Page 203 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

P1: GSS Final Pages
Encyclopedia of Physical Science and Technology EN009F-398 July 6, 2001 20:34

12 Main Group Elements

The calcium oxide (quicklime) produced by the thermal heat
CO + 2H 2 −−−→ CH 3 OH (5)
decomposition of limestone and chalk exceeds 13 bil- catalyst Methanol
lion kg annually and earns lime sixth place among the
manufactured chemicals. CH 3 OH + CO → HC 2 H 3 O 2 (6)
Acetic acid
Ranked eleventh among chemicals, Na 2 CO 3 is impor-
Methanol (number 22) and acetic acid (number 34) both
tant in glassmaking and the preparation of detergents. This
rank among the top 50 chemicals produced in the United
chemical was once produced synthetically in vast quanti-
States.
ties by the Solvay process.
Carbon dioxide, the other important oxide of carbon,
Carbonation:
earns the rank of seventeenth among chemicals produced.
NaCl + H 2 O + NH 3 + CO 2 → NaHCO 3 + NH 4 Cl It is recovered mainly as a by-product of other processes,
for example, the thermal decomposition of carbonates and
Ammonia recovery:
the fermentation of carbohydrates.
2NH 4 Cl + Ca(OH) → 2NH 3 + CaCl 2 + 2H 2 O Carbon dioxide is a colorless, odorless gas. When
2
Heating: cooled at atmospheric pressure, it solidifies to form dry
ice. This solid sublimes at atmospheric pressure and a
2NaHCO 3 (s) → Na 2 CO 3 (s) + H 2 O (g) + CO 2 (g) ◦
temperature of −78 C. All three physical states of CO 2
Now, however, this process is being phased out because of are useful: more than half the solid CO 2 produced an-
the cost of ammonia and the availability of huge natural nually is used as a refrigerant, liquid CO 2 is used as an
deposits of natron in Wyoming. A major disadvantage of aerosol propellant, and gaseous CO 2 is used primarily to
the Solvay process involves the ammonia recovery step. carbonate beverages.
The by-product CaCl 2 finds some use as a deicer on high- A significant amount of CO 2 is used to manufacture
ways and sidewalks and as a concrete additive, but the urea, which ranks thirteenth by weight among the top 50
quantities of CaCl 2 produced far exceed the demand. The chemicals produced in the United States:
disposal of excess CaCl 2 from the Solvay process creates O
a serious environmental problem. pressure ||
CO 2 + 2NH 3 −−−→ H 2 N C NH 2 + H 2 O
E. Oxides

When carbon is burned in a sufficiency of air or oxygen, IV. SILICON
CO 2 results; but if the supply of oxygen is limited, highly
toxic CO is the major combustion product. Other, higher After oxygen (45.5 wt%), silicon (27.2 wt%) is the most
oxides of carbon can be formed by indirect methods; for abundant element in the earth’s crust. It is never found
example, C 3 O 2 , carbon suboxide (tricarbon dioxide), re- free in nature, and it is usually found combined with oxy-
sults from the dehydration of malonic acid, a three-carbon gen. Unlike carbon, silicon does not readily concatenate;
dicarboxylic acid. The Lewis structures of these three ox- instead, it forms stable Si O Si bonds, and it is these
ides indicate the presence of multiple bonds: bonds that are found in the bewildering and complex va-
riety of minerals called silicates.
C O O C O O C C C O Relatively crude silicon can be prepared by the reduc-
tion of silicon dioxide (sand or quartzite) with coke in an
At room temperature, carbon monoxide is a colorless,
◦
electric furnace at ∼3000 C:
odorless, and tasteless gas; its boiling point is −192 C,
◦
and its melting point is −205 C. The extreme toxicity SiO 2 (s) + 2C (g) → 2CO (g) + Si (s)
◦
of CO arises from its capacity to bind to the iron ion
The crystal structure of silicon is similar to that of dia-
in hemoglobin molecules, thus reducing the capacity of
mond; however, the Si Si bonds (226 kJ/mol) are weaker
hemoglobin to bind and carry O 2 .
than the C C bonds (356 kJ/mol), and silicon is not
Carbon monoxide has a number of important industrial
nearly as hard. There is no graphitic allotrope of silicon.
uses as a fuel [Eq. (3)], as a reducing agent in metallurgy
Crystalline silicon is a blue–gray, somewhat shiny, brit-
[Eq. (4)], and as a reactant in the preparation of important
tle element that certainly appears metallic; however, it is
organic compounds [Eqs. (5) and (6)]:
classified as a nonmetal or metalloid because it is a semi-
2CO (g) + O 2 (g) → 2CO 2 (g) + heat conductor; that is, at low temperatures it is an insulator.
However, when heated sufficiently, its electrical conduc-
H =−566 kJ (3)
tivity increases markedly. Very pure silicon for transistors
Fe 2 O 3 (s) + 3CO (g) → 2Fe (s) + 3CO 2 (g) (4) is produced by reducing silicon tetrachloride prepared by