Hydrogen chalcogenide

Water, hydrogen sulfide, and hydrogen selenide, three simple hydrogen chalcogenides

Hydrogen chalcogenides (also chalcogen hydrides or hydrogen chalcides) are binary compounds of hydrogen with chalcogen atoms (elements of group 16: oxygen, sulfur, selenium, tellurium, and polonium). 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 triatomic. They take on a bent structure and as such are polar molecules. Water is an essential compound to life on Earth today,[2] covering 70.9% of the planet's surface. The other hydrogen chalcogenides are usually extremely toxic, and have strong unpleasant scents usually resembling rotting eggs or vegetables. Hydrogen sulfide is a common product of decomposition in oxygen-poor environments and as such is one chemical responsible for the smell of flatulence. It is also a volcanic gas. Despite its toxicity, the human body intentionally produces it in small enough doses for use as a signaling molecule.

Water can dissolve the other hydrogen chalcogenides (at least those up to hydrogen telluride), forming acidic solutions known as hydrochalcogenic acids. Although these are weaker acids than the hydrohalic acids, they follow a similar trend of acid strength increasing with heavier chalcogens, and also form in a similar way (turning the water into a hydronium ion H3O+ and the solute into a XH ion). It is unknown if polonium hydride forms an acidic solution in water like its lighter homologues, or if it behaves more like a metal hydride (see also hydrogen astatide).

Compound As aqueous solution Chemical formula Geometry pKa model
hydrogen oxide
oxygen hydride
water H2O H2O 2D labelled.svg 13.995 Water molecule 3D.svg
hydrogen sulfide
sulfur hydride
hydrosulfuric acid H2S Hydrogen-sulfide-2D-dimensions.svg 7.0 Hydrogen-sulfide-3D-vdW.svg
hydrogen selenide
selenium hydride
hydroselenic acid H2Se Hydrogen-selenide-2D-dimensions.svg 3.89 Hydrogen-selenide-3D-vdW.svg
hydrogen telluride
tellurium hydride
hydrotelluric acid H2Te Hydrogen-telluride-2D-dimensions.svg 2.6 Hydrogen-telluride-3D-vdW.svg
hydrogen polonide
polonium hydride
hydropolonic acid H2Po ? Polonium-hydride-3D-vdW.svg

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 ?
Comparison of the boiling points of the hydrogen chalcogenides and hydrogen halides; it can be seen that hydrogen fluoride similarly exhibits anomalous effects due to hydrogen bonding. Ammonia also misbehaves similarly.

Many of the anomalous properties of water compared to the rest of the hydrogen chalcogenides may be attributed to significant hydrogen bonding between hydrogen and oxygen atoms. Some of these properties are the high melting and boiling points (it is a liquid at room temperature), as well as the high dielectric constant and observable ionic dissociation. Hydrogen bonding in water also results in large values of heat and entropy of vaporisation, surface tension, and viscosity.[5]

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.[6] Since they are both gases at STP, hydrogen can be simply burned in the presence of oxygen to form water in a highly exothermic reaction; such a test can be used in beginner chemistry to test for the gases produced by a reaction as hydrogen will burn with a pop. Water, hydrogen sulfide, and hydrogen selenide may be made by heating their constituent elements together above 350 °C, but hydrogen telluride and polonium hydride are not attainable by this method due to their thermal instability; hydrogen telluride decomposes in moisture, in light, and in temperatures above 0 °C. Polonium hydride is unstable, and due to the intense radioactivity of polonium (resulting in self-radiolysis upon formation), only trace quantities may be obtained by treating dilute hydrochloric acid with polonium-plated magnesium foil. Its properties are somewhat distinct from the rest of the hydrogen chalcogenides, since polonium is a metal while the other chalcogens are not, and hence this compound is intermediate between a normal hydrogen chalcogenide or hydrogen halide such as hydrogen chloride, and a metal hydride like stannane. Like water, the first of the group, polonium hydride is also a liquid at room temperature. Unlike water, however, the strong intermolecular attractions that cause the higher boiling point are van der Waals interactions, an effect of the large electron clouds of polonium.[3]