Lewis definition
Main article: Lewis acid-base theory
The Lewis definition of acid base reactions, devised by Gilbert N. Lewis in
1923
is an encompassing theory to the Brønsted-Lowry and solvent-system
definitions
with regards to the premise of a donation mechanism,
which conversely
attributes the donation of electron pairs from bases and the acceptance by acids,
rather than protons or other bonded substance and spans both aqueous and non-
aqueous reactions.
Ag
+
+ 2 :NH
3
→ [H
3
N:Ag:NH
3
]
+
A silver cation reacts as an acid with ammonia which acts as an electron-pair
donor, forming an ammonia-silver adduct
In reactions between Lewis acids and bases, there is the formation of an adduct
when the highest occupied molecular orbital (HOMO) of a molecule, such as NH
3
with available lone electron pair(s) donates lone pairs of electrons to the electron-
deficient molecule's lowest unoccupied molecular orbital (LUMO) through a co-
ordinate covalent bond; in such a reaction, the HOMO-interacting molecule acts as a
base, and the LUMO-interacting molecule acts as an acid.
In highly-polar molecules,
such as Boron Tri-fluoride (BF
3
),
the most electronegativeelement pulls electrons
towards its own orbitals, providing a more positive charge on the less-electronegative
element and a difference in its electronic structure due to the axial or equatorial
orbiting positions of its electrons, causing repulsive effects from Lone pair-bonding
pair (Lp-Bp) interactions between bonded atoms in excess of those already provided
by Bonding pair-bonding pair (Bp-Bp) interactions.
Adducts involving metal ions are
referred to as co-ordination compounds.
Solvent-system definition
This definition is based on a generalization of the earlier Arrhenius definition
to all autodissociating solvents. In all such solvents there is a certain concentration of
a positive species, solvonium cations and negative species, solvate anions, in
equilibrium with the neutral solvent molecules. For example:
2H
2
O
⇌ H
3
O
+
(hydronium) + OH
-
(hydroxide)
2NH
3
⇌ NH
4
+
(ammonium) + NH
2
−
(amide)
or even some aprotic systems
N
2
O
4
⇌ NO
+
(nitrosonium) + NO
3
−
(nitrate)
2SbCl
3
⇌ SbCl
2
+
(dichloroantimonium) + SbCl
4
-
(tetrachloroantimonate)
A solute causing an increase in the concentration of the solvonium ions and a
decrease in the solvate ions is an acidand one causing the reverse is a base. Thus, in
440
liquid ammonia, KNH
2
(supplying NH
2
-
) is a strong base, and NH
4
NO
3
(supplying
NH
4
+
) is a strong acid. In liquid sulfur dioxide (SO
2
), thionyl compounds (supplying
SO
2+
) behave as acids, and sulfites (supplying SO
3
2−
) behave as bases.
Here are some nonaqueous acid-base reactions in liquid ammonia
2NaNH
2
(base) + Zn(NH
2
)
2
(amphiphilic amide)
→ Na
2
[Zn(NH
2
)
4
]
2NH
4
I (acid) + Zn(NH
2
)
2
(amphiphilic amide)
→ [Zn(NH
3
)
4
)]I
2
Nitric acid can be a base in liquid sulfuric acid:
HNO
3
(base) + 2H
2
SO
4
→ NO
2
+
+ H
3
O
+
+ 2HSO
4
-
And things become even stranger in the aprotic world, for example in liquid
N
2
O
4
:
AgNO
3
(base) + NOCl (acid)
→ N
2
O
4
+ AgCl
Since solvent-system definition depends on the solvent as well as on the
compound itself, the same compound can change its role depending on the choice of
the solvent. Thus, HClO
4
is a strong acid in water, a weak acid in acetic acid, and a
weak base in fluorosulfonic acid.
Other acid-base theories
Usanovich definition
The most general definition is that of the Russian chemist Mikhail Usanovich,
and can basically be summarized as defining an acid as anything that accepts negative
species or donates positive ones, and a base as the reverse. This tends to overlap the
concept of redox (oxidation-reduction), and so is not highly favored by chemists.
This is because redox reactions focus more on physical electron transfer processes,
rather than bond making/bond breaking processes, although the distinction between
these two processes is somewhat ambiguous.
Lux-Flood definition
This definition, proposed by German chemist Hermann Lux in 1939, further
improved by Håkon Flood circa 1947
and now commonly used in modern
geochemistry and electrochemistry of molten salts, describes an acid as an oxide ion
acceptor and a base as an oxide ion donor. For example:
MgO (base) + CO
2
(acid)
→ MgCO
3
CaO (base) + SiO
2
(acid)
→ CaSiO
3
NO
3
-
(base) + S
2
O
7
2-
(acid)
→ NO
2
+
+ 2SO
4
2-
Pearson definition
Main article: HSAB concept
In 1963 Ralph Pearson proposed an advanced qualitative concept known as
Hard Soft Acid Base principle, later made quantitative with help of Robert Parr in
1984. 'Hard' applies to species which are small, have high charge states, and are
weakly polarizable. 'Soft' applies to species which are large, have low charge states
and are strongly polarizable. Acids and bases interact and the most stable interactions
are hard-hard and soft-soft. This theory has found use in both organic and inorganic
chemistry.
Earth Chemistry
441
The Earth and its Lithospher
The earth has been in a state of continual change since its formation. The major
part of this change, involving volcanism and tectonics, has been driven by heat
produced from the decay of radioactive elements within the earth. The other source of
change has been solar energy, which acts as the driving force of weathering and is the
ultimate source of energy for living organisms.
The solar system was probably formed about 4.6 billion years ago, and the
oldest known rocks have an age of 3.8 billion years. There is thus a gap of 0.8 billion
years for which there is no direct evidence. It is known that the earth was subjected to
extensive bombardment earlier in its history; recent computer simulations suggest
that the moon could have resulted from an especially massive collision with another
body. Although these major collisions have diminished in magnitude as the matter in
the solar system has become more consolidated, they continue to occur, with the most
recent one being responsible for the annihilation of the dinosaurs and much of the
other life on Earth. The lack of many overt signs of these collisions (such as craters,
for example) testifies to the dynamic processes at work on the Earth’s surface and
beneath it.
Chemical composition of the Earth
The earth is composed of 90 chemical elements, of which 81 have at least one
stable isotope. The unstable elements are
43
Tc and
61
Pm, and all elements heavier than
83
Bi.
Note that the vertical axis is logarithmic, which has the effect of greatly
reducing
the visual impression of the differences between the various elements.
The chart gives the abundances of the elements present in the solar system, in
the earth as a whole, and in the various geospheres. Of particular interest are the
differences between the terrestrial and cosmic abundances, which are especially
notable in the cases of the lighter elements (H, C, N) and the noble gas elements (He,
Ne, Ar, Xe, Kr).
Given the mix of elements that are present in the earth, how might they
combine so as to produce the chemical composition we now observe?
442
Thermodynamics allows us to predict the composition that any isolated system will
eventually reach at a given temperature and pressure. Of course the earth is not an
isolated system, although most parts of it can be considered approximately so in
many respects, on time scales sufficient to make thermodynamic predictions
reasonably meaningful. The equilibrium states predicted by thermodynamics differ
markedly from the observed compositions. The atmosphere, for example, contains
0.03% CO
2
, 78% N
2
and 21% O
2
; in a world at equilibrium the air would be 99%
CO
2
.
Similarly, the oceans, containing about 3.5% NaCl, would have a salt content
of 35% if they were in equilibrium with the atmosphere and the lithosphere. Trying to
understand the mechanisms that maintain these non-equilibrium states is an important
part of contemporary environmental geochemistry.
Structure of the Earth
Studies based on the reflection and refraction of the acoustic waves resulting
from earthquakes show that the interior of the earth consists of four distinct regions.
A combination of physical and chemical processes led to the differentiation of the
earth into these major parts. This is believed to have occurred approximately 4 billion
years ago.
The Earth's Core
The Earth’s core is believed to consist of two regions. The inner core is solid,
while the outer core is liquid. This phase difference probably reflects a difference in
pressure and composition, rather than one of temperature. Density estimates obtained
from seismological studies indicate that the core is metallic, and mainly iron, with 8-
10 percent of lighter elements.
Hypotheses about the nature of the core must be consistent with the the core’s
role as the source of the earth’s magnetic field. This field arises from convective
motion of the electrically conductive liquid comprising the outer core. Whether this
convection is driven by differences in temperature or composition is not certain. The
estimated abundance of radioactive isotopes (mainly U
238
and K
40
in the core is
sufficient to provide the thermal energy required to drive the convective dynamo.
Laboratory experiments on the high-pressure behavior of iron oxides and sulfides
indicate that these substances are probably metallic in nature, and hence conductive,
at the temperatures (4000-5000K) and pressures (1.3-3.5 million atm) that are
estimated for the core. Their presence in the core, alloyed with the iron, would be
443
consistent with the observed density, and would also resolve the apparent lack of
sulfur in the earth, compared to its primordial abundance.
The mantle
The region extending from the outer part of the core to the crust of the earth is
known as the mantle. The mantle is composed of oxides and silicates, i.e., of rock. It
was once believed that this rock was molten, and served as a source of volcanic
magma. It is now known on the basis of seismological evidence that the mantle is not
in the liquid state. Laboratory experiments have shown, however, that when rock is
subjected to the high temperatures and pressures believed to exist in the mantle, it can
be deformed and flows very much like a liquid.
The upper part of the mantle consists of a region of convective cells whose
motion is driven by the heat due to decay of radioactive potassium, thorium, and
uranium, which were selectively incorporated in the crystal lattices of the lower-
density minerals that form the mantle. There are several independent sources of
evidence of this motion. First, there are gravitational anomalies; the force of gravity,
measured by changes in elevation in the sea surface, is different over upward and
downward moving regions, and has permitted the mapping of some of the convective
cells. Secondly, numerous isotopic ratio studies have traced the exchange of material
between oceanic sediments, upper mantle rock, and back into the continental crust,
which forms from melting of the upper mantle. Thirdly, the composition of the basalt
formed by upper mantle melting is quite uniform everywhere, suggesting complete
mixing of diverse materials incorporated into the mantle over periods of 100 million
years.
High-pressure studies in the laboratory have revealed that olivine, a highly
abundant substance in the mantle composed of Fe, Mg, Si, and O (and also the
principal constituent of meteorites) can undergo a reversible phase change between
two forms differing in density. Estimates of conditions within the upper mantle
suggest that the this phase change could occur within this region in such as way as to
contribute to convection. The most apparent effect of mantle convection is the motion
it imparts to the earth’s crust, as evidenced by the the external topography of the
earth.
Cross section of the crust and
upper mantle. The dark- and
medium-brown
represent
the
continental and oceanic crust,
respectively,
while
the
light
brown is the upper part of the
mantle.
Arrows
show
the
direction of movement. Click on
the link below for a more detailed
description.
From a Columbia U. course Web
site of Prof. Paul Polson
444
The crust
The outermost part of the earth, known also as the lithosphere, is broken up
into plates that are supported by the underlying mantle, and are moved by the
convective cells within the mantle at a rate of a few centimetres per year. New crust
is formed where plates move away from each other under the oceans, and old crust is
recycled back into the mantle as where plates moving in opposite directions collide.
This dynamic earth: the story of plate
tectonics is an excellent, graphics-rich site
maintained by the U.S. Coast and Geodetic
Survey.
Click on the image at the right to see an
expanded map of the world's crustal plates from
the USCGS site.
The oceanic crust
The parts of the crust that contain the world’s oceans are very different from
the parts that form the continents. The continental crust is 10-70 km thick, while
oceanic crust averages only 5-7 km in thickness. Oceanic crust is more dense (3.0-3.1
g cm
–3
) and therefore “floats” on the mantle at a greater depth than does continental
crust (density 2.7-2.8 ). Finally, oceanic crust is much younger; the oldest oceanic
crust is about 200 million years old, while the most ancient continental rocks were
formed 3.8 billion years ago.
New crust is formed from molten
material in the upper mantle at the
divergent boundaries that exist at
undersea ridges. The melting is due to
the rise in temperature associated with
the nearly adiabatic decompression of
the upper 50-70 km of mantle material
as separation of the plates reduces the
pressure below. The molten material
collects in a magma pocket which is
gradually exuded in undersea lava
flows.
The
solidified
lava
is
transformed into crust by the effects of
heat and the action of seawater which
selectively dissolves the more soluble
components.
An
animated
view
of
seafloor spreading can be
seen
at
the
PBS
site
Mountain
maker,
earth
shaker, which has a lot of
good stuff on plate tectonics.
Plate collisions
445
Where two plates collide, one
generally plunges under the other and
returns to the mantle in a process known
as subduction. Since the continental plates
have a lower density, they tend to float
above the oceanic plates and resist
subduction. At continental boundaries
such as that of the North American west
coast where an oceanic plate pushes under
the continental crust, oceanic sediments
may be sheared off, resulting in a low
coastal mountain range (see here for a nice
animation of this process.) Also, the
injection of water into the subducting
material lowers its melting point, resulting
in the formation of shallow magma
pockets and volcanic activity. Divergent
plate boundaries can cross continents,
however; temporary divergences create
rift valleys such as the Rhine and Rio
Grande, while permanent ones eventually
lead to new oceanic basins.
Collision of two continental plates
can also occur; the most notable example
is the one resulting in the formation of the
Himalayan mountain chain.
(See the USCGS Plate Motions
page for much more about these and other
diagrams.)
Carbon and its Compounds
Carbon is an element of immense significance in both its elemental and
combined form. We are surrounded by compounds made up of carbon and its
substituents. The list given below illustrates the importance of carbon compounds in
our daily life:
1.
Foods [starch, sugar, fats, vitamins, proteins]
2.
Fuels [wood, coal, alcohol, petrol]
3.
Household and commercial articles [paper, soap, cosmetics, oils, paints]
4.
Textile fabrics [cotton, wool, silk, linen, rayon, nylon]
5.
Drugs and disinfectants [penicillin, quinine, aspirin, sulfa drugs]
6.
Poisons [opium, strychnine]
7.
Perfumes [vanillin, camphor]
8.
Explosives [nitroglycerine, dynamite, picric acid, TNT]
9.
Dyes [indigo, congo red, malachite green]
446
10.
War gases [mustard gas, chloropicrin, lewisite]
The list above consists of compounds having plant or animal origin like sugar,
starch, proteins, acetic acid, urea, etc. These are classified as organic compounds and
their chemistry is known as "Organic Chemistry". Modern organic chemistry
comprises the chemistry of carbon compounds which are natural as well as man
made. Much of organic chemistry is devoted to studying compounds of carbon and
hydrogen, i.e., hydrocarbons and their derivatives.
Carbon compounds are of a second type which can be prepared from minerals
such as oxygen, halogens and metals. These are "inorganic" and result in compounds
like carbon dioxide, sodium chloride, copper sulphate, potassium nitrate, sodium
carbonate, etc. Their chemistry is referred to as "Inorganic Chemistry".
Carbon
Back to Top
Carbon undergoes oxidation by combining with oxygen at higher temperature
to form oxides, viz., carbon monoxide (CO) and carbon dioxide (CO
2
). Carbon
monoxide is formed when incomplete combustion of carbon or carbon containing
fuels take place.
C + 1212 O
2
→→ CO
(g)
CO is present in automobile exhausts (when there is incomplete combustion),
volcanic gases, chimney gases, etc.
C + O
2(g)
→→ CO
2(g)
+ Heat
2CO + O
2(g)
→→ 2CO
2(g)
+ Heat
CH
4
+ 2O
2(g)
→→ CO
2(g)
+ 2H
2
O
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