Introduction to Soil Chemistry

energy considerations 37
side-by-side, as depicted in the p x–p x representation. This is possible because
the p and d orbitals are directed in space along the x, y, and z axes.
In some atoms the p and s orbitals are mixed together to form several equivalent
orbitals. The most common example is carbon, where four orbitals are
formed by mixing one s orbital with three p orbitals to give four equivalent
orbitals designated as sp 3 orbitals.
Silicon as SiO 2, or its polymeric form [SiO 4] n, has only two types of orbitals
available for bonding. In the following scheme, sp 3 bonding occurs in SiO 4 and
[SiO 4] n; however they are tetrahedral, not planar as shown:
O Si O O
O
Si
(2.1)
In SiO 2 the Si and O atoms have sp 2 -hybridized orbitals 5 forming sigma bonds
and p orbitals forming p bonds. The oxygen also has p orbitals containing lone
pairs of electrons. Polymeric silicon has sp 3 -hybridized obitals available for
bonding, while oxygen still has lone pairs of electrons in p orbitals. These then
are the orbitals available for bonding to elements or compounds that come
into contact with the particle’s surface. This type of interaction could be of
either end-on-end or p type (i.e., side-by-side). In all cases steric hindrances
may limit the type of interaction occurring.
Clays contain aluminum oxides in addition to silica as SiO 2 and its polymeric
forms. Again there are the p orbitals of oxygen and sp-hybridized
orbitals from aluminum, which may result in end-on-end or side-by-side
bonding with the same restrictions encountered with silicon.
For removal of compounds bonded to silicon, aluminum or oxygen bonded
to silicon or aluminum p- and sp-hybridized orbitals must be broken and new
bonds formed. This will require energy and the correct orientation of attacking
groups along with an effective attacking species. In addition, the rate of
species removal will depend on the reaction path and the steric factors
involved. All of these put together will determine the overall rate of the reaction
and the time needed for extraction.
2.4. ENERGY CONSIDERATIONS
The first question in terms of the stability or strength of any bonding interaction
is energy. A general equation for energy is DH = H i - He. This equation
5 sp 2 -hybridized orbitals are formed by mixing one s orbital with two p orbitals to produce three
sp 2 orbitals.
O
O