Obtaining and properties of hydrogen sulfide
Hydrogen sulfide (H₂S) is a colorless gas with the smell of rotten eggs. It is heavier than hydrogen by its density. Hydrogen sulfide is a deadly poison for humans and animals. Even a small amount in the air causes headaches and nausea, but the worst thing is that it is inhaled for a lengthy period, you cease to detect the smell.
However, as in the case with other poisons, there is a simple antidote: in cases of hydrogen sulfide poisoning you should sniff a handkerchief soaked in acetic acid, with a piece of lime chloride wrapped in it.
H₂ + S → H₂S↑
This is an oxidation-reduction reaction, as the degrees of oxidation of the elements change.
In the laboratory, hydrogen sulfide is obtained by reacting iron sulfide with sulfuric or hydrochloric acid:
FeS + 2HCl → FeCl₂ + H₂S
This is an exchange reaction, as the interacting substances exchange ions. This process is usually carried out with Kipp’s apparatus.
Properties of hydrogen sulfide
In the combustion of hydrogen sulfide, sulfur oxide 4 and water vapor form:
2H₂S + 3О₂ → 2Н₂О + 2SO₂
H₂S burns with a bluish flame, and if you hold an upturned beaker over this flame, transparent drops of condensing water from the water vapor will form.
However, if the temperature is reduced slightly, this reaction takes place somewhat differently – and a yellow coating of free sulfur appears on the walls of the cooled beaker:
2H₂S + О₂ → 2Н₂О + 2S
The industrial method of obtaining sulfur is based on this reaction. When heating a prepared gaseous mixture of hydrogen sulfide with oxygen, an explosion takes place. The reaction of hydrogen sulfide and sulfur oxide 4 also makes it possible to obtain free sulfur:
2H₂S + SО₂ → 2Н₂О + 3S
Hydrogen sulfide is soluble in water, and three volumes of this gas can dissolve in one volume of water, forming weak and unstable hydrosulfuric acid (H₂S). This acid is also called hydrogen sulfide water. As you can see, the formula of hydrogen sulfide gas and hydrosulfuric acid are written in the same way.
If you add a solution of lead salt to hydrosulfuric acid, a black sediment of lead sulfide forms:
H₂S + Pb(NO₃)₂ → PbS + 2HNO₃
This is the qualitative reaction for detecting hydrogen sulfide. It demonstrates the ability of hydrosulfuric acid to enter into an exchange reaction with solutions of salts. Thus, any soluble lead salt is a reagent for hydrogen sulfide. Some other metal sulfides also have a characteristic color, for example: zinc sulfide (ZnS) is white, cadmium sulfide (CdS) is yellow, copper sulfide (CuS) is black, and antimony sulfide (Sb₂S₃) is red.
Incidentally, hydrogen sulfide is an unstable gas, and it breaks down almost completely into hydrogen and free sulfur on heating:
H₂S → Н₂ + S
Hydrogen sulfide interacts intensely with aqueous solutions of halogens:
H₂S + 4Cl₂ + 4H₂O→ H₂SO₄ + 8HCl
Hydrogen sulfide in nature and practical application
Hydrogen sulfide is part of volcanic gases, natural gas and gases found in oil fields. There is a great deal of it in natural mineral waters, for example in the Black Sea it lies at a depth from 150 meters and below.
Hydrogen sulfide is used:
- in medicine – treatment with hydrogen sulfide baths and mineral waters;
- in industry – obtaining sulfur, sulfuric acid and sulfides;
- in analytical chemistry – for the precipitation of sulfides of heavy metals, which are usually insoluble;
- in organic synthesis – to obtain sulfurous equivalents of organic alcohols – mercaptans, and thiophen – an aromatic hydrocarbon containing sulfur.
Another direction in science that has appeared recently is hydrogen sulfide energy. At present serious research is being conducted on obtaining energy from deposits of hydrogen sulfide at the bottom of the Black Sea.
The nature of the oxidation-reduction reactions of sulfur and hydrogen
Hydrogen sulfide is formed by sulfur and hydrogen. The reaction of its formation is an oxidation-reduction reaction:
Н₂⁰ + S⁰→ H₂⁺S²⁻
The process of the interaction of sulfur with hydrogen can be easily explained by the structure of their atoms: hydrogen holds first place on the periodic table, and accordingly the charge of its atomic nucleus is equal to (+1), and 1 electron revolves around its nucleus. Hydrogen easily gives up its electron to atoms of other elements, turning into a positively charged hydrogen atom – a proton.
Н⁰ -1е⁻= Н⁺
Sulfur holds 16th place on the period table. So the charge of its nucleus is equal to (+16), and the number of electrons in each atom is also 16е-. The position of sulfur in the third period shows that its 16 electrons revolve around the nucleus, forming 3 layers, and in the outer layer there are 6 valent electrons. The number of valent electrons of sulfur corresponds to group VI, where it is located in the periodic system.
So, sulfur can give up all six of its valent electrons, as in the case of the formation of sulfur oxide (VI):
2S⁰ + 3O₂⁰ → 2S⁺⁶O₃⁻²
Additionally, as a result of the oxidation of sulfur, 4е ̅ can be given by the atom to another element with the formation of sulfur oxide (IV):
S⁰ + О₂⁰ → S⁺₄ O₂⁻²
Sulfur can also give up two electrons with the formation of sulfur chloride (II):
S⁰ + Cl₂⁰ → S⁺² Cl₂⁻
In all three of the above reactions, sulfur gives up electrons, and accordingly it oxidizes, but it acts as a reducer for oxygen (O) and chlorine (Cl) atoms.
However, in the case of the formation of H₂S, it is the hydrogen atoms that oxidize, as they lose electrons, reducing the outer energy level of sulfur from six electrons to eight. As a result, each hydrogen atom in its molecule becomes a proton:
Н₂⁰-2е⁻ → 2Н⁺,
and the sulfur molecule, on the contrary, is reduced, turning into a negatively charged anion (S2):
S⁰ + 2е⁻ → S⁻²
Thus, in the chemical reaction of the formation of hydrogen sulfide, it is sulfur that is the oxidizer. From the standpoint of sulfur displaying differing degrees of oxidation, it is also interesting to see the interaction of sulfur oxide 4 and hydrogen sulfide – the reaction for obtaining free sulfur:
2H₂⁺S-²+ S⁺⁴О₂-²→ 2H₂⁺O-²+ 3S⁰
As we can see from the equation of the reaction, both the oxidizer and the reducer in it are sulfur ions, and namely two sulfur anions ( 2-) give two of their electrons each to the sulfur atom in the molecule of sulfur oxide (II), as a result of which all three sulfur atoms are reduced to free sulfur.
S-² - 4е⁻→ 2S⁰ — reducer, oxidizes
S⁺⁴ + 4е⁻→ S⁰ — oxidizer, reduces.