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Hydrogen Sulphide

Hydrogen sulphide or hydrogen sulfide is one of the most distinctive chemical compounds that can be identified quite easily. It has a smell that is obnoxiously foul and closely resembles the smell of rotten eggs. Hydrogen sulphide is a colourless chalcogen hydride gas with the formula H2S. However, it is a highly corrosive, flammable and poisonous gas.

Hydrogen sulphide is most commonly formed due to the microbial breakdown of organic matter in the absence of oxygen. This occurs mostly in swamps and sewers and the process is known as anaerobic digestion. This is mainly carried out by sulfate-reducing microorganisms. Hydrogen sulphide is also found in volcanic gases, crude petroleum, natural gas and some sources of well water or hot springs. Interestingly, the human body also produces small amounts of hydrogen sulphide which is further used as a signalling molecule.

Hydrogen sulfide gas is mainly used in industrial areas. It can be a byproduct of industrial activities such as petroleum refineries, food processing, tanneries, coke ovens and kraft paper mills. Meanwhile, the deliberate mixture of household chemicals to form hydrogen sulfide are extensively used as means of suicide and the cases pose a threat for the first responders. The emergency physicians need to be aware of the management of hydrogen sulfide since rapid identification and treatment is critical for recovery.

The chemical composition of hydrogen sulfide was discovered in 1777 by a Swedish chemist named Carl Wilhelm Scheele.

Overview

Properties

Chemical formula H2S
Molar mass 34.08 g·mol-1
Appearance Colourless gas
Odour Pungent, like that of rotten eggs
Density 1.363 g dm-3
Melting point −82 °C (−116 °F; 191 K)
Boiling point −60 °C (−76 °F; 213 K)
Solubility in water 4 g dm-3 (at 20 °C)
Vapour pressure 1740 kPa (at 21 °C)
Acidity (pKa) 7.0
Conjugate acid Sulfonium
Conjugate base Bisulfide
Magnetic susceptibility (χ) −25.5·10-6 cm-3/mol

Hydrogen Sulphide

Structure of Hydrogen Sulphide

The structure of hydrogen sulfide is similar to that of water. If we look at the Lewis structure of hydrogen sulphide it is a combination of two hydrogen atoms and one sulfur atom. Sulfur is the centre particle and contains 2 solitary hydrogens linked with the help of a single bond.

Hydrogen Sulphide

Molecular geometry

Bent

Electronic geometry

Tetrahedral

Hybridization

sp3

Bond angle

92.1º

Total valence electrons

8

Formal charge

0

However, sulfur is not as electronegative as oxygen as such that hydrogen sulfide is not as polar as water. Due to this, comparatively weak intermolecular forces exist in H2S and the boiling points and melting points are way lower than in water.

Properties of Hydrogen Sulphide

Let us look at some of the properties of hydrogen sulfide.

Physical Properties of Hydrogen Sulphide

  • The boiling points of water and hydrogen sulfide are 100oC and -60oC respectively.
  • Hydrogen sulfide is slightly denser or heavier than air but fairly soluble in water.
  • Hydrogen sulphide is a colourless, poisonous gas that can lead to headaches even if it is inhaled in small quantities.
  • It has a distinct rotten egg smell.
  • Hydrogen sulfide is a flammable gas.

Chemical Properties of Hydrogen Sulphide

  • A mixture of air and H2S can lead to an explosion.
  • Oxygen and Hydrogen sulfide catch fire with a blue flame to produce sulfur dioxide (SO2) and water.
  • Hydrogen sulfide serves as a reducing agent and mostly in the presence of the base that makes SH.
  • In the presence of catalysts or at higher temperatures, sulfur dioxide reacts with hydrogen sulfide to produce water and elemental sulfur. This reaction is an essential industrial method to dispose of hydrogen sulfide, exploited in the Claus process.
  • It is acidic in nature as it turns blue litmus paper to red.
  • Hydrogen sulfide can react with metal ions to form metal sulfides that are insoluble and often dark-coloured solids.
  • At pressures above 90 GPa, hydrogen sulphide becomes a metallic conductor of electricity.
  • Hydrogen sulphide has a tendency to decompose on heating. It usually dissociates into hydrogen and sulphur.
  • It also has reducing properties due to which it can bring about changes in the colour of the substance.

Production of Hydrogen Sulphide

The most common method that can be used to obtain hydrogen sulfide is by separating it from sour gas. Now, this gas is basically natural gas with a high content of H2S.

Hydrogen sulphide can also be produced by using molten elemental sulfur to treat hydrogen at about 450 °C. In this, hydrocarbons serve as a source of hydrogen in this process.

Hydrogen sulfide is mainly obtained as a waste product when sulfate-reducing bacteria oxidize organic compounds or hydrogen to generate usable energy under low-oxygen conditions.

Lab Preparation of Hydrogen Sulphide

A standard lab preparation involves treating ferrous sulfide with a strong acid in a Kipp generator:

FeS + 2 HCl → FeCl2 + H2S

Thioacetamide can be used to generate H2S during qualitative inorganic analysis.

CH3C(S)NH2 + H2O → CH2C(O)NH2 + H2S

Many metal and nonmetal sulfides are exposed to water they liberate hydrogen sulphide gas.

6 H2O + Al2S3 → 3 HS + 2 Al(OH)3

Hydrogen sulphide gas is also produced by heating sulfur with solid organic compounds and by reducing sulfurated organic compounds with hydrogen.

Uses Hydrogen sulphide

  • It is used for the detection of cations which is studied in analytical chemistry.
  • Hydrogen sulfide is a precursor to elemental sulfur. Several organosulfur compounds such as methanethiol, ethanethiol, and thioglycolic acid are produced using hydrogen sulfide.
  • Hydrogen sulphide is used to prepare metallic sulphides many of which are used in the paint industry.
  • It is used to separate deuterium oxide or heavy water from normal water via the Girdler sulfide process.
  • In the medical field wherein cell exposure to small amounts of hydrogen sulfide gas led to the prevention of mitochondrial damage.

Safety Hazards

Hydrogen sulfide is a highly explosive and flammable gas and is capable of causing life-threatening conditions if handled without care. Moreover, hydrogen sulfide gas burns readily and forms other toxic gases and vapours like sulfur dioxide.

If we are exposed to low concentrations of hydrogen sulphide then it might cause some irritation to the nose, throat or eyes,  nausea, shortness of breath. Some may experience difficulty in breathing especially people with conditions like asthma. While a low concentration of hydrogen sulphide is not worrisome it can cause headaches, memory loss, tiredness and body balancing problems.

On the other hand, brief exposures even to high concentrations of hydrogen sulphide (usually greater than 1000 ppm) can cause loss of consciousness. In some cases, individuals might suffer from permanent or long-term effects such as headaches, low attention span, dizziness, poor memory and motor function.

Apart from exposure to hydrogen sulfide, exposure to fluid hydrogen sulfide can lead to frostbite or “blue skin”.

Frequently Asked Questions on Hydrogen sulphide

Is Hydrogen sulfide toxic to humans?

Yes, hydrogen sulfide is highly toxic to humans. It causes respiratory depression, skin irritation or even coma.

Where is hydrogen sulfide found?

Hydrogen sulfide is found in natural gas and crude petroleum. Decomposition of animal and human waste also produces hydrogen sulfide.

What is the smell of hydrogen sulfide?

Hydrogen sulfide has a bad odor of rotten eggs.

How is hydrogen sulfide removed from water?

Hydrogen sulfide is removed from water by a process called aeration. In this process, the air is circulated through the water. The hydrogen sulfide dissolved in the air is vented outside, removing hydrogen sulfide from the water.

Is hydrogen sulfide flammable?

Yes, hydrogen sulfide is a highly flammable gas.
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