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The hydrides of sulfur, selenium and tellurium are all extremely toxic gases with repulsive smells. Hydrogen sulfide (H 2 S) is very toxic, in fact it is more than 5x as toxic as HCN ( [link] ). Hydrogen sulfide is considered a broad-spectrum poison, meaning that it can poison several different systems in the body, although the nervous system is most affected. It forms a complex bond with iron in the mitochondrial cytochrome enzymes, thereby blocking oxygen from binding and stopping cellular respiration. Exposure to low concentrations can result in eye irritation, a sore throat and cough, nausea, shortness of breath, and fluid in the lungs. Long-term, low-level exposure may result in fatigue, loss of appetite, headaches, irritability, poor memory, and dizziness.
| Concentration (ppm) | Biological effect |
| 0.00047 | Threshold. |
| 10–20 | Borderline concentration for eye irritation. |
| 50–100 | Eye damage. |
| 100–150 | Olfactory nerve is paralyzed and the sense of smell disappears, often together with awareness of danger. |
| 320–530 | Pulmonary edema with the possibility of death. |
| 530–1000 | Stimulation of the central nervous system and rapid breathing, leading to loss of breathing. |
| 800 | Lethal concentration for 50% of humans for 5 minutes exposure (LC50). |
| +1000 | immediate collapse with loss of breathing, even after inhalation of a single breath. |
Each of the hydrides is prepared by the reaction of acid on a metal chalcogenide, e.g., [link] and [link] . The unstable H 2 Po has been prepared by the reaction of HCl on Po metal.
The thermal stability and bond strength of the dihydrides follows the trend:
While H 2 Se is thermodynamically stable to 280 °C, H 2 Te and H 2 Po are thermodynamically unstable.
All the dihydrides behave as weak acids in water. Thus, dissolution of H 2 S is water results in the formation of the conjugate bases, [link] and [link] , with dissociation constants of 10 -7 and 10 -17 , respectively.
The propensity of sulfur for catenation means that while the hydrides of oxygen are limited to water (H 2 O) and hydrogen peroxide (H 2 O 2 ), the compounds H 2 S n where n = 2 - 6 may all be isolated. Higher homologs are also known, but only as mixtures. All of the sulfanes are yellow liquids whose viscosity increases with increased chain length.
A mixture of lower sulfanes is prepared by the reaction of sodium sulfides (Na 2 Sn) with HCl, [link] . From this mixture the compounds H 2 S n where n = 2 - 5 are purified by fractional distillation. However, higher sulfanes are made by the reaction of either H 2 S or H 2 S 2 with sulfur chlorides, [link] and [link] .
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