DR. VITUS BLOG

Physical chemistry
Many chemical disciplines, such as those
already discussed, focus on certain classes
of materials that share common structural
and chemical features. Other specialties
may be centred not on a class of
substances but rather on their interactions
and transformations. The oldest of these
fields is physical chemistry, which seeks to
measure, correlate, and explain the
quantitative aspects of chemical processes.
The Anglo-Irish chemist Robert Boyle , for
example, discovered in the 17th century
that at room temperature the volume of a
fixed quantity of gas decreases
proportionally as the pressure on it
increases. Thus, for a gas at constant
temperature, the product of its volume V
and pressure P equals a constant number—
i.e., PV = constant. Such a simple
arithmetic relationship is valid for nearly all
gases at room temperature and at
pressures equal to or less than one
atmosphere. Subsequent work has shown
that the relationship loses its validity at
higher pressures, but more complicated
expressions that more accurately match
experimental results can be derived. The
discovery and investigation of such
chemical regularities, often called laws of
nature, lie within the realm of physical
chemistry. For much of the 18th century the
source of mathematical regularity in
chemical systems was assumed to be the
continuum of forces and fields that
surround the atoms making up chemical
elements and compounds. Developments in
the 20th century, however, have shown that
chemical behaviour is best interpreted by a
quantum mechanical model of atomic and
molecular structure. The branch of physical
chemistry that is largely devoted to this
subject is theoretical chemistry. Theoretical
chemists make extensive use of computers
to help them solve complicated
mathematical equations. Other branches of
physical chemistry include chemical
thermodynamics, which deals with the
relationship between heat and other forms
of chemical energy, and chemical kinetics ,
which seeks to measure and understand the
rates of chemical reactions.
Electrochemistry investigates the
interrelationship of electric current and
chemical change. The passage of an
electric current through a chemical solution
causes changes in the constituent
substances that are often reversible— i.e.,
under different conditions the altered
substances themselves will yield an electric
current. Common batteries contain
chemical substances that, when placed in
contact with each other by closing an
electrical circuit, will deliver current at a
constant voltage until the substances are
consumed. At present there is much
interest in devices that can use the energy
in sunlight to drive chemical reactions
whose products are capable of storing the
energy. The discovery of such devices
would make possible the widespread
utilization of solar energy.
There are many other disciplines within
physical chemistry that are concerned more
with the general properties of substances
and the interactions among substances than
with the substances themselves.
Photochemistry is a specialty that
investigates the interaction of light with
matter. Chemical reactions initiated by the
absorption of light can be very different
from those that occur by other means.
Vitamin D, for example, is formed in the
human body when the steroid ergosterol
absorbs solar radiation; ergosterol does not
change to vitamin D in the dark.
A rapidly developing subdiscipline of
physical chemistry is surface chemistry. It
examines the properties of chemical
surfaces, relying heavily on instruments that
can provide a chemical profile of such
surfaces. Whenever a solid is exposed to a
liquid or a gas, a reaction occurs initially
on the surface of the solid, and its
properties can change dramatically as a
result. Aluminum is a case in point: it is
resistant to corrosion precisely because the
surface of the pure metal reacts with
oxygen to form a layer of aluminum oxide ,
which serves to protect the interior of the
metal from further oxidation. Numerous
reaction catalysts perform their function by
providing a reactive surface on which
substances can react.