Skills to develop
- Describe the chemistry of the oxygen group.
- Give the trend of various properties.
- Remember the names of Group 6A elements.
- Explain the Frasch process.
- Describe properties and applications of H2SO4.
- Explain properties and applications of H2S.
Sulfur and Group 6A Chemistry
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.
|Group 6A Elements
|Oxygen || O2
|Sulfur || S6
|Polonium || Po
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
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:
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
Electronegativity, ionization energy (or ionization
potential IP), and electron affinity decrease for the
group as atomic weight increases.
The atomic radii and melting point increase.
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.
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.
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
Sulfur is an essential element of life in sulfur-containing
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 is produced by the contact process in three steps:
+O2 +H2SO4 +H2O
SO2 --> SO3 -----> H2S2O7 ---> H2SO4
Applications of sulfuric acid
- as a strong acid for making HCl and HNO3.
- as an oxidizing agent for metals.
- as a dehydrating agent.
- 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
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
Which reaction is responsible for the destruction of limestone and marble
statues and buildings?
a. CaCO3 --> CaO + CO2
b. SO2 + H2O --> H2SO3
c. BaO + CO2 --> BaCO3 --> BaSO3 upon reaction with SO2
d. CaCO3 + H2O --> Ca(OH)2 + CO2
e. CaCO3 + SO2 --> CaSO3 + CO2 --> CaSO4 upon oxidation
SO2 in H2SO3 is the acid in acid rain,
which attacks CaCO3, marbles.
SO2 reduces pigments in organic mater
Give the formula of thiosulfate ion:
Sulfate is SO42-; Replacement of an O by an S to give
The two S in S2O32- have different oxidation states:
one is +6 the other is (-2), average +2.
What is the oxidation state of S in SF6,
H2SO4, NaHSO4, SO42-,
Oxidation state for S in H2SO3,
SO32-, SO2, etc is 4. The oxidation state
of S is the same for all in the list.
What is the phase of sulfur at 298 K. Enter the type of crystals.
Answer rhombic sulfur
The term rhombic describe a type of crystal.
The monoclinic sulfur is meta stable at 298 K.
Give the name of the process, by which sulfur is forced out of the
ground using hot water and air.
Answer Frasch process
The Frasch process is used to mine elemental sulfur.