Encyclopedia of Chemistry

For simple elementary reactions, e.g., A – B + C →
A + B – C, the surface can show the potential energy
for all values of the A, B, C geometry, providing that
the ABC angle is fixed.
For more-complicated reactions, a different choice
of two coordinates is sometimes preferred, e.g., the
BOND ORDERs of two different BONDs. Such a diagram
is often arranged so that reactants are located at the
bottom left corner and products at the top right. If the
trace of the representative point characterizing the
route from reactants to products follows two adjacent
edges of the diagram, the changes represented by the
two coordinates take place in distinct succession; if the
trace leaves the edges and crosses the interior of the
diagram, the two changes are concerted. In many quali-
tative applications, it is convenient (although not
strictly equivalent) for the third coordinate to represent
the standard Gibbs energy rather than potential energy.
Using bond orders is, however, an oversimplifica-
tion, since these are not well defined, even for the
TRANSITION STATE. (Some reservations concerning the
diagrammatic use of Gibbs energies are noted under
GIBBS ENERGY DIAGRAM.)
The energetically easiest route from reactants to
products on the potential-energy contour map defines
the POTENTIAL-ENERGY PROFILE.
See also CONCERTED PROCESS; GIBBS ENERGY OF
ACTIVATION; REACTION COORDINATE.

potentiometer(pot) A continuously variable resistor
or instrument used to determine the precise measure-
ment of electromotive forces; used to vary, or control,
the amount of current that flows through an electronic
circuit.

potentiometry The measurement of potential for
quantitative electrochemical analysis.

Pourbaix diagram Pourbaix diagrams are often
used to grab quickly an idea of which species predomi-
nates at specific pH and Eh (oxidation-reduction) con-
ditions. The Pourbaix diagram is commonly used by
corrosion engineers to determine the pH and potential
where a metal will either be stable to corrosion, will
corrode, or will form a passivating layer.
See alsoPREDOMINANCE AREA DIAGRAM.

power saturation A phenomenon used in ELEC-

TRON PARAMAGNETIC RESONANCE SPECTROSCOPYto

estimate the electron-spin relaxation times, providing

information about distances between PARAMAGNETIC

centers.

pre-association A step on the REACTION PATH of

some STEPWISE REACTIONs in which the MOLECULAR

ENTITYC is already present in an encounter pair or

ENCOUNTER COMPLEXwith A during the formation of

B from A, e.g.,

In this mechanism, the CHEMICAL SPECIESC may

(but does not necessarily) assist in the formation of B

from A, which may itself be a BIMOLECULARreaction

with some other reagent.

Pre-association is important when B is too short-

lived to permit B and C to come together by diffusion.

See alsoMICROSCOPIC DIFFUSION CONTROL; SPEC-

TATOR MECHANISM.

precipitate An insoluable solid that forms from a liq-

uid suspension as a result of a chemical reaction, and

usually settles out.

precursor complex SeeENCOUNTER COMPLEX.

predominance area diagram Diagrams that delin-

eate the relative stabilities of a chemical species in a

given aqueous environment. Used to understand and

predict equilibrium reactions in aqueous systems. The

most well-known of such stability or predominance

area diagrams is the Eh-pH diagram introduced by the

work of Pourbaix. Others include the log {M}-pH and

log {Ligand}-pH diagrams.

See alsoPOURBAIX DIAGRAM.

pre-equilibrium (prior equilibrium) A rapidly

reversible step preceding the RATE-LIMITING STEPin a

STEPWISE REACTION. For example:

A+C↽⇀Pre-association (AC))BC

encounter

complex

encounter

complex

→() →rrapid Product

pre-equilibrium 219