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 ?
10 and 11 the temperature variation of the activation enthalpy deduced form TSC elementary spectra for the [Beta] and [Alpha] modes, respectively.
It is constituted of elementary processes situated between -138 and -180 [degrees] C and the activation enthalpy ranging between 0.
Since the compensation temperature is close to the maximum temperature of the TSC peak, the width of this peak is restricted despite the broad distribution of activation enthalpy.
g] relaxations, [Beta] and [Gamma], showing distinct behavior were observed: the [Gamma] relaxation, located at low temperature, is related to simple noncooperative motions while the [Beta] relaxation, occurring close to 0 [degrees] C, has the characteristic of a complex and cooperative relaxation with a distribution of activation enthalpy.
Three parameters are used: n and D in the pre-exponential factor, and a third parameter equivalent to shear activation volume in the activation enthalpy [Delta][G.
For the reversible volume increment dV of the developing shear nucleus, the activation enthalpy increases by an amount:
a] = Activation enthalpy, or Gibbs free enthalpy respectively.
The activation enthalpy and activation energy both increased with modifier content up to 10 or 15 phr, which again supports that the molecular motion associated with the [Beta] relaxation was restricted by the modifier.