activation energy

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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.
Segen's Medical Dictionary. © 2012 Farlex, Inc. All rights reserved.

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.
Medical Dictionary for the Health Professions and Nursing © Farlex 2012

activation energy

The energy needed to form chemical bonds during a chemical reaction or to break existing ones.
Collins Dictionary of Medicine © Robert M. Youngson 2004, 2005
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.

Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005
References in periodicals archive ?
At a conversion rate [alpha] less than 0.16, the activation enthalpy and entropy curves exhibit a sharp rise after a decline.
The activation enthalpy reflects the heat possessed by the activated molecules of the activated complex.
It can be seen that when the proportion of IA in the blend augments, the value of the activation volume [V.sup.*] seems to increase, whereas the value of the activation enthalpy [DELTA]H seems to decrease.
where [[Tau].sub.oi] is the pre exponential factor, and [Delta][H.sub.i] is the activation enthalpy.
The shift of the [Alpha] peak can be approximated by an Arrhenius equation: the corresponding apparent activation enthalpy is 4.2 eV.
Therefore the low value of the activation enthalpy [E.sub.a[Beta]] noticed for the most crosslinked network can be explained by the low value of the activation entropy.
Application to tensile and compressive tests under a confinement pressure P is developed with the aim of showing how the strain-rate law can be determined from measurements of yield characteristics (yield stress, activation enthalpy and volumes).
The properties that define this arrangement are the activation enthalpy, the activation volume, and the entropy.
where [Delta]H is the activation enthalpy, R is the gas constant and T is the absolute temperature.
The data were further analyzed by calculating the activation energy, [Delta]Ea, and the activation enthalpy, [Delta]Ha, of the [Beta] relaxation process.