the axon of a nerve, it causes a localized and transient switch in electric
potential across the cell membrane
from –60 mV (the resting potential)
to +45 mV. The change in electric potential is caused by an inÛux of
sodium ions. Nervous stimulation of
a muscle Übre has a similar effect.
action spectrum A graphical plot
of the efÜciency of electromagnetic
radiation in producing a photochemical reaction against the wavelength
of the radiation used. For example,
the action spectrum for photosynthesis using light shows a peak in the region 670–700 nm. This corresponds
to a maximum absorption in the absorption spectrum of chlorophylls in
this region.
activated adsorption *Adsorption that involves an activation energy. This occurs in certain cases of
chemisorption.
activated alumina See aluminium
hydroxide.
activated charcoal See charcoal.
activated complex See activatedcomplex theory.
activated-complex theory (ACT)
A theory enabling the rate constants
in chemical reactions to be calculated using statistical thermodynamics. The events assumed to be taking
place can be shown in a diagram
with the potential energy as the vertical axis, while the horizontal axis,
called the reaction coordinate, represents the course of the reaction. As
two reactants A and B approach each
other, the potential energy rises to a
maximum. The collection of atoms
near the maximum is called the activated complex. After the atoms have
rearranged in the chemical reaction,
the value of the potential energy falls
as the products of the reaction are
formed. The point of maximum potential energy is called the transition
state of the reaction, as reactants
passing through this state become
products. In ACT, it is assumed that
the reactants are in equilibrium with
the activated complex, and that this
decomposes along the reaction coordinate to give the products. ACT was
developed by the US chemist Henry
Eyring and colleagues in the 1930s.
See also eyring equation.
activated sludge process A
sewage and waste-water treatment.
The sludge produced after primary
treatment is pumped into aeration
tanks, where it is continuously
stirred and aerated, resulting in the
formation of small aggregates of suspended colloidal organic matter
called Ûoc. Floc contains numerous
slime-forming and nitrifying bacteria, as well as protozoans, which
decompose organic substances in the
sludge. Agitation or air injection
maintains high levels of dissolved
oxygen, which helps to reduce the
*biochemical oxygen demand.
Roughly half the sewage in Britain is
treated using this method.
activation analysis An analytical
technique that can be used to detect
most elements when present in a
sample in milligram quantities (or
less). In neutron activation analysis
the sample is exposed to a Ûux of
thermal neutrons in a nuclear reactor. Some of these neutrons are captured by nuclides in the sample to
form nuclides of the same atomic
number but a higher mass number.
These newly formed nuclides emit
gamma radiation, which can be used
to identify the element present by
means of a gamma-ray spectrometer.
Activation analysis has also been employed using charged particles, such
as protons or alpha particles.
activation energy Symbol E a . The
action spectrum
10
a
www.AzShimi.ir www.AzShimi.com
potential across the cell membrane
from –60 mV (the resting potential)
to +45 mV. The change in electric potential is caused by an inÛux of
sodium ions. Nervous stimulation of
a muscle Übre has a similar effect.
action spectrum A graphical plot
of the efÜciency of electromagnetic
radiation in producing a photochemical reaction against the wavelength
of the radiation used. For example,
the action spectrum for photosynthesis using light shows a peak in the region 670–700 nm. This corresponds
to a maximum absorption in the absorption spectrum of chlorophylls in
this region.
activated adsorption *Adsorption that involves an activation energy. This occurs in certain cases of
chemisorption.
activated alumina See aluminium
hydroxide.
activated charcoal See charcoal.
activated complex See activatedcomplex theory.
activated-complex theory (ACT)
A theory enabling the rate constants
in chemical reactions to be calculated using statistical thermodynamics. The events assumed to be taking
place can be shown in a diagram
with the potential energy as the vertical axis, while the horizontal axis,
called the reaction coordinate, represents the course of the reaction. As
two reactants A and B approach each
other, the potential energy rises to a
maximum. The collection of atoms
near the maximum is called the activated complex. After the atoms have
rearranged in the chemical reaction,
the value of the potential energy falls
as the products of the reaction are
formed. The point of maximum potential energy is called the transition
state of the reaction, as reactants
passing through this state become
products. In ACT, it is assumed that
the reactants are in equilibrium with
the activated complex, and that this
decomposes along the reaction coordinate to give the products. ACT was
developed by the US chemist Henry
Eyring and colleagues in the 1930s.
See also eyring equation.
activated sludge process A
sewage and waste-water treatment.
The sludge produced after primary
treatment is pumped into aeration
tanks, where it is continuously
stirred and aerated, resulting in the
formation of small aggregates of suspended colloidal organic matter
called Ûoc. Floc contains numerous
slime-forming and nitrifying bacteria, as well as protozoans, which
decompose organic substances in the
sludge. Agitation or air injection
maintains high levels of dissolved
oxygen, which helps to reduce the
*biochemical oxygen demand.
Roughly half the sewage in Britain is
treated using this method.
activation analysis An analytical
technique that can be used to detect
most elements when present in a
sample in milligram quantities (or
less). In neutron activation analysis
the sample is exposed to a Ûux of
thermal neutrons in a nuclear reactor. Some of these neutrons are captured by nuclides in the sample to
form nuclides of the same atomic
number but a higher mass number.
These newly formed nuclides emit
gamma radiation, which can be used
to identify the element present by
means of a gamma-ray spectrometer.
Activation analysis has also been employed using charged particles, such
as protons or alpha particles.
activation energy Symbol E a . The
action spectrum
10
a
www.AzShimi.ir www.AzShimi.com