Reactivity


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Related to Reactivity: Reactivity series

reactivity

[‚rē·ak′tiv·əd·ē]
(chemistry)
The relative capacity of an atom, molecule, or radical to combine chemically with another atom, molecule, or radical.
(nucleonics)
A measure of the deviation of a nuclear reactor from the critical state at any instant of time such that positive and negative values correspond to reactors above and below critical, respectively; measured in percent k, millikays, dollars, or in-hours.

Reactivity

 

a measure of the chemical activity of substances accounting for the variety and rate of the reactions possible for a given substance. For example, noble metals (Au, Pt) and inert gases (He, Ar, Kr, Xe) are chemically inactive; that is, they possess low reactivity. Alkali metals (Li, Na, K, Cs) and halogens (F, Cl, Br, I), on the other hand, are chemically active; that is, they possess high reactivity.

In organic chemistry, alkanes are characterized by low reactivity and therefore can undergo only reactions (radical halo-genation and nitration, dehydrogenation, dissociation with rupture of C—C bond) occurring under such vigorous conditions as elevated temperatures and ultraviolet irradiation. For the alkyl halides, additional reactions, such as dehydrohalogen-ation, nucleophilic substitution for the halogen, and the formation of organomagnesium compounds, are possible under mild conditions. The presence of double or triple bonds or functional groups (hydroxyl —OH, carboxyl —COOH, amino —NH2) in a molecule promotes a further increase in reactivity.

Reactivity is quantitatively expressed by specific rate constants or, in the case of reversible processes, by equilibrium constants. Modern concepts of reactivity are based on the valence-bond theory and on the study of the distribution (and displacement upon action of a reagent) of the electron density in a molecule. Electron displacements are defined qualitatively through inductive and mesomeric effects and quantitatively through quantum-mechanical calculations. The key factor determining the relative reactivity of a number of related compounds is the molecular structure, which encompasses the nature of the substituents, the electronic and steric effect of these substituents on the reaction center, and the geometry of the molecule. Reactivity is dependent on the reaction conditions, for example, the nature of the medium, presence of catalysts or inhibitors, pressure, temperature, and irradiation. Depending on the mechanism of a given reaction, these factors all have a varied and sometimes contrary effect on the reaction rate. The quantitative relationship between rate (or equilibrium) constants within one reaction series may be represented by correlation equations, which describe the change in the constant as a function of the change in a given parameter. Examples are seen in the Hammett-Taft equation, which deals with the effect of a substituent, and the Brønsted equation, which deals with solvent polarity.

References in periodicals archive ?
Produced antibodies had reactivity in two different epitopes; antibodies recognizing conformational epitopes and antibodies recognizing linear epitopes (7).
The reactor is returned to the critical state by adding the opposite reactivity variation at the expense of increasing or decreasing BA concentration in the coolant 1st loop.
Patients were followed for one year to determine the relationship between platelet reactivity and subsequent events.
Only when the juice is contaminated does it cause immune reactivity with alpha-gliadin antibody.
Future research may examine the change in cognitive reactivity (both positive and negative) throughout the course of illness before, during, and following different modalities of treatment," they wrote.
One such model, a Quality Improvement and Value Assessment Tool, incorporates platelet reactivity testing to estimate and model the potential impact of integrating platelet reactivity testing into cardiovascular patient care.
With the aim of measuring task evoked cardiovascular reactivity EKG and BP were continuously recorded during experimental session.
12] reactivity index (PRI) while on placebo and 30% while on cilostazol, a 23% absolute, average drop in PRI attributable to cilostazol, a statistically significant effect.
A simple morning walk to school could reduce stress reactivity in children, curbing increases in heart rate and blood pressure that can lead to cardiovascular disease later in life, according to a study by the University at Buffalo (N.
Interventions included hypnosis and body awareness techniques designed to decrease autonomic reactivity and muscle tension.
More specifically, social support has been linked to positive affect, improved emotional adjustment (Chou, 1999; Curtis, Groarke, Coughlan, & Gsel, 2004), decreased levels of psychological distress, decreased cardiovascular reactivity (CVR) in the face of stressors (Fritz, Nagurney, & Helgeson, 2003; Uchino, Uno, & Holt-Lunstad, 1999), and enhanced adjustment to coronary heart disease (Helgeson, 1993).