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 ?
* However, the strain rale sensitivity and absorbed energy of all lesied specimens significantly increased wilh increasing strain rale, whereas the thermal activation values showed a contrary trend.
Thus, in this case the excess current has thermal activation dependence of [J.sub.ot] (with activation energy [E.sub.a] = [E.sub.g]/[A.sub.[beta]] and ideality factor [A.sub.[beta]] = E/(kT/q) ~1.5) at high temperatures T [??] 200 K, but the current acquires the tunneling character, when temperature is decreasing ([E.sub.tt0] = 0,016 /0,019 V) (Figure 8).
The thermal activation energy of the process ([E.sub.T] = 0.45 eV) is lower than in case of the hole release from a trap, and the frequency factor is lower as well ([W.sub.T0] = 1.14 x [10.sup.5] [s.sup.-1]).
In particular, at high temperatures (typically several kelvin) thermal activation across an energy gap generally explains the experimental data satisfactorily.
Thermal activation, however, can cause conformers to interconvert, acting as what Pomes describes a "push switch," which can effectively control proton passage.
Thermal activation of AIY occurs via AFD in an environment that is colder than culture temperature and produces an "up gradient" thermotaxis toward warmer temperatures.
It is, therefore, not surprising to find that a particular type of IC is known to fail with different failure mechanisms characterized by their thermal activation energy and associated parameters.
However, it should be noted that the important factor is not a temperature holding time but an exposure time to thermal activation or sintering time, which can induce the densification during sintering process.
The chemical and thermal activation of peach nut shells enhances the removal efficiency for all metal ions.
In this work, bentonite from Nkalagu was modified by thermal activation, acid activation and combined acid and thermal activation.