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Sulfur Chemistry

Skills to develop

Sulfur and Group 6A Chemistry

Group 6A Elements
Oxygen O2
Sulfur S6
Selenium Se
Tellurium Te
Polonium Po
The group 6A elements are listed in the Table on the side here. This goup of elements are intimately related to our lives. We need oxygen all the time throughout our lives. Did you know that sulfur is also one of the essential elements of life. It is responsible for some of the protein structures in all living organisms. Many industries utilize sulfur, but emission of sulfer compounds is often seen more as a problem than the natural phenomenon. The matallic properties increase as the atomic number increases. The element polonium has no stable isotopes, and the isotope with mass number 209 has the longest half life of 103 years.

Properties of oxygen are very different from other elements of the group, but they all have 2 electons in the outer s orbital, and 4 electrons in the p orbitals, usually written as

s2p4

The trends of their properties in this group are interesting. Knowing the trend allows us to predict their reactions with other elements. Most trends are true for all groups of elements, and the group trends are due mostly to the size of the atoms and number of electrons per atom.

The trends are described below:

  1. The metallic properties increase in the order oxygen, sulfur, selenium, tellurium, or polonium. Polonium is essentially a metal. It was discovered by M. Curie, who name it after her native country Poland.

  2. Electronegativity, ionization energy (or ionization potential IP), and electron affinity decrease for the group as atomic weight increases.

  3. The atomic radii and melting point increase.

  4. Oxygen differs from sulfur in chemical properties due to its small size. The differences between O and S are more than the differences between other members.

Sulfur - a commodity

Sulfuris recovered by the Frasch process. This process has made sulfur a high purity (up to 99.9 percent pure) chemical commodity in largequantities.

Natural Sources of Sulfur

Most sulfur containing minerals are metal sulfides, and the best known is perhaps pyrite, (FeS2, known as fools gold because of its golden color). The most common sulfate containing mineral is gypsum, CaSO4.2H2O, also known as plaster of paris.

Mining Method - Frasch process

Frasch process force (99.5% pure) sulfur out by using hot water and air. In this process, superheated water is forced down the outer most of three concentric pipes. Compressed air is pumped down the center tube, and a mixture of elemental sulfur, hot water, and air comes up the middle pipe. Sulfur is melted with superheated water (at 170 degrees C under high pressure) and forced to the surface of the earth as a slurry.

Applications

Sulfur is mostly used for the production of sulfuric acid, H2SO4. Most sulfur mined by Frasch process is used in industry for the manufacture of sulfuric acid.

Sulfuric acid, the most abundantly produced chemical in the United States, is manufactured by the Contact process.

Most (about 70%) of the sulfuric acid produced in the world is used in the fertilizer industry.

Sulfuric acid can act as a strong acid, a dehydrating agent, and an oxidizing agent. It's applications use these properties.

Sulfur is an essential element of life in sulfur-containing proteins.

Elemental Sulfur

Rhombic and monoclinic sulfur are known as allotropes. The crystals of these have the molecules S8. In these molecules, S form two S-S bonds. The lone pairs of electrons make the S-S-S bend (108 deg), resulting in S8 having the shape of a crown.

At 298 K, rhombic sulfur is stable, whereas at at 368 K, monoclinc sulfur is formed. The latter is meta-stable at room temperature for some time. In sulfur vapor, S8, S6, and S2 molecules are present.

What happens at when the solid sulfur melts? The S8 molecules bread up. When suddenly cooled, long chain molecules are formed in the plastic sulfur which, behave as rubber. Plastic sulfur transform into rhombic sulfur over time.

Reactions of Sulfur

Reading the following reactions, figure out and notice the change of the oxidation state of S in the reactants and products. Common oxidation states of sulfur are -2, 0, 4, and 6.

Sulfur (brimstone, stone that burns) reacts with O2 giving a blue flame:

S + O2 = SO2

SO2 is produced whenever metalsulfide is oxidized. It is recovered and oxidized further to give SO3, for production of H2SO4. SO2 reacts with H2S to form H2O and S.

2 SO2 + O2 = 2 SO3
SO3 + H2O = H2SO4 <- a valuable commodity
SO3 + H2SO4 = H2S2O7 <- pyrosulfuric acid

Sulfur reacts with sulfite ions in solution to form thiosulfate,

S + SO32- = S2O32-,

but the reaction is reversed in an acidic solution.

Sulfuric Acid

Sulfuric acid is produced by the contact process in three steps:

       +O2      +H2SO4         +H2O
  SO2  -->  SO3 ----->  H2S2O7  --->  H2SO4

Applications of sulfuric acid

  1. as a strong acid for making HCl and HNO3.
  2. as an oxidizing agent for metals.
  3. as a dehydrating agent.
  4. for manufacture of fertilizer and other commodities.

Hydrogen Sulfide H2S

hydrogen sulfide, H2S is a diprotic acid. The equilibria below. H2S = HS- + H+
HS- = S2- + H+
have been discussed in connection with Polyprotic Acids

Structures of Some Sulfur Compounds

In the DOS version, a Demonstration shows you the rotation of S8, H2S, SO2, SO3, SF6, etc. Draw the molecular structures for these substance yourself, so that you will get some sense about the beauty of molecules.

Confidence Building Questions

cchieh@uwaterloo.ca