Page 216 - Academic Press Encyclopedia of Physical Science and Technology 3rd InOrganic Chemistry

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Main Group Elements 25

small amounts can reduce crop yields without apparent in- 2. Basic. (a) Oxides of electropositive representative
jury to plant foliage. How ozone produces these effects is elements (Na 2 O, K 2 O, MgO, BaO, Tl 2 O); (b) oxides of
not understood, but research has established that ambient electropositive transition elements in lower oxidation
levels of ozone can significantly reduce net photosynthesis states (Sc 2 O 3 ,O 3 ,TiO 2 , ZrO 2 ).
in both tree and crop species, with a concomitant reduction 3. Amphoteric. (a) Oxides of less electropositive rep-
in growth. The deleterious effects of ozone on terrestrial resentative elements (BeO, Al 2 O 3 , GeO, SnO, Sb 2 O 3 ); (b)
systems may ultimately prove to be as serious as those of oxides of less electropositive transition elements or those
acid rain on aquatic systems. In contrast, studies indicate in intermediate oxidation states (Cr 2 O 3 , ZnO).
that acid rain has little negative effect on photosynthesis
in certain arboreal species. When active metals react with oxygen, they do not al-
ways form the simple, “normal” oxides. Elements from
B. Water and Hydrogen Peroxide Groups 1 (IA) and 2 (IIA) may also form peroxides and
superoxides. The Lewis structures for these ions are as
Water and hydrogen peroxide (H 2 O 2 ) are the two impor-
follows:
tant hydrides of oxygen. Water is abundant and widely
2 2
dispersed over the earth. It is indispensable to industry O O O O O
and life, and its unique nature and importance were rec- Oxide ion Peroxide ion Superoxide ion
ognized by the ancients, who regarded it as one of the four Oxidation Oxidation Oxidation
state 2 state 1 state 2 1
“elements”: water, air, fire, and earth.
Some of the important physical and molecular proper- K 2 O is potassium oxide, K 2 O 2 is potassium peroxide,
ties of water and hydrogen peroxide are compared in the and KO 2 is potassium superoxide.
following tabulation: The term superoxide is something of a misnomer if it
leads one to assume that the ion is somehow superreactive.
H 2 O H 2 O 2 1
Both the unusual oxidation state of − for oxygen and the
2
odd number of electrons have prompted many to expect an
Structure O O H
H H H H O O unusual chemistry for this species; however, superoxide is
H not a reactive electron transfer oxidant of either organic or
Boiling point ( C) 100 150 inorganic substrates. Despite its odd electron, superoxide
◦
◦
Melting point ( C) 0 −0.43 does not resemble organic radicals in terms of reactivity.
3
Density (g/cm ) 1.00 (liquid) 1.44 (liquid) The superoxide ion appears as a common intermediate
0.92 (solid) 1.64 (solid) of cellular reduction of oxygen and has become the subject
Color Colorless Very pale blue of special biochemical studies dealing with respiration in
both normal and cancerous cells. If O 2 is only partially
Water is an anomalous substance with many unusual reduced by gaining two electrons, highly toxic hydrogen
properties, most of which result from hydrogen bonding. peroxide is produced:
Water has an abnormally high melting point and boiling
−
+
point, a high specific heat, a high enthalpy of vaporization, O 2 + 2H + 2e → 2H 2 O 2
a high entropy of vaporization, and a high surface tension.
If, however, O 2 in a cell gains only one electron, the prod-
It is a superior solvent, and much of the value of water
uct is the toxic superoxide ion:
results from its extraordinary capacity to dissolve so many
substances—both ionic and covalent. O 2 + e → O −
−
2
C. Oxides, Peroxides, and Superoxides In 1968 it was discovered that respiring cells are
protected from superoxide by the action of an enzyme
All the elements form oxides except the three lightest no-
called superoxide dismutase (SOD). Superoxide dismu-
ble gases, helium, neon, and argon, which form no stable
tase (EC 1.15.1.1) converts the toxic superoxide ion to
compounds with any element. Oxides are often classified
hydrogen peroxide, which is then rapidly decomposed to
as acidic, basic, or amphoteric on the basis of their acid–
wateranddioxygenbytheactionofcatalase(EC1.11.1.6),
base properties in aqueous solution.
an extremely active enzyme whose function has been
known for several decades:
1. Acidic. (a) Most oxides of nonmetallic elements
SOD
+
(Cl 2 O, SO 2 ,SO 3 ,NO 2 ,N 2 O 5 ,P 4 O 10 ,CO 2 ); (b) oxides of 2H + 2O − −→ H 2 O 2 + O 2
2
transition elements in high oxidation states (CrO 3 , MoO 3 ,
catalase
WO 3 ,Mn 2 O 7 ). 2H 2 O 2 −−−→ 2H 2 O + O 2