HYDROGEN SULFIDE
[7783-06-4]
Formula: H2S; MW 34.08
Synonyms: sulfur hydride; sulfureted hydrogen
Occurrence and Uses
Hydrogen sulfide occurs in natural gas. It also is found in many sewer gases. It is a by-product of many industrial processes. Trace amounts of
dissolved H2S are found in wastewaters in equilibrium with dissolved sulfides and hydrosulfides. It also is found in volcanic eruptions, hot springs
and in troposphere. The average concentration of H2S in the air is about 0.05 ppb. The most important applications of hydrogen sulfide involve the
production of sodium sulfide and other inorganic sulfides. Hydrogen sulfide obtained as a by-product often is converted into sulfuric acid. It also is
used in organic synthesis to make thiols or mercaptans. Other applications are in metallurgy for extracting nickel, copper, and cobalt as sulfides
from their minerals; and in classical qualitative analytical methods for precipitation of many metals (see Reactions). It also is used in producing
heavy water for nuclear reactors.
Physical Properties
Colorless gas; characteristic odor of rotten eggs; odor threshold 1ppm; sweetish taste; fumes in air; flammable gas, burns with a pale blue flame;
refractive index at 589.3nm, 1.000644 at 0°C and 1 atm; density 1.539 g/L at 0°C; critical temperature 100.4°C; critical pressure 88.9 atm;
liquefies at –60.7°C; solidifies at –85.5°C; velocity of sound 289 m/sec in H2S gas; slightly soluble in water (0.4% at 20°C); pH of a
saturated aqueous solution 4.5; slightly acidic; diffusivity in water at 16°C, 1.77x105 cm2/sec; soluble in carbon disulfide, methanol, acetone; very
soluble in N-methylpyrrolidinone and alkanolamines (salt formation occurs: salt dissociates on heating); liquid H2S dissolves sulfur and SO2.
Thermochemical Properties
ΔHf° –4.93 kcal/mol
ΔG° –8.02 kcal/mol
S° 49.16 cal/degree mol
Cρ 8.18 cal/degree mol
Thermal conductivity (15°C) 33.3x10–6 cal/sec.cm2.°C/cm
Production
Hydrogen sulfide may be prepared conveniently in the laboratory in a Kipp apparatus by the reaction of iron(II) sulfide or zinc sulfide with dilute
hydrochloric or sulfuric acid:
ZnS + 2HCl → H2S + ZnCl2
A steady supply of hydrogen sulfide may be maintained by adding acid from time to time.
H2S is produced in large scale by several processes, which are:
1. Refining crude oil by hydrodesulfurization. The crude oil contains varying amounts of sulfur that may range from 0.05 to about 5%. The sulfur rich
fractions, the coke-distillate and the gas-oil fractions of the crude oil are passed through a fixed-bed catalyst along with hydrogen.
2. It also is produced as a by-product by hydrodesulfurization of coal or liquefaction of coal.
3. It also is obtained by the reaction of methane with sulfur vapor to produce carbon disulfide which on hydrolysis yields H2S:
CH4 + 2S --high temp--> CS2 + 2H2
CS2 + 2H2O --high temp--> 2H2S + CO2
4. Heating hydrogen and sulfur vapor at 500°C in the presence of a catalyst, such as, bauxite or cobalt molybdate, which produces a high purity H2S:
H2 + S --500C-catalyst--> H2S
Reactions
There are four types of H2S reactions: decomposition, precipitation, oxidation, and organic addition. Thermal dissociation of H2S is rapid above
850°C, producing hydrogen and sulfur. The reaction is endothermic.
2H2S -- >850C → 2H2 + S2
The dissociation also is rapid below this temperature (between 450°C to 850°C), however, only in the presence of a catalyst, such as silica,
platinum sulfide or cobalt molybdate. Other sulfur species are also produced in the reaction.
Hydrogen sulfide forms precipitates of several metal sulfides when passed through an aqueous solution of metal salts. Under acid conditions, several
metals including arsenic, antimony, bismuth, cadmium, copper, lead, mercury, and tin are precipitated as their sulfide; e.g.;
Cd2+(aq) + 2Cl¯(aq) + H2S (g) --acid--> CdS (s) + 2H+(aq) + 2Cl¯(aq)
Under ammoniacal conditions, iron, cobalt, nickel, zinc and manganese precipitate
as sulfides.
NaOH + H2S --base--> NaHS + H2O
2NaOH + H2S --base--> Na2S + 2H2O
Na2CO3 + H2S --base--> NaHS + NaHCO3
In the presence of moisture at ordinary temperatures, H2S reacts with some metals, such as copper and silver, forming sulfides:
4Ag + 2H2S + O2 --moisture--> 2Ag2S + 2H2O
Hydrogen sulfide undergoes thermal or catalytic oxidation with oxidizing agents forming sulfur, sulfur oxides, or sulfur derivatives. The products
formed depend on reaction conditions and the nature of oxidizing agents.
Combustion in air in the presence of flame primarily produces sulfur dioxide:
2H2S + 3O2 --high temp--> 2SO2 + 2H2O
Sulfur trioxide is a minor product in such oxidation:
H2S + 2O2 → SO3 + H2O
When a mixture of H2S and SO2 gases are passed over a catalyst such as silica gel at elevated temperatures, sulfur and water are formed:
2H2S + SO2 --high temp-catalyst--> 3S + 2H2O
Stronger oxidizing agents such as hydrogen peroxide or ozone readily oxidize H2S forming sulfur and various other sulfur products. For example, H2O2
reacts with H2S under neutral conditions forming sulfur and water:
H2S + H2O2 → S + 2H2O
In alkaline solution however, the reactions are more complex and the products include thiosulfate and sulfate.
Oxidation with ozone in aqueous conditions yields sulfur and sulfuric acid:
3H2S + O3 → 3S + 3H2O
S + O3 + H2O → H2SO4
In aqueous solution, H2S and iodine react to form sulfur and hydriodic acid:
H2S + I2 → S + 2HI
In gaseous phase, chlorine and H2S react at equimolar ratio, forming sulfur and hydrogen chloride:
H2S + Cl2 → S + 2HCl
In excess chlorine, the product is sulfur dichloride:
H2S + 2Cl2 → SCl2 + 2HCl
In aqueous solution, however, chlorine in higher molar ratios oxidizes H2S to sulfuric acid:
H2S + 4Cl2 + 4H2O → H2SO4 + 8HCl
In aqueous solution nitric acid also oxidizes H2S to sulfuric acid.
H2S + 2HNO3 → H2SO4 + 2HNO2
Reaction with concentrated sulfuric acid yields sulfur and sulfur dioxide:
H2S + H2SO4 → S + SO2 + 2H2O
In aqueous solutions, metal ions in their higher oxidation states oxidize H2S, forming a lower-valence sulfide and sulfur:
2CoCl3 + H2S → 2CoS2 + S + 6HCl
2Fe(NO3)3 + 3H2S → 2FeS + S + 6HNO3
Hydrogen sulfide also reacts with many types of organic substances. Reaction with methanol at high temperatures in the presence of an acidic catalyst
yields methanethiol and dimethylsulfide:
H2S + CH3OH --acid catalyst-heat--> CH3SH + H2O
CH3SH + CH3OH --acid catalyst-heat--> CH3SCH3 + H2O
H2S adds to olefins in the presence of acid catalysts forming thiols (mercaptans):
H2S + CH3CH=CH2 --acid catalyst-heat--> (CH3)2CHSH
In the presence of a free radical initiator, the product is 1-propanethiol:
H2S + CH3CH=CH2 --free radical initiator--> CH3CH2 CH2SH
Reaction with cyanamide in the presence of basic catalyst yields thiourea:
H2S + NH2CN --basic alumina--> NH2CSNH2
Under similar conditions, reactions with nitriles yield thioamides:
H2S + CH3CN --basic catalyst--> CH3CSNH2
Reaction with chlorobenzene at elevated temperatures yields thiophenol as major product and dibenzyl sulfide in smaller amounts:
H2S + C6H5Cl --600C--> C6H5SH + HCl
H2S + 2C6H5Cl --600C--> C6H5SC6H5 + 2HCl
Analysis
Elemental composition: H 5.92%, S 94.08. Hydrogen sulfide may be distinguished by its characteristic odor. The gas turns a paper soaked in lead
acetate solution black. Many infrared sensors are commercially available for in-situ measurements of H2S. It may be monitored semiquantitatively by
Draeger tubes. It also may be analyzed by GC following trapping over molecular sieves and thermal desorption. Either a flame photometric detector or a
sulfur chemiluminescence detector may be used for GC analysis. It may be separated on a capillary column such as Carboxen 1006 PLOT ™ or SPB-1
SULFUR™ (Supelco Catalog 1999: Supelco Inc., Bellefonte, PA).
Hazard
Hydrogen sulfide is a highly toxic and flammable gas. A 5-minute exposure to 1,000 ppm concentration in air can be fatal to
humans (Patnaik, P. 1999. A Comprehensive Guide to the Hazardous Properties of Chemical Substances, 2nd ed. New York: John Wiley). The
symptoms are headache, nausea, nervousness, cough, eye irritation, and insomnia. High doses can produce unconsciousness
and respiratory paralysis. Hydrogen sulfide forms explosive mixtures with air; the LEL and UEL are 4.3 and 45.0% by volume in air, respectively. Its
autoignition temperature is 260°C. Its reaction with soda-lime in oxygen can be explosive. Reactions with strong oxidizing agents can progress to
incandescence.
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Long list of
interesting experiments I want to try, but I never get around do actually doing them.