tized vibrational and rotational mo-
tion.Engel’s salt See potassium carbon-
ate.enolate ionA negative ion ob-
tained from an enol, by removal of a
hydrogen atom. Enolate ions can
have two forms: one with a single
C–C bond and the negative charge on
the beta carbon atom and the other
with a double C–C bond and the neg-
ative charge on the oxygen atom.enolsCompounds containing the
group –CH=C(OH)– in their mol-
ecules. See also keto–enol tau-
tomerism.enrichmentThe process of increas-
ing the abundance of a speciÜed iso-
tope in a mixture of isotopes. It is
usually applied to an increase in the
proportion of U–235, or the addition
of Pu–239 to natural uranium for use
in a nuclear reactor or weapon.ensembleA set of systems of parti-
cles used in *statistical mechanics to
describe a single system. The concept
of an ensemble was put forward by
the US scientist Josiah Willard Gibbs
(1839–1903) in 1902 as a way of cal-
culating the time average of the sin-
gle system, by averaging over the
systems in the ensemble at aÜxed
time. An ensemble of systems is con-
structed from knowledge of the sin-
gle system and can be represented as
a set of points in phase space with
each system of the ensemble repre-
sented by a point. Ensembles can be
constructed both for isolated systems
and for open systems.
enthalpySymbol H. A thermody-
namic property of a system deÜned
by H = U + pV, where H is the en-
thalpy, U is the internal energy of the
system, p its pressure, and V its vol-
ume. In a chemical reaction carried
out in the atmosphere the pressureremains constant and the enthalpy of
reaction, ∆H, is equal to ∆U + p∆V.
For an exothermic reaction ∆H is
taken to be negative.
entropy Symbol S. A measure of
the unavailability of a system’s en-
ergy to do work; in a closed system,
an increase in entropy is accompa-
nied by a decrease in energy avail-
ability. When a system undergoes a
reversible change the entropy (S)
changes by an amount equal to the
energy (Q) transferred to the system
by heat divided by the thermody-
namic temperature (T) at which this
occurs, i.e. ∆S = ∆Q/T. However, all
real processes are to a certain extent
irreversible changes and in any
closed system an irreversible change
is always accompanied by an increase
in entropy.
In a wider sense entropy can be in-
terpreted as a measure of disorder;
the higher the entropy the greater
the disorder (see boltzmann
formula). As any real change to a
closed system tends towards higher
entropy, and therefore higher disor-
der, it follows that the entropy of the
universe (if it can be considered a
closed system) is increasing and its
available energy is decreasing. This
increase in the entropy of the uni-
verse is one way of stating the sec-
ond law of *thermodynamics.envelopeSee ring conformations.
enyl complexA type of complex
in which there is a link between the
metal atom or ion and the pi elec-
trons of a double bond. *Zeise’s salt
was theÜrst known example.enzymeA protein that acts as a
*catalyst in biochemical reactions.
Each enzyme is speciÜc to a particu-
lar reaction or group of similar reac-
tions. Many require the association of
certain nonprotein *cofactors in
order to function. The molecule un-Engel’s salt 206e