Molecular structure and physical properties
In terms of its geometry, the hydrogen sulfide ($H_2S$) molecule largely resembles the water ($H_2O$) molecule, yet it has significant differences. The bond angle is noticeably smaller at $93^\circ$. The fundamental difference lies in intermolecular interaction: hydrogen bonds are not formed between $H_2S$ molecules. Precisely for this reason, under standard conditions, the substance exists in a gaseous state, whereas water is a liquid.
Divalent sulfur is capable of forming stable compounds with the vast majority of elements. The only exceptions are oxygen and fluorine. Many organosulfur compounds in chemistry are traditionally viewed as substituted derivatives of hydrogen sulfide.
Chemical reactions and preparation
In laboratory practice, hydrogen sulfide is usually obtained by the action of hydrochloric acid on solid iron(II) sulfide: $$FeS + 2HCl = FeCl_2 + H_2S\uparrow$$
In aqueous solutions, the gas exhibits the properties of a weak diprotic acid, forming two series of salts: normal (sulfides) and acid (hydrosulfides).
Hydrogen sulfide acts as a strong reducing agent. The products of its oxidation depend directly on the reaction conditions:
- In oxygen deficiency, incomplete oxidation occurs with the release of elemental sulfur:
$$2H_2S + O_2 = 2H_2O + 2S\downarrow$$
- In oxygen excess, sulfur(IV) oxide is formed:
$$2H_2S + 3O_2 = 2H_2O + 2SO_2$$
- Reactions with halogens (e.g., with iodine) are also accompanied by the precipitation of sulfur:
$$H_2S + I_2 = 2HI + S\downarrow$$ (A similar turbidity of the solution due to the release of sulfur is observed when passing the gas through potassium dichromate).
Metal sulfides and their characteristics
Metal sulfides can be obtained by several basic methods: neutralization of hydrogen sulfide with hydroxides, direct synthesis from metals and sulfur, and via exchange reactions.
Regarding their interaction with water, these salts are clearly divided into two groups. Ammonium and alkali metal sulfides are readily soluble, with ammonium sulfide existing in solution exclusively as an acid salt ($NH_4HS$). Soluble salts undergo strong hydrolysis, which causes their solutions to acquire a characteristic hydrogen sulfide odor. Certain compounds, such as $Al_2S_3$, decompose completely and irreversibly with water.
Most other sulfides precipitate. Their specific coloration serves as an excellent qualitative test for metal cations:
| Precipitate formula | Color | Additional properties |
|---|---|---|
| FeS, CuS | Black | — |
| CdS | Yellow | Used as a pigment in painting |
| ZnS | White | Capable of luminescence under an electron beam |
Polysulfides, thiosulfates, and non-metal compounds
Sulfur atoms possess the ability to extend the carbon backbone by attaching to already existing sulfides. As a result, polysulfides are formed: $$Na_2S + (n - 1)S = Na_2S_n$$ In such molecules, $n$ can take values from 1 to 8. Structurally, they represent zigzag anionic chains of sulfur atoms with negative charges at the ends.
Upon boiling a solution of sodium sulfite with sulfur, sodium thiosulfate ($Na_2S_2O_3$) is synthesized. Its structural feature lies in the fact that in the sulfate ion ($SO_4^{2-}$) one oxygen atom is replaced by sulfur. Thus, instead of one $S=O$ double bond, an $S=S$ bond appears.
Sulfur also readily enters into combination reactions with non-metals possessing lower electronegativity:
- When heated with carbon, carbon disulfide ($CS_2$) is formed, which is actively used as a solvent.
- Interaction with phosphorus leads to the formation of phosphorus sulfides ($P_2S_3$, $P_2S_5$), which are used as pyrophoric substances.
Briefly: answers to topical questions
Which metal sulfides undergo complete and irreversible hydrolysis in aqueous solution?
In aqueous solutions, certain metal sulfides undergo complete and irreversible hydrolysis with the formation of a hydroxide precipitate and the release of gaseous hydrogen sulfide. Such compounds include aluminum sulfide, which upon the addition of water forms a turbid mixture with a characteristic hydrogen sulfide odor.
- Aluminum sulfide (Al2S3) — complete irreversible decomposition by water with the formation of aluminum hydroxide and hydrogen sulfide.
In which diseases is balneological treatment with hydrogen sulfide mineral waters indicated?
Balneological treatment using hydrogen sulfide mineral waters and therapeutic baths is used in the comprehensive rehabilitation and spa treatment of patients with various musculoskeletal disorders. Such therapeutic baths are indicated for the following pathologies:
- Gonarthrosis (gonarthrosis) — a disease of the knee joint for which balneotherapy has a high level of recommendation validity.
- Degenerative spine diseases (morbi degenerativi columnae vertebralis) — pathologies in which hydrogen sulfide baths are used at the final stage of conservative treatment.
- Psoriatic arthritis (arthritis psoriatica) — inflammatory joint involvement during periods of minimal arthritis severity or its absence.
What is the biological role of sulfhydryl groups in the formation of the tertiary structure of proteins?
Sulfhydryl groups of cysteine residues play a key role in stabilizing the spatial structure of protein molecules through the formation of covalent disulfide bonds. During the oxidation of thiol groups of two cysteine amino acid residues, a strong disulfide bridge is formed. The presence of such S-S bonds is most characteristic of secreted proteins exported from the cell, where they ensure high molecule stability and protect it from denaturation in extracellular conditions.
To remember the colors of sulfide precipitates: Iron and copper resemble coal (black), cadmium is bright like the sun in a painting (yellow pigment), and zinc is light like the screen of an old television (white phosphor).
Class and exam questions
Why is hydrogen sulfide a gas under normal conditions, while water is a liquid?
Unlike water, hydrogen sulfide molecules do not form hydrogen bonds with each other. This deprives them of the ability to tightly bind to one another, which determines their gaseous aggregate state.
How to structurally distinguish the thiosulfate ion from the sulfate ion?
The thiosulfate ion ($S_2O_3^{2-}$) has a similar geometry to the sulfate ion ($SO_4^{2-}$), but one of the oxygen atoms is replaced by a sulfur atom, meaning one $S=O$ bond is replaced by $S=S$.
Why do aqueous solutions of soluble sulfides smell like rotten eggs?
Soluble alkali metal sulfides undergo strong hydrolysis in water. As a result of this process, gaseous hydrogen sulfide is formed, which volatilizes and produces a characteristic unpleasant odor.
What else to study on this topic
- Mechanisms of step-by-step neutralization reactions of hydrogen sulfide
- Structure and chemical properties of organosulfur compounds
- Physiological mechanisms of action of therapeutic hydrogen sulfide baths
- Detailed analysis of the redox reaction between hydrogen sulfide and potassium dichromate
- Thermodynamics of the irreversible hydrolysis of aluminum sulfide in aqueous solutions
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