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Hydrogen Sulfide

What is hydrogen sulfide?

Hydrogen sulfide (chemical formula H2S) is a colorless, flammable, toxic, or poisonous gas molecule that can be detected by its smell of rotten eggs. In 1977, Swedish chemist Carl Wilhelm Scheele first discovered and proved that hydrogen sulfide is a molecule that contains hydrogen and sulfur. Hydrogen sulfide uses mainly for the production of sulfur and sulfuric acid. It is found naturally in crude petroleum, natural gas, and hot springs. It burns in oxygen with blue flame by the formation of sulfur dioxide and water. Hydrogen sulfide poisoning causes different types of health problems like eye irritation, cough, nausea, shortness of breath, etc. Like carbon monoxide, hydrogen sulfide poisoning also prevents cellular respiration in our bodies.

Hydrogen sulfide poisoning

Hydrogen sulfide is a highly toxic gas or broad-spectrum poison for humans and animals. It can affect the respiration or nervous system of our body. The effects of hydrogen sulfide poisoning are comparable with carbon monoxide poisoning. Carbon monoxide binds to iron in hemoglobin to form carboxyhemoglobin complexes to prevent the transport of oxygen from the lungs to our cells. Hydrogen sulfide also binds to iron in the mitochondrial cytochrome enzymes to prevent cellular respiration in our body.

  • The low explosion of hydrogen sulfide causes different types of health problems such as eye irritation, cough, nausea, shortness of breath, etc.
  • The low-level long-term explosion causes the health problems such as fatigue, loss of appetite, headaches, irritability, poor memory, etc.
  • A short-term high-level explosion of hydrogen sulfide affects immediately on our cellular respiration which may be caused to death.

Hydrogen sulfide structure

Hydrogen sulfide structure formula and uses with poisoning or toxic effects on humans

The structure of the polar hydrogen sulfide molecule is given above the picture. It is formed by sp3 hybridization of the sulfur atom which binds with two hydrogen atoms by forming a bond angle of 92.1° and bond length 136.3 pm.

Where is hydrogen sulfide found?

It is found naturally in crude petroleum, natural gas, and hot springs. The H2S occurring in the hot springs arise due to the hydrolysis of sulfide minerals. Bacterial breakdown of organic materials and human wastes produced some part of H2S in our environment. Industrial activities such as petroleum or natural gas refining, wastewater treatment, and paper mills are also the other sources of H2S to our environment.

How to make hydrogen sulfide?

In the laboratory, hydrogen sulfide can be made by the action of non-oxidizing acids such as HCl on iron sulfide.
FeS + 2 HCl → FeCl2 + H2S

Various metal and nonmetal sulfides such as aluminum sulfide, phosphorus pentasulfide, silicon disulfide can be liberated hydrogen sulfide gas when exposed to water.
Al2S3 + 6 H2O → 3 H2S + 2 Al(OH)3

A direct combination of hydrogen and sulfur at elevated temperatures also gives hydrogen sulfide. The reaction mixture is cooling to 60°C. It may be liquefied and then purified by distillation.

It can be prepared by heating a mixture of solid sulfur with hydrocarbon like alkane or paraffin (CnH2n+2). It is a convenient method for the making of hydrogen sulfide in the laboratory.

Chemical properties

The poisonous gas hydrogen sulfide is slightly denser than air. It burns in oxygen to form a blue flame. Some common properties are given below the table,

IUPAC name hydrogen sulfide
Chemical formula H2S
Molar mass 34.08 g mol-1
Appearance Colorless gas
Odor Smell like rotten eggs
Density 1.363 g dm-3
Melting point -82 °C
Boiling point -60 °C
Solubility in water 4 g dm-3
Conjugate acid Sulfonium
Conjugate base Bisulfide
Dipole moment 0.67 D
Heat capacity 1.003 J K-1 g-1

Hydrogen sulfide in water

The hydrogen sulfide gas is soluble in water to form a 0.1M solution. Therefore, H2S is a weak dibasic acid with pka1 = 10-7 and pka2 = 10-15. The concentration of H2S, HS and S depends on the addition of acid or alkali to the saturated solution of H2S.

  • The addition of H+ ions or acid to a saturated solution of H2S decreases the concentration of HS and S-2.
  • The addition of OH ions or alkali to a saturated solution of H2S increases the concentration of HS and S-2.

Reducing properties

In an acid solution, the standard potential of the S/H2S couple is +0.14 volt. Therefore, H2S can be readily oxidized to sulfur by the addition of a large number of oxidants like KMnO4, K2Cr2O7, HNO3, etc.
H2S + 2 HNO3 → S + 2 NO2 + 2H2O

Precipitating properties

Separation of metal ions becomes possible by precipitating the metal sulfides with hydrogen sulfide. Experiments show that the S-2 concentration in a saturated solution of H2S and 0.3 M HCl is just high enough to precipitate insoluble sulfides like HgS, CuS.

Polysulfide formation

Polysulfides are formed when sulfur is boiled with a solution of alkali sulfide. The solution changes its color from yellow to dark red. Disulfide (S2-2), trisulphide (S3-2) tetrasulfide (S4-2), and pentasulphide (S5-2) are the common examples of polysulfide. An acidified polysulfide solution gives free hydrides such as H2S2, H2S3, H2S4, etc. The density, viscosity, and boiling point of such hydrides increase with the increasing chain length.

Hydrogen sulfide uses

  • It is used mainly for the production of sulfur and sulfuric acid.
  • In analytical chemistry, hydrogen sulfide is used for the qualitative analysis of metal ions. The heavy metal ions such as Pb(II), Cu(II), Hg(II), As(III)) are precipitated from the solution in presence of H2S.
  • It is also used for the production of various inorganic sulfides which are used in various industries. Inorganic sulfides produced from hydrogen sulfide are used mainly in the leather and pharmaceuticals industries.
  • Hydrogen sulfide is used for the production of heavy water which is used widely in nuclear power reactors.