. Heavier than air, it collects in sewers, pits and basements. The PubChem Laboratory Chemical Safety Summary and the NIOSH card are the authoritative sheets.
Questions
What causes the rotten egg smell of sewers and hot springs?
Hydrogen sulfide, the colourless gas OpenStax names as responsible for the odour of rotten eggs and many hot springs. Sewage sludge, liquid manure and decaying organic matter release it, and in sewers it collects because it is heavier than air. Bacteria in the mouth and gut also make it while breaking down material that holds vegetable or animal protein, and people can smell it at low levels.
Why do people stop smelling hydrogen sulfide?
OpenStax calls the gas particularly deceptive because it paralyses the olfactory nerves, so after a short exposure the smell is gone. The NIOSH Pocket Guide describes the same effect as rapid fatigue of the sense of smell and warns that smell cannot be relied on to signal that the gas is still present. A fading odour therefore says nothing about whether the air has cleared.
Is H2S a linear molecule?
No. OpenStax classes H2S as nonlinear, and a contrast with carbon dioxide shows why that matters. In CO2 the two bond moments point in opposite directions along one straight line, so they cancel and the molecule is nonpolar. In H2S the two equal S–H moments point in different directions, so they add up to a polar molecule.
Why is hydrogen sulfide a gas when water is a liquid?
Down group 16, from H2S to H2Te, the molecules grow much larger while their polarity drops only slightly. Stronger dispersion forces win, so boiling points climb steadily. Water breaks the pattern: the trend predicts about −80 °C, but it boils far higher. OpenStax takes that gap as evidence of hydrogen bonding, a particularly strong dipole–dipole attraction that forms when hydrogen is bonded to fluorine, oxygen or nitrogen. Sulfur is not one of the three.
Appears in these articles
- Why water is bent and carbon dioxide is straight: the VSEPR model Water's two bonds meet at 104.5°; carbon dioxide's lie in a straight line.
Sources
- PubChem CID 402, hydrogen sulfide
- IUPAC/CIAAW, Abridged Standard Atomic Weights 2024
- PubChem Laboratory Chemical Safety Summary for hydrogen sulfide (CID 402)
- NIOSH Pocket Guide to Chemical Hazards, Hydrogen sulfide
- OpenStax, Chemistry 2e, §7.6 Molecular Structure and Polarity
- OpenStax, Chemistry 2e, §10.1 Intermolecular Forces
- 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, §18.10 Occurrence, Preparation, and Properties of Sulfur
- OpenStax, Biology 2e, §7.5 Metabolism without Oxygen