What Are Sulfides?
Compounds of sulfur in the −2 oxidation state; metal sulfides can be regarded as salts of hydrosulfuric acid. Metal sulfides can be prepared by the direct reaction of a metal with sulfur, by passing hydrogen sulfide gas into a metal-salt solution, or by adding sodium sulfide to a salt solution.
Alkali-metal sulfides and ammonium sulfide are readily soluble in water; their solutions are alkaline due to hydrolysis. Sulfides of alkaline-earth metals, scandium, yttrium, and lanthanides are relatively insoluble. When the cation has an outer-electron configuration of 18 or 18+2 electrons, insoluble, colored sulfides are often formed due to strong polarization. Most water-insoluble sulfides dissolve in acid with release of hydrogen sulfide; a few extremely insoluble metal sulfides (e.g., CuS, HgS) can be dissolved by oxidizing acids, in which case S is oxidized to sulfur and precipitates from solution. Insoluble metal sulfides exhibit a dissolution–precipitation equilibrium in solution. By controlling acidity, the concentration of S²⁻ ions in solution can be changed, thereby precipitating insoluble metal sulfides of differing solubilities separately. This is the basis of using hydrogen sulfide to separate and identify metal ions in qualitative analysis. [1]
Inorganic sulfides are usually synthesized by the following methods: (Note: K is the international unit of temperature, kelvin)
1. Direct combination of elements, e.g.:
C + 2S CS2
2. Reduction of sulfates or higher-valence sulfides, e.g.:
Na2SO4 + 4C→ Na2S + 4CO 1373K
In2S3 + 2 → In2S + 2H2S
3. Metathesis reactions in solution or at high temperature, e.g.:
3SiO2 + 2Al2S3→ 3SiS2 + 2Al2O3 1373K
4. Preparation from thio-salts, e.g.:
(NH4)2MoO4+ 4(NH4)2S + 4H2O → (NH4)2[MoS4] + 8NH3·H2O
(NH₄)₂MoO₄ + 2HCl --(heat)→ MoS₃ + H₂S + 2NH₄Cl
5. Thermal decomposition of higher-valence sulfides, e.g.:
MoS₃ --(heat)→ MoS₂ + S [1]
Most sulfides have bright colors, as shown in the table above. In addition, transition-metal sulfides such as MoS₂, Re₂S₇, FeS, CoS₂, NiS, PtS₂, Cu₂S, CuS, and Ag₂S are all black.
Acid sulfides of metals are all soluble in water, but among normal salts only alkali-metal sulfides and ammonium sulfide are soluble. Generally, the solubility of metal sulfides can be predicted from the cation's polarizing power (ionic charge / ionic radius, Z/r). Stronger cation polarization leads to greater covalency, lower polarity, and thus lower solubility.
Solubility of metal sulfides in acid
Metal sulfides undergo hydrolysis to varying degrees in water:
S2-+ H2O ⇌ HS-+ OH-
HS-+ H2O ⇌ H2S + OH-
The pKa values of H₂S are about pKa₁ = 6.89 and pKa₂ = 15±2; therefore metal-sulfide solutions show varying degrees of alkalinity, and the alkalinity of alkali-metal sulfide solutions can rival that of the corresponding hydroxides.
Two types of reactions may occur when roasting sulfide minerals:
The sulfide is converted to the corresponding oxide, and sulfur to sulfur dioxide. For example, one step in obtaining lead from galena is: 2PbS + 3O₂ → 2PbO + 2SO₂. The sulfide is oxidized to the corresponding soluble sulfate.
Both steps above are important methods for converting sulfide ores when smelting metals.
The sulfur in the −2 state in sulfides is reducing and, depending on conditions, can be oxidized to sulfur, sulfites, sulfates, etc.
S2-- 2e-= S; -0.407V
Like the corresponding oxides, the acidity/basicity of sulfides varies with period and group similarly to oxides, but sulfides are less basic than oxides.
Among elements of the same period, the highest-oxidation-state sulfide becomes more acidic from left to right; among elements of the same group, the same-oxidation-state sulfide becomes less acidic from top to bottom; and among sulfides of the same element, the higher-oxidation-state sulfide is more acidic. Thus As₂S₅ is more acidic than Sb₂S₅, and Sb₂S₅ is more acidic than SnS₂ and Sb₂S₃. [1]
Hydrogen sulfide is a colorless, toxic gas with a rotten-egg odor; its permissible concentration in air is no more than 0.01 mg/L. H₂S can combine with the ferrous ions in human heme to form ferrous sulfide, rendering it inactive. Frequent exposure to H₂S causes chronic poisoning such as dulled smell, emaciation, and headaches. In the lab, H₂S is commonly prepared by reacting a metal sulfide with acid. The aqueous solution of H₂S is hydrosulfuric acid, a diprotic weak acid. Whether in acidic or alkaline media, H₂S is strongly reducing. [1]
Polysulfides are compounds containing polysulfide ions Sₙ (n = 2, 3, 4, 5, 6, …, 9). They can be prepared by boiling sulfur in a sulfide solution; their solutions are generally yellow, and the color deepens as n increases.
Polysulfide ions contain persulfide bonds, similar to peroxides; they are oxidizing but less so than peroxide ions, and undergo disproportionation:
Polysulfides are very unstable in acidic solution and, upon acidification, release hydrogen sulfide and sulfur:
+ 2H → H2S + (n-1)S
Polysulfide ions can also act as ligands. For example, when Na₂Sₙ acts on (η-C₅H₅)₂TiCl₂, a coordination compound containing a TiS₅ ring is formed. [1]
The spot-test method is a sensitive way to identify sulfide and hydrosulfide ions. The procedure: on a spot plate, mix the alkaline solution of a soluble sulfide with a 1% solution of sodium nitroprusside Na₂[Fe(CN)₅NO] (sodium nitroferricyanide); if S²⁻ ions are present in the sample, a red-purple color of varying depth appears, with a sensitivity of 1:50,000. The mechanism is formation of the [Fe(CN)₅(NOS)]⁴⁻ ion. In addition, adding the test solution, concentrated HCl, a few crystals of p-aminodimethylaniline, and 0.1 mol/L ferric chloride solution to a spot plate; if a blue color appears after 2–3 minutes, it also proves the presence of sulfide ions. The mechanism is formation of blue methylene blue. [1]
Sulfides and their analogues include a series of minerals formed by metals and metalloids combined with S, Se, Te, As, Sb, and Bi. There are about 350 mineral species, of which sulfides account for more than two-thirds; the rest are selenides, tellurides, arsenides, and a few antimonides and bismuthides.
This mineral class occupies only 0.15% of the Earth's crust by mass, most of it iron sulfides; sulfides and analogous compounds of other elements account for only about 0.001%. Although their abundance is limited, they can be enriched into industrially significant deposits—mainly non-ferrous metals such as Cu, Pb, Zn, Hg, Sb, Bi, Mo, Ni, and Co—which all derive mainly from this mineral class, giving it great significance to the national economy.
According to the valence state of the sulfur ion and the presence of complex anions, sulfide minerals are divided into three categories: simple sulfides, where sulfur combines with cations as S²⁻ and are mostly black; persulfides, where sulfur combines with cations as the dumbbell-shaped [S₂]²⁻ counter-anion;
sulfosalts, where sulfur and the metalloids arsenic, antimony, or bismuth form pyramidal complex anions [AsS₃]³⁻, [BiS₃]³⁻, and complex anions formed by linking these pyramidal units, which then combine with cations. [2]
The hydrogen-sulfide system is a traditional and fairly widely used method for analyzing cations, based mainly on the marked differences in sulfide solubility, which divides common cations into five groups.
0.3 mol/L HCl, H2S
or 0.2–0.6 mol/L HCl
NH3 + NH4Cl
Because H₂S gas is highly toxic and inconvenient to store, a thioacetamide (CH₃CSNH₂, TAA) aqueous solution is generally used as the precipitant.
Molybdenum disulfide is a catalyst in organic synthesis. Because sulfur-containing organic compounds (e.g., thiophene) poison ordinary hydrogenation catalysts, molybdenum disulfide can catalyze the hydrogenation of sulfur-containing organic substances. Cadmium sulfide is used to make photoelectric cells. Lead sulfide is used to make infrared sensors. Calcium polysulfide, barium polysulfide, and ammonium polysulfide are fungicides and insecticides. Carbon disulfide is used industrially as a solvent; it is also used to make carbon tetrachloride, and in organic chemistry to insert the –C(=S)–S– group. Zinc sulfide and cadmium sulfide are used to make phosphors; high-purity cadmium sulfide is a good semiconductor. Tetraphosphorus trisulfide is used to make matches and fireworks. Tetraphosphorus decasulfide is used to make insecticides, lubricant additives, and flotation agents. Sodium sulfide is used extensively in manufacturing sulfur dyes, organic drugs, and pulp. Calcium sulfide and barium sulfide are used to make luminous paints. [3-4]
In acidic solution TAA hydrolyzes to produce H₂S, replacing H₂S: CH₃CSNH₂ + H⁺ + 2H₂O ⇌ CH₃COOH + NH₄⁺ + H₂S↑. In ammoniacal solution it hydrolyzes to HS⁻, replacing (NH₄)₂S: CH₃CSNH₂ + 2NH₃ ⇌ CH₃–C(=NH)NH₂ + NH₄⁺ + HS⁻. In alkaline solution it hydrolyzes to S, replacing Na₂S: CH₃CSNH₂ + 3OH⁻ ⇌ CH₃COO⁻ + NH₃ + H₂O + S. Other applications of sulfides include:
