Hydrogen chalcogenide
Hydrogen chalcogenides (also chalcogen hydrides or hydrogen chalcides) are binary compounds of hydrogen with chalcogen atoms (elements of group 16: oxygen, sulfur, selenium, tellurium, polonium, and livermorium). Water, the first chemical compound in this series, contains one oxygen atom and two hydrogen atoms, and is the most common compound on the Earth's surface.[1]
Dihydrogen chalcogenides
The most important series, including water, has the chemical formula H2X, with X representing any chalcogen. They are therefore
Water can
Compound | As aqueous solution | Chemical formula | Geometry | pKa | model |
---|---|---|---|---|---|
hydrogen oxide oxygen hydride water (oxidane) |
water | H2O | 13.995 | ||
hydrogen sulfide sulfur hydride (sulfane) |
hydrosulfuric acid |
H2S | 7.0 | ||
hydrogen selenide selenium hydride (selane) |
hydroselenic acid |
H2Se | 3.89 | ||
hydrogen telluride tellurium hydride (tellane) |
hydrotelluric acid |
H2Te | 2.6 | ||
hydrogen polonide polonium hydride (polane) |
hydropolonic acid |
H2Po | ? | ||
hydrogen livermoride livermorium hydride (livermorane) |
hydrolivermoric acid | H2Lv | ? |
Some properties of the hydrogen chalcogenides follow:[3]
Property | H2O | H2S | H2Se | H2Te | H2Po |
---|---|---|---|---|---|
Melting point (°C) | 0.0 | −85.6 | −65.7 | −51 | −35.3 |
Boiling point (°C) | 100.0 | −60.3 | −41.3 | −4 | 36.1 |
−285.9 | +20.1 | +73.0 | +99.6 | ? | |
Bond angle (H–X–H) (gas) | 104.45° | 92.1° | 91° | 90° | 90.9° (predicted)[4] |
Dissociation constant (HX−, K1) | 1.8 × 10−16 | 1.3 × 10−7 | 1.3 × 10−4 | 2.3 × 10−3 | ? |
Dissociation constant (X2−, K2) | 0 | 7.1 × 10−15 | 1 × 10−11 | 1.6 × 10−11 | ? |
Many of the anomalous properties of water compared to the rest of the hydrogen chalcogenides may be attributed to significant
The other hydrogen chalcogenides are highly toxic, malodorous gases. Hydrogen sulfide occurs commonly in nature and its properties compared with water reveal a lack of any significant hydrogen bonding.
Dihydrogen dichalcogenides
Dihydrogen dichalcogenides have the chemical formula H2X2, and are generally less stable than the monochalcogenides, commonly decomposing into the monochalcogenide and the chalcogen involved.
The most important of these is hydrogen peroxide, H2O2, a pale blue, nearly colourless liquid that has a lower volatility than water and a higher density and viscosity. It is important chemically as it can be either oxidised or reduced in solutions of any pH, can readily form peroxometal complexes and peroxoacid complexes, as well as undergoing many proton acid/base reactions. In its less concentrated form hydrogen peroxide has some major household uses, such as a disinfectant or for bleaching hair; much more concentrated solutions are much more dangerous.
Compound | Chemical formula | Bond length | Model |
---|---|---|---|
hydrogen peroxide (dioxidane) |
H2O2 | ||
hydrogen disulfide (disulfane) |
H2S2 | ||
hydrogen diselenide[7] (diselane) |
H2Se2 | — | |
hydrogen ditelluride[8] (ditellane) |
H2Te2 | — |
Some properties of the hydrogen dichalcogenides follow:
Property | H2O2 | H2S2 | H2Se2 | H2Te2 |
---|---|---|---|---|
Melting point (°C) | -0.43 | −89.6 | ? | ? |
Boiling point (°C) | 150.2 (decomposes) | 70.7 | ? | ? |
An alternative structural isomer of the dichalcogenides, in which both hydrogen atoms are bonded to the same chalcogen atom, which is also bonded to the other chalcogen atom, have been examined computationally. These H2X+–X– structures are ylides. This isomeric form of hydrogen peroxide, oxywater, has not been synthesized experimentally. The analogous isomer of hydrogen disulfide, thiosulfoxide, has been detected by mass spectrometry experiments.[9]
It is possible for two different chalcogen atoms to share a dichalcogenide, as in hydrogen thioperoxide (H2SO); more well-known compounds of similar description include sulfuric acid (H2SO4).
Higher dihydrogen chalcogenides
All straight-chain hydrogen chalcogenides follow the formula H2Xn.
Higher
Beyond H2S and H2S2, many higher polysulfanes H2Sn (n = 3–8) are known as stable compounds.[12] They feature unbranched sulfur chains, reflecting sulfur's tendency for catenation. Starting with H2S2, all known polysulfanes are liquids at room temperature. H2S2 is colourless while the other polysulfanes are yellow; the colour becomes richer as n increases, as do the density, viscosity, and boiling point. A table of physical properties is given below.[13]
Compound | Density at 20 °C (g·cm−3) | mmHg )
|
Extrapolated °C )
|
---|---|---|---|
H2S | 1.363 (g·dm−3) | 1740 (kPa, 21 °C) | −60 |
H2S2 | 1.334 | 87.7 | 70 |
H2S3 | 1.491 | 1.4 | 170 |
H2S4 | 1.582 | 0.035 | 240 |
H2S5 | 1.644 | 0.0012 | 285 |
H2S6 | 1.688 | ? | ? |
H2S7 | 1.721 | ? | ? |
H2S8 | 1.747 | ? | ? |
However, they can easily be oxidised and are all thermally unstable, disproportionating readily to sulfur and hydrogen sulfide, a reaction for which alkali acts as a catalyst:[13]
- 8 H2Sn → 8 H2S + (n − 1) S8
They also react with sulfite and cyanide to produce thiosulfate and thiocyanate respectively.[13]
An alternative structural isomer of the trisulfide, in which the two hydrogen atoms are attached to the central sulfur of the three-sulfur chain rather than one on each end, has been examined computationally.[11] Thiosulfurous acid, a branched isomer of the tetrasulfide, in which the fourth sulfur is bonded to the central sulfur of a linear dihydrogen trisulfide structure ((HS)2S+−S−), has also been examined computationally.[14] Thiosulfuric acid, in which two sulfur atoms branch off of the central of a linear dihydrogen trisulfide structure has been studied computationally as well.[15]
Higher polonium hydrides may exist.[16]
Other hydrogen-chalcogen compounds
Some monohydrogen chalcogenide compounds do exist and others have been studied theoretically. As radical compounds, they are quite unstable. The two simplest are hydroxyl (HO) and hydroperoxyl (HO2). The compound hydrogen ozonide (HO3) is also known,[17] along with some of its alkali metal ozonide salts are (various MO3).[18] The respective sulfur analogue for hydroxyl is sulfanyl (HS) and HS2 for hydroperoxyl.
One or both of the protium atoms in water can be substituted with the isotope deuterium, yielding respectively semiheavy water and heavy water, the latter being one of the most famous deuterium compounds. Due to the high difference in density between deuterium and regular protium, heavy water exhibits many anomalous properties. The radioisotope tritium can also form tritiated water in much the same way. Another notable deuterium chalcogenide is deuterium disulfide. Deuterium telluride (D2Te) has slightly higher thermal stability than protium telluride, and has been used experimentally for chemical deposition methods of telluride-based thin films.[19]
Hydrogen shares many properties with the halogens; substituting the hydrogen with halogens can result in chalcogen halide compounds such as oxygen difluoride and dichlorine monoxide, alongside ones that may be impossible with hydrogen such as chlorine dioxide.
Hydrogen Ions
One of the most well-known hydrogen chalcogenide ions is the
The hydronium (H3O+) ion is present in aqueous acidic solutions, including the hydrochalcogenic acids themselves, as well as pure water alongside hydroxide.
References
- ^ "CIA – The world factbook". Central Intelligence Agency. Retrieved 18 August 2016.
- ^ "About the International Decade for Action 'Water for Life' 2005-2015".
- ^ a b Greenwood and Earnshaw, pp. 766–7
- doi:10.1063/1.458298.
- ^ Greenwood and Earnshaw, p. 623
- ^ Greenwood and Earnshaw, p. 682
- .
- .
- .
- ^ Greenwood and Earnshaw, pp. 633–8
- ^ .
- doi:10.1007/b13182
- ^ a b c Greenwood and Earnshaw, p. 683
- .
- S2CID 95470892.
- arXiv:1503.08587 [cond-mat.supr-con].
- PMID 10390365.
- ^ Wiberg 2001, p. 497
- ^ Xiao, M. & Gaffney, T. R. Tellurium (Te) Precursors for Making Phase Change Memory Materials. (Google Patents, 2013) (https://www.google.ch/patents/US20130129603)
Bibliography
- ISBN 978-0-08-037941-8.