tized vibrational and rotational motion.
Engel’s salt See potassium carbonate.
enolate ion A negative ion obtained 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 negative charge on the oxygen atom.
enols Compounds containing the
group –CH=C(OH)– in their molecules. See also keto–enol tautomerism.
enrichment The process of increasing the abundance of a speciÜed isotope 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.
ensemble A set of systems of particles 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 calculating the time average of the single system, by averaging over the
systems in the ensemble at a Üxed
time. An ensemble of systems is constructed from knowledge of the single system and can be represented as
a set of points in phase space with
each system of the ensemble represented by a point. Ensembles can be
constructed both for isolated systems
and for open systems.
enthalpy Symbol H. A thermodynamic property of a system deÜned
by H = U + pV, where H is the enthalpy, U is the internal energy of the
system, p its pressure, and V its volume. In a chemical reaction carried
out in the atmosphere the pressure
remains 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 energy to do work; in a closed system,
an increase in entropy is accompanied by a decrease in energy availability. 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 thermodynamic 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 interpreted 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 disorder, 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 universe is one way of stating the second law of *thermodynamics.
envelope See ring conformations.
enyl complex A type of complex
in which there is a link between the
metal atom or ion and the pi electrons of a double bond. *Zeise’s salt
was the Ürst known example.
enzyme A protein that acts as a
*catalyst in biochemical reactions.
Each enzyme is speciÜc to a particular reaction or group of similar reactions. Many require the association of
certain nonprotein *cofactors in
order to function. The molecule unEngel’s salt
206
e
www.AzShimi.ir www.AzShimi.com
Engel’s salt See potassium carbonate.
enolate ion A negative ion obtained 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 negative charge on the oxygen atom.
enols Compounds containing the
group –CH=C(OH)– in their molecules. See also keto–enol tautomerism.
enrichment The process of increasing the abundance of a speciÜed isotope 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.
ensemble A set of systems of particles 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 calculating the time average of the single system, by averaging over the
systems in the ensemble at a Üxed
time. An ensemble of systems is constructed from knowledge of the single system and can be represented as
a set of points in phase space with
each system of the ensemble represented by a point. Ensembles can be
constructed both for isolated systems
and for open systems.
enthalpy Symbol H. A thermodynamic property of a system deÜned
by H = U + pV, where H is the enthalpy, U is the internal energy of the
system, p its pressure, and V its volume. In a chemical reaction carried
out in the atmosphere the pressure
remains 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 energy to do work; in a closed system,
an increase in entropy is accompanied by a decrease in energy availability. 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 thermodynamic 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 interpreted 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 disorder, 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 universe is one way of stating the second law of *thermodynamics.
envelope See ring conformations.
enyl complex A type of complex
in which there is a link between the
metal atom or ion and the pi electrons of a double bond. *Zeise’s salt
was the Ürst known example.
enzyme A protein that acts as a
*catalyst in biochemical reactions.
Each enzyme is speciÜc to a particular reaction or group of similar reactions. Many require the association of
certain nonprotein *cofactors in
order to function. The molecule unEngel’s salt
206
e
www.AzShimi.ir www.AzShimi.com