Selenhydric acid: formula, properties and synthesis

He Selenic acid , Hydrogen selenide or hyposelenious hydride forms like the rest of the elements of group 16, a hydride of the covalent type, which in general have molecule structure.

They are nonconductive volatile compounds with low melting and boiling points, except in cases where hydrogen bonds can be formed, which is not the case with this particular compound.

Selenic acid Fig. 1. Comparison of the boiling points of the different covalent hydrides.

It is also part of the so-called"hydrogen calcogenides", binary compounds that arise from the union of hydrogen with a calcogen atom, which are the elements that form part of group 16 of the periodic table such as oxygen, sulfur, selenium and tellurium .

Hydrogen selenide, selenium hydride or selenide acid is a colorless, flammable gas under standard conditions (0 ° C = 273,15 K and 1 amt = 1.01325.10 5 Pa), but with a characteristic odor of a rotten egg, as is also the case with compounds having sulfur, which is also found in the same group as the periodic table.

It is the most toxic of selenium compounds, restricting their use or exposure limit to 0.05 ppm / 8 hours. Even when it is in very low concentrations, its smell can become very unpleasant.


Its formula is H2Se, acting selenium with valence -2, and hydrogen with valence +1.


The structure adopted by the H 2 It is approximately angular in space, the angle forming the H-Se-H bond of 91 °, and the distance between the Se-H is 146 pm.
Selenhydric acid: formula, properties and synthesis


Hydrogen selenide is significantly more acidic than hydrogen sulfide (Holleman & Wiberg, 2001). The dissociation of the first proton is in the order of other compounds such as nitrous acid (HNO2) whose Ka = 4.5.10 -4 And of hydrogen fluoride whose Ka = 7.2.10 -4 . Following reaction with its determined acidity constant:
Selenhydric acid: formula, properties and synthesis 1 The second ionization constant of the hydride is of the order of 10 -eleven .

As diprotic acid, it forms both hydrogen selenides of the formula M¹HSe , And the normal seleniuros (or neutral seleniuros) M½Se. Like sulfur homologs, selenium metals are more or less colored, insoluble in water, and sometimes insoluble even in acids, and can be synthesized by the reaction of a selenium hydrogen with a solution of metal salt.

H2Se is a weaker base than H2S, and can be protonated in liquid hydrofluoric acid in the presence of SbF 5 (But is not AsF presence 5 ), At -78 ° C.

Due to its low stability, hydrogen selenide is a potent reducing agent, even more so than hydrogen sulfide.

H's accusative solutions 2 They are oxidized rapidly by atmospheric oxygen, giving a characteristic red precipitate. Due to this sensitivity to oxygen, when you want to synthesize H 2 It must, in the absence of O 2 .

Therefore, it can be said that H2Se has similar properties to H2S, with the difference that the former is more acidic than the latter, and its solubility in water is a reflection of this.

Contact with oxidants, acids, water, and halogenated hydrocarbons may cause explosions (Centers for Disease Control and Prevention, 1978). As for the risk associated with the decomposition of the products, it can emit toxic gases and steam, such as selenium dioxide smoke, which can be released in the form of a fire in which there is hydrogen selenide.

Data of its properties

  • Molecular mass = 80.98 g / mol.
  • Colorless gas.
  • PKa 1 = 3.89; PKa 2 = 11.0 at 25 ° C.
  • Melting and boiling points = - 65.73 ºC and - 41.25 ºC respectively.
  • Solubility in water = 0.70 g / 100 mL.
  • Also soluble in compounds such as CS 2 , And phosgene.


To produce hydrogen selenide or hydrogen selenide on an industrial scale, it is carried out by treating elemental selenium at temperatures above 300 ° C with hydrogen in gaseous form.

Although it is possible to speak of many synthetic routes for the production of selenic acid, both long and small scale, the H 2 It is normally prepared in the laboratory by Al action 2 HE 3 With water, which occurs simultaneously with the formation of hydrated alumina, as seen in the following reaction
Selenhydric acid: formula, properties and synthesis 2

The aluminum selenide needed for this reaction can be prepared by direct synthesis as shown in the following chemical equation (, 2016):

Selenhydric acid: formula, properties and synthesis 3

Being the selenium, a toxic element, it would have to work with him under the hood.

Hydrogen selenide may also be prepared in situ, in aqueous solution (Tlv, 2001), using boron hydride.


Hydrogen selenide is used to prepare (New Jersey Department of Health and Senior Services, 1999), metal selenides and organoselenized compounds. It is also used for doping gas mixtures for the preparation of semiconductor materials containing controlled quantities of significant impurities.


  1. Centers for Disease Control and Prevention. (1978). Occupational health guideline for hydrogen selenide.
  2. Cohen, V. I. (1980). A Convenient Synthesis of Mono-, N , N '-Di-, and Trisubstituted Selenoureas from Methyl Carbamimidothioates ( S -Methylpseudothioureas). Synthesis , 1980 (1), 60-63.
  3. Holleman, A. F., & Wiberg, E. (2001). Inorganic chemistry. In Inorganic chemistry (Page 578). San Diego, California.
  4. New Jersey Department of Health and Senior Services. (1999). Hydrogen Selenide. Hazardous Substance Fact Sheet .
  5. (2016). Synthesis of HYDROGEN SELENIDE. Retrieved January 10, 2017, from
  6. PubChem. (2016). Dihydrogen selenide | H2Se. Retrieved January 9, 2017, from
  7. Tlv, S. (2001). Hydrogen Selenide.
  8. NIOSH Pocket Guide to Chemical Hazards # 0336 . (N.d.). National Institute for Occupational Safety and Health (NIOSH). Retrieved from
  9. Hydrogen selenide. (N.d.). Immediately Dangerous to Life and Health . Retrieved from
  10. Hydrogen Selenide - H2Se. (N.d.). Retrieved January 9, 2017, from

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