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 ?
Furthermore, as the vertical activation energy of the equilibrated samples is not reduced irreversibly by processing, it is stated that the degree of branching is not affected by mechanical degradation.
And hot deformation apparent activation energy of the AZ81E alloy is 166.15 kJ/mol, approximately.
The activation energy and ash content of raw coal were determined as 41.24 kJ/mol and 35.84%, respectively.
The Arrhenius equation was used in a modified form to illustrate the relationship between the effective moisture diffusion and the ratio of the microwave output power to sample weight (m/P) instead of the temperature for calculation of the activation energy. The modified Arrhenius equation (6) derived by Dadali et al.
After fitting is conducted over the fixed viscosity and temperature reciprocal, the activation energy of the viscous flow of a slag component was obtained according to the slope.
The f([alpha]) function can be determined from a nonisothermal curve if activation energy is known (Malek 1989).
Where Ea, R, Ko and T are activation energy for the hydration process in kJ.mol-1, universal gas constant in 8.314x10-3 kJ mol-1 K-1, frequency factor or pre-exponential constant in % s-1, the soaking temperatures (K), respectively.
The activation energy values we got for FeAc and FeAc-FD are smaller than those ones found in scientific literature for some transition metal acetates [8].
Several models [11-14] employed single activation energy value to estimate kinetic parameters, assuming a first order of reaction, whilst other methods [5, 7, 15] made use of the distribution of activation energy values over the entire range of pyrolysis process.
where d[alpha]/dt is the reaction speed, [s.sup.-1]; k is the speed constant; E is the activation energy, J/mol; R is the universal gas constant, J/(mol x K); T is the temperature, K; [alpha] is the degree of transformation, %.