Compounds of sulfur (-2)
Hydrogen sulfide, sulfides. At ordinary conditions sulfur does not react with Н2, the reverse reaction of the formation of H2S proceeds at heating:
H2
+ S
H2S Н0
= -20.6
kJ/mol
This reaction is exothermic, therefore an equilibrium at 3500C is displaced to the right, and at an increase of temperature to the left.
Preparation
In laboratory: 1). Action of the diluted acids on FeS:
FeS + 2HCl = FeCl2 + H2S .
2). Hydrolysis of aluminium sulfide - a pure product forms:
Al2S3 + 6H2О = 2Al(OH)3 + 3H2S.
In industry: it is obtained as a by-product at cleansing of oil, natural and coke gases.
Physical properties
U
It must be checked that the loss of its smell is the sign of acute poisoning H2S, when the smell H2S is no longer felt.
The gaseous state of H2S (under ordinary conditions) is explained by the fact that, unlike H2O, its molecules practically do not form hydrogen bonds among themselves. The absence of hydrogen bonds accounts for comparatively low solubility H2S in water: under ordinary conditions 2.58 volumes H2S can be dissolved in 1 volume of water. Solution of about 0.1 mole∙L-1 (~ 0.45% by mass) concentration which is named hydrogen sulfide water.
H2S is a covalent compound. Its molecule has a bent structure:
It is polar ( = 0.93 D = 0.34.10-29 Cm).
Comparative with water, structure of which molecule is possible to explain by sp3-hybridization of oxygen valency orbitals (104.50), in a molecule of H2S hybridization is absent ( 920).
Chemical properties.
In water solution H2S is weak two-basic hydrogen sulfide acid:
H2S H+ + HS-; K1 = 8.7.10-8
HS- H+ + S2-; K2 = 10-14
The neutral salts of H2S are sulfides, acidic salts – hydrogen sulfides.
Great majority of sulfides are insoluble in water. Soluble are only the sulfides of alkaline metals and NH4+, and also hydrogen sulfides, most of which exist only in solution.
Preparation of sulfides. Sulfides can be obtained in a number of different ways:
1. Reactions between simple substances (sulfur at to vigorously reacts with metals).
2. Action of H2S on soluble salts of metals. This way is not always suitable. A reason is a very considerable differences in solubility products of sulfides, e.g.: SPHgS = 4•10-53, SPFeS = 3,7•10-19.
Such reactions are reversible in principle, as one of initial substances - H2S is a weak acid, and one of products of reaction is MS - sparingly soluble salt.
Equilibrium:
MCl2 + H2S MS + 2HCl ( M2+ + H2S MS + 2H+ )
will be displaced depending on that, which from two compounds - weak electrolyte H2S (KH2S = K1•K2 = 8,7•10–22) or the sparingly soluble sulfide MS is stronger.
Here can be two variants:
1). In case of sulfides with very low solubility (SPMS << KH2S) the reactions proceed toward their formation, for example:
Cu2+ + H2S = CuS + 2H+ (SPCuS = 4•10-38)
Hg2+ + H2S = HgS + 2H+ (DPHgS = 4•10-53)
They are not soluble in water and acids, its preparation is possible by an action of H2S on the solutions of such metals salt.
2). in cases of SPMS << KH2S formation of sulfides practically does not take place (the equilibrium of reaction is displaced to the left). Such sulfides (FeS, MnS, BaS and others like that) are easily dissolved in acids, for example:
FeS + 2HCl = FeCl2 + H2S
The sulfides of both types are insoluble in water and are prepared applying instead of the weak electrolyte H2S the soluble sulfides, for example:
FeSO4 + Na2S = FeS + Na2SO4
At such conditions creates a possibility for the deep proceeding of direct reaction – up to concentration of ions in solution, which is determined by solubility of the obtained sulfide, that is its SPMS.
On different solubility of sulfides of metals in water and in the solutions of acids is based qualitative analysis of cations in analytical chemistry. A part from them (Na+, K+, NH4+ and etc.) form soluble sulfides, the second
1 ca – abbreviation meaning circa (приблизно)
2 ppm — abbreviation parts per million
3 селенових випрямлячів