Chemical kinetics half-life

The time (hours, days or years) required for the chemical concentration in a medium to be reduced by half. If the elimination rate involves transport and transformation processes that follow first order kinetics, the half-life time is related to the total elimination rate constant k by 0.693 /k. 

In addition to the elimination rate constant, the half-life (T/i) another important parameter that characterizes the time-course of chemical compounds in the body. The elimination half-life (t-1/2) is the time to reduce the concentration of a chemical in plasma to half of its original level. The relationship of half-life to the elimination rate constant is ti/2 = 0.693/ki,i and, therefore, the half-life of a chemical compound can be determined after the determination of k j from the slope of the line. The half-life can also be determined through visual inspection from the log C versus time plot (Fig. 5.40). For compounds that are eliminated through first-order kinetics, the time required for the plasma concentration to be decreased by one half is constant. It is impottant to understand that the half-life of chemicals that are eliminated by first-order kinetics is independent of dose. ... 

The most prominent chemical property of HOF is its instability. It decomposes spontaneously (sometimes explosively) to HF and O2 with a half-life of ca. 30 min in a Teflon apparatus at room temperature and lOOmmHg. It reacts rapidly with water to produce HF, H2O2 and O2 in dilute aqueous acid H2O2 is the predominant product whereas in alkaline solution O2 is the principal O-containing product. The kinetics of these processes have been studied and, by use of 0-enriched H2O2, it has... 

Haber process (Haber-Bosch process) The catalyzed synthesis of ammonia at high pressure, half-cell One compartment of an electrochemical cell consisting of an electrode and an electrolyte, half-life (f1/2) (1) In chemical kinetics, the time needed for... 

The most reliable kinetic data are for atmospheric oxidation by hydroxyl radicals. These data are usually reported as second-order rate constants applied to the concentration of the chemical and the concentration of hydroxyl radicals (usually of the order of 10s radicals per cm3). The product of the assumed hydroxyl radical concentration and the second-order rate constant is a first-order rate constant from which a half-life can be deduced. 

Chemical/Physical. Under laboratory conditions, carbon tetrachloride partially hydrolyzed in aqueous solutions forming chloroform and carbon dioxide (Smith and Dragun, 1984). Complete hydrolysis yields carbon dioxide and HCl (Ellington et al., 1993 Kollig, 1993). The estimated hydrolysis half-life in water at 25 °C and pH 7 is 7,000 yr (Mabey and Mill, 1978) and 40.5 yr (Jeffers et al., 1989 Ellington et al, 1993). The estimated hydrolysis half-life reported by Mabey and Mill (1978) was based on second-order neutral kinetics. Jeffers et al. (1996) reported that hydrolysis of carbon tetrachloride is first-order, contrary to findings of Mabey and Mill (1978). Jeffers et al. (1996) report that the extrapolated environmental half-life at 25 °C is 40 years. 

V = V max [S]// m- A reaction of higher order is called pseudo-first-order if all but one of the reactants are high in concentration and do not change appreciably in concentration over the time course of the reaction. In such cases, these concentrations can be treated as constants. See Order of Reaction Half-Life Second-Order Reaction Zero-Order Reaction Molecularity Michaelis-Menten Equation Chemical Kinetics... 

CHEMICAL KINETICS First-order rate behavior, AUTOPHOSPHORYLATION FIRST-ORDER REACTION KINETICS ORDER OF REACTION HALF-LIFE... 

The ratio of the enantiomeric benzyl amide products was determined by analyzing a diluted aliquot of the quenched reaction mixture by HPLC using a chiral stationary phase column (Chiralcel OD, Daicel Chemical Co.). Since racemization is a pseudo-first-order kinetic process, these data (along with the time zero value) are sufficient for determination of the intrinsic rate of racemization kR. The half-life for racemization lRU2 can be directly calculated from the l/d ratio (or % enantiomeric excess, %ee) where t was the time of benzylamine addition (the delay time) ... 

In Chapter 12, the concept of half-life was used in connection with the time it took for reactants to change into products during a chemical reaction. Radioactive decay follows first order kinetics (Chapter 12). First order kinetics means that the decay rate... 

Radioactive decay follows the same rate equation as first-order chemical kinetics (Section 2.5) the half-life t j2, the time required for one-half of the sample to decay, is given by (In 2)/(rate constant). Decay of a sample is considered arbitrarily to be practically complete after 10ti/2, which is 240,000 years for 239Pu. 

A complete discussion of reaction half-life is given in Chap. 2 of K. J. Laidler, Chemical Kinetics, Harper Row, New York, 1987. 

For the development of concepts of kinetics and of thermodynamics, considered in this chapter, the single atom was assumed to be stable. In fact, the probability for a unique atom to exist in specific specie can only be determined in a chemical experiment if the atom is radioactive. Therefore, if the atom is stable or has a large half-life, any radiochemical techniques can be used for single atom chemistry. 

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