H2S is the chemical formula for the compound hydrogen sulfide. Hydrogen sulfide is a covalent compound that is composed out of 2 hydrogen atoms bonded to a central sulfur atom. Like water (H20), hydrogen sulfide is a hydrogen chalcogenide—a compound made from hydrogen and a group 16 element (oxygen, sulfur, selenium, tellurium. Hydrogen sulfide is non-polar on account of its nonpolar H–S bonds. The EN difference between hydrogen and sulfur is 0.4, so hydrogen and sulfur form non-polar bonds. Although it has an asymmetrical molecular geometry, the entire molecule is non-polar dues to the absence of any polar bonds.
Hydrogen sulfide is most commonly encountered as a product of the anaerobic respiration of sulfidogenic organisms. For instance, some bacteria that operate in the absence of oxygen use sulfate ions (SO4–) as the terminal electron acceptor during cellular respiration which reduces it into H2S. In other words, sulfidogenic organisms breathe sulfur and exhale hydrogen sulfide. Conversely, in aerobic organisms, molecular oxygen (O2) acts as the terminal electron acceptor during respiration, which is reduced into H2O. It is also the product of processes in volcanoes and natural gas formations.
Hydrogen sulfide is known for its pungent odor that is described as rotting eggs. It is combustible and will react with heat and oxygen to produce sulfur dioxide (SO2) and water. Hydrogen sulfide is poisonous to humans in large amounts. its level of toxicity is comparable to that of carbon monoxide (CO). When inhaled, hydrogen sulfide will bind to enzymes in the mitochondria, which prevents cellular respiration.
Polarity In A Nutshell
Essentially, polarity in chemistry is a measure of how evenly distributed electrons in a molecule are. When two atoms form a covalent bond, they do so by sharing valence electrons. Each element has an electronegativity which is a measure of how hard they pull on electrons. When two elements that differ greatly in their electronegativities form a covalent bond, the more electronegative element will pull harder of the shared electrons than the less electronegative element. The result is that the shared electrons are pulled closer to the more electronegative element.
The uneven displacement of electric charges in the molecule gives the more electronegative element a partial negative charge and the less electronegative element a partial positive charge. This is what it means for a molecule to be polar; it has a partially charged dipole across its structure on account of the uneven spatial distribution of electrons.
Whether or not two atoms will form a polar or non-polar bond is dependent on those elements’ respective electronegativities. If two elements have an EN difference between 0.5 and 2, the bond is generally considered polar. If the difference is less than 0.5, it is considered functionally non-polar. If the difference is greater than 2, then the bond is completely polar, and is more properly referred to as an ionic bond.
For instance, a molecule of water is polar in virtue of its H-O bonds. Hydrogen has an EN of 2.1 and oxygen has an EN value of 3.5. the difference between these two values is 1.4, so H-O bonds are considered polar, with a partial negative charge on the oxygen.
The Polarity Of Hydrogen Sulfide
Applying the previous lesson on polarity, we can find out if hydrogen sulfide is a polar compound. Hydrogen has an EN value of 2.1 and sulfur has an EN value of 2.5. the difference between these two values is less than 0.5, so H-S bonds are classified as non-polar. Since hydrogen sulfide consists entirely of non-polar H-S bonds, the entire molecule is non-polar.
Strictly speaking, H-S bonds are not completely non-polar. Sulfur is slightly more electronegative than hydrogen, so it does pull slightly harder on the shared electrons. This polarity is very weak though, and practically, it is useful to treat very weakly polar bonds as if they are not polar at all. So even though H-S bonds are technically a little bit polar, most of the time it is safe to treat them as if they are non-polar. The only truly non-polar bonds are formed between atoms with identical EN values (like the diatomic molecules) The very slight polarity of hydrogen sulfide has significant effects at small scales, so in certain circumstances, it would be appropriate to treat H-S bonds as polar.
Hydrogen Sulfide As A Compound
Hydrogen sulfide is a triatomic (3-atom) molecule that consists of a central sulfur atom and 2 terminal hydrogen atoms. Like a molecule of water, hydrogen sulfide has a bent geometric structure with a bond angle of 92.1° and bond lengths of 136 picometers (1 picometer = 1 trillionth of a meter). It is a bit denser than air and is explosive in the presence of oxygen and heat. Hydrogen sulfide is slightly soluble in water, and will disassociate into a lone proton (H+) and a hydrosulfide ion (HS−). This behavior makes hydrogen sulfide a weak acid.
Hydrogen sulfide is combustible and will react with oxygen and heat to form sulfur dioxide and water. Under high temperature, sulfur dioxide will convert to elemental sulfur and water, so the combustion of hydrogen sulfide is often used as one of the steps to produce pure elemental sulfur. It reacts with metal ions to form metal sulfides, most commonly with lead (Pb) to form lead(II) sulfide (PbS). Conversely, treating metal sulfides with a strong acid results in the production of hydrogen sulfide.
Occurrences Of Hydrogen Sulfide
One of the primary natural sources of hydrogen sulfide is the activity of sulfidogenic bacteria. Sulfidogenic bacteria use sulfur instead of oxygen for their metabolisms. During sulfidogenic respiration, bacteria will use sulfate ions as a reducing agent to carry electrons on the electron transport train. At the end of this reaction, the sulfate ions are reduced into hydrogen sulfide which is released into the environment. The activity of sulfidogenic bacteria and their hydrogen sulfide products are responsible for the rotting smell associated with places with large quantities of decaying organic matter, like marshes or sewers.
The activity of sulfidogenic bacteria is of crucial importance to the sulfur cycle on earth. Thus, hydrogen sulfide is one of the main constituents of the sulfur cycle. The sulfur cycle is the process by which sulfur is cycled through the environment, into living organisms, and back into the environment. Sulfur is a necessary trace element for living organisms, so the sulfur cycle is what keeps a constant supply of elemental sulfur for living organisms to use. The production of hydrogen sulfide by sulfidogenic bacteria represents an important step in this cycle; the production of the sulfur that will eventually make its way into living organisms.
Small amounts of hydrogen sulfide are also produced in geochemical reactions in the Earth’s crust. The earth;s crust contains large quantities of sulfur and sulfur-containing minerals. Under the presence of heat and pressure, metal sulfide compounds will undergo hydrolysis with water to form a metal oxide and hydrogen sulfide gas. As such, hydrogen sulfide is a natural product of the process that creates natural gas. In fact, a large amount of hydrogen sulfide is produced via the separation of it from natural gas deposits. Similar mechanisms also result in the formation of hydrogen sulfide in thermal ocean vents.
Although hydrogen sulfide is extremely toxic to humans in large quantities, small amounts of hydrogen sulfide play a crucial role in human biology. Hydrogen sulfide in the body often acts a signaling molecule that regulates the amount of ATP production during cellular respiration. Hydrogen sulfide also seems to be implicated in the vasoconstriction of animal blood vessels and the rate of seed germination in plants.
Toxicity Of Hydrogen Sulfide
In general, hydrogen sulfide is very toxic to obligate oxygen breathers. Its mechanisms of action are similar to that of carbon monoxide. Hydrogen sulfide will bind to important enzymes and cofactors, preventing them from doing their job during cellular respiration. Since hydrogen sulfide is naturally produced in the human body, the body does have mechanisms for removing hydrogen sulfide, though these mechanisms can be outpaced by a large enough dose.
The symptoms of hydrogen sulfide poisoning are similar to those of carbon monoxide poisoning; fatigue, dizziness, inability to concentrate, loss of memory, and irritability. Though initially a pungent odor, the body quickly acclimates to the smell, which can make people unaware of its presence. It is slightly denser than air, so it has a tendency to accumulate near the bottom of poorly ventilated spaces. The human body can tolerate low concentrations of hydrogen sulfide for some time. In high concentrations, inhalation of hydrogen sulfide can be immediately fatal or cause serious brain damage.
Historically, doctors have diagnosed extreme cases of hydrogen sulfide poisoning by placing a copper coin in the victim’s pocket. If the patient has high quantities of hydrogen sulfide in their body, it will react with the copper coin in their pocket, oxidizing it and turning it green.