activation energy

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activation energy

Etymology: L, activus, active
the energy required to convert reactants to transition-state species or an activated complex that will spontaneously proceed to products.

Activation Energy

The energy in joules needed to convert a mole of a substance from a ground state to a transition state, which allows a chemical reaction to occur.

ac·ti·va·tion e·ner·gy

(ak'ti-vā'shŭn en'ĕr-jē)
Minimum amount of energy to convert a stable molecule to a reactive molecule.

activation energy

The energy needed to form chemical bonds during a chemical reaction or to break existing ones.
Activation energyclick for a larger image
Fig. 10 Activation energy . (a) Activation energy required without enzymes. (b) Activation energy required with enzymes. (c) Energy from exergonic reaction.

activation energy

the energy required to initiate a reaction. Chemical bonds holding molecules together are difficult to break, requiring extra ‘activation’ energy to push the bonded atoms apart. This extra energy makes the bonds less stable so that the molecule releases not only the activation energy but also the energy unlocked when the chemical bonds break, forming an EXERGONIC REACTION.

Activation energy can be applied externally as heat, but this is inappropriate for living organisms. Instead, they rely on biological catalysts (ENZYMES) which decrease the activation energy needed for the reaction to take place. See Fig. 10 . See also ENDERGONIC REACTION.

activation

the process of activating.

activation analysis
a method of analyzing the content of elements in samples of biological material. The sample is bombarded with nuclear particles and the elements in it measured by the radiation emitted by their radioactive daughter products. Called also radioactivation analysis.
activation energy
the difference in energy between the ground state of the reactants in a reaction and the point of maximum energy or transition state of the reactions. Usually lowered by enzyme catalysts.
activation factor
activation unit
the combination of complement (C4, C2 and C3) that binds to the antigen-antibody complex in the initial reaction step in the classical pathway of complement activation. See also complement.
References in periodicals archive ?
The energy barrier difference between a 2-D disordered structure (at A = 0.
98), permitting thus the calculation of the effective energy barrier at different degrees of crystallinily.
Therefore, in this situation, once the energy barrier has been overcome, the slit between the two planes will completely fill up with liquid.
Apart from the Ozawa macroscopic kinetic models it is also interesting to evaluate the effective energy barrier, [DELTA]E, for a nonisothermal crystallization process.
2] increases the free volume of amorphous regions in polymers, reduces the energy barrier for the motion of polymer chains, and hence facilitates the crystallization of polymers.
More recently, Camesano and Liu have shown that chemical changes caused by cranberry juice also create an energy barrier that keeps the bacteria from getting close to the urinary tract lining in the first place.
Feibelman started by theoretically determining the energy required for an adsorbed aluminum atom to "hop" over the energy barrier separating one location from another on an aluminum surface.
The migration of an electron from site i to site j is determined by thermally activated hopping above an energy barrier of height [DELTA]E = [E.
By converting this kinetic energy into potential energy, the reactants get closer to each other until they surmount the reaction's energy barrier and finally exchange an atom.
o] is a constant, [DELTA]H is the activation energy required to overcome the potential energy barrier for a given process, R is the gas constant, and T is the absolute temperature.
In the arcane and microscopic world of quantum mechanics, electrons routinely vanish from one side of an energy barrier and reappear on the other.
In the recent study (7), the energy barrier [DELTA]H controlling the time temperature dependence of the fracture process has been estimated using the data of [T.