Half decomposition time

Besides the reaction rate k previously defined in Section 3.2, which provides an instantaneous description of how fast a certain process is at a given temperature, some integral parameters (within a time range) were defined for the same purpose. One such parameter is the half decomposition time tio, which is the time required to get W/Wo = 1/2. Making the approximation that k does not vary with the heating time (extremely short TRT or very high values for E ), the formula for v2 calculated from rel. (7) Section 3.2 is the following ... 

The constant k has been determined during the half-decomposition time ... 

Useful constants that can be measured in the course of the experiment are time of full foaming, tf and time of CBA half-decomposition, t1/2. Both values fall exponentially with temperature. 

The thermal decomposition of FCIO 2 in Monel was studied by Macheteau and Gillardeau (183). Decomposition to CIF and O2 was observed at 100°C (2.5% in 144 hr) and 200°C (10% in 235 hr), but a temperatme >250°C was required for rate measurements. It was found that the decomposition is of first order and monomolecular at temperatures up to 285°C. At 300°C the reaction becomes second-order. The calculated rate constants and half-life times are summarized in Table XVI. The... 

The rate of decomposition of initiators usually follows first-order kinetics and is dependent on the solvent present and the temperature of polymerization. The rate is usually expressed as a half-life time (h/2), where ti/2 = In 2/A d = 0.693/fcd- The rate constant ( d) changes with temperature in accordance with the Arrhenius equation as shown below ... 

The half-life time, Tm, of decomposition of di(t-butyl)peroxide at a temperature of 463 K and ambient pressure is 50 s. One may calculate the rate constant and the half-life time for decomposition at 300 MPa and the same temperature, when the activation volume is Av = +13 cm3 mol"1. 

Half-Life. Once these activation parameters have been determined lor a initiator, half-life times tit a given temperature, i.e.. the lime required for 50 decomposition at a selected temperature, and half-life temperatures for a given period, i.e.. the temperature required for 509f decomposition of an initiator over a given time, can be calculated. Half life data arc useful for comparing the activity or one initiator with another when the half-life data arc determined in the same solvent and at the same concentration and. preferably, when the initiators are of the same class. 

In contrast to the (E)-isomer, (Z)-alkenyl(phenyl)-A3-iodane 41 is labile and decomposes with a half-life time of 20 min to terminal alkynes in chloroform solution at room temperature [64]. Stereo electronically preferable reductive anti / -elimination accounts for this facile decomposition. In fact, the kinetic results for E2-type dehydrohalogenation of vinyl halides show that the relative rates of elimination decrease in the order anti /3->syn / - a-elimination [65]. Similar anti -elimination of vinyl-A3-iodane was proposed in the oxidation of methoxyallene with (diacetoxyiodo)benzene 4 to 3-acetoxy-3-methoxypropyne [66]. 

The use of solar powered systems for destruction of a mixture of pesticides in well water including dichloroaniline, benzopyran, atrazine, propazine, alachlor, prometryn, bromacil and cyanobenzoate has been achieved [124], Variable efficiencies for the decomposition of these compounds was observed with over 90% of dichloroaniline destroyed in 10 h compared to 50% of propazine in the same period. In general for most of the pesticides examined the half life time for the process in the 100 ppb concentration range was less than 1 h. 

Figure 1 shows the effect of reaction time on tlie productions of gasses from pulp. Decomposition ratio was slightly increased with increasing reaction time. The influence of reaction time on the decomposition of pulp is low compared with Naphthalene [16]. In the case of Naphthalene, half reaction time led to approximately half decomposition ratio. Pulp seems to be easy to decomposition. The volume of produced hydrogen was virtually the same as one among three difference reaction time parameters. 

Chlorfenvinphos can be grouped among the moderately toxic insecticides. Its action is lasting, but after a few months it is decomposed to harmless compounds. When environmental conditions are known, the decomposition time can be predicted. When used for soil treatment, its half-life in peat soil is 22 weeks, in clay... 

Chemical initiation involves the decomposition of initiator molecules A to form very active primary radicals R" capable of initiating new polymer chains. The stability of A is measured by the half-life time (fx ) defined in terms of its dissociation constant as follows ... 

Recently, we found that calixspherands 14b and 14c form kinetically stable complexes with Ag, in CDCI3 saturated with D2O, with half-life times of decomposition of 51 and 131 h, respectively [19]. 

TFE and monomer (I) were also copolymerized in supercritical carbon dioxide using a free radical initiator such as bis(perfluoro-2-propoxypropionyl)peroxide (III) at 35°C (the half-life time of the initiator is 40 minutes at 35°C) [9], The decomposition of the initiator proceeds through a single-bond homolysis mechanism [10], resulting in the formation of perfluorinated end group that yields thermally stable polymers [9] (Scheme 16.1). The reaction conditions and properties of the copolymers of (I) and TFE obtained in supercritical carbon dioxide are shown in Table 16.1. PTFE is crystalline, so that when the amount of TFE increases in copolymers, the polymer has some microcrystalline regions. The polymers obtained in carbon dioxide have similar properties with the commercial polymers. 

Organic peroxides and hydroperoxides are very commonly used at the laboratory scale as well as at the industrial level. Their instability can be characterized by their half-life time (/1/2)—that is, the time necessary to their half-decomposition at a given temperature —or by the temperature at which they exhibit a given half-life time (see Table 8.2) from these half-life times it is possible to easily And the corresponding value of kd using the kinetic equation of decomposition of the initiator... 

In general, free radicals initiators are used under conditions of half-life times of about 10 hours. The decomposition of peroxides can be single-step or multistep for example, dicumyl peroxide (DICUP) decomposes as follows ... 

Approximately 19% of 8 has decomposed at the end of the intermediate period. The decomposition products are TEMPO in an amount corresponding to the decomposed 8, and the dead products of the cumyl radical. Without the back-reaction between 2 and 9, the decomposition of 8 would obey classical first order kinetics described by eqn (4.6), and 8 would behave as a classical radical initiator, such as azo-isobutyronitrile (AIBN), i.e., its half life time (ti/2 = ln2/kd) would be only 301 s at 83 °C. 

Alitame (trade name Adame) is a water-soluble, crystalline powder of high sweetness potency (2000X, 10% sucrose solution sweetness equivalence). The sweet taste is clean, and the time—intensity profile is similar to that of aspartame. Because it is a stericaHy hindered amide rather than an ester, ahtame is expected to be more stable than aspartame. At pH 2 to 4, the half-life of aUtame in solution is reported to be twice that of aspartame. The main decomposition pathways (Fig. 6) include conversion to the unsweet P-aspartic isomer (17) and hydrolysis to aspartic acid and alanine amide (96). No cyclization to diketopiperazine or hydrolysis of the alanine amide bond has been reported. AUtame-sweetened beverages, particularly colas, that have a pH below 4.0 can develop an off-flavor which can be avoided or minimized by the addition of edetic acid (EDTA) [60-00-4] (97). 

On exposure to ultraviolet irradiation over a period of 25 hours, specific elongation of normal polyethylene decreases by half, and complete decomposition is observed after a 50-hour exposure. At the same time, specific elongation of quenched polyethylene, when exposed to irradiation not greater than 50 hours, is little affected, but when exposed over a period of 100 hours, its specific elongation amounts to 100%. 

The crude product is dissolved in five times its weight of water, and after clearing with a little Norite the solution is diluted with one and one-half volumes of 95 per cent alcohol. The product separates in well-formed, snow-white crystals, and after standing for several days in an ice chest is collected with suction on a Buchner funnel. The yield of purified histidine monohydrochloride is 75-80 g. (Note 5). The compound melts at 251-2520, with decomposition. The amino acid is not race-mized by the procedure employed, and shows the characteristic optical activity, [a]n6° = +8.00, in the presence of three moles of... 

Self-Tfst 13.9A Calculate (a) the number of half-lives and (b) the time required for the concentration of N20 to fall to one-eighth of its initial value in a first-order decomposition at 1000. K. Consult Table 13.1 for the rate constant. 

Dinitrogen pentoxide, N2Os, decomposes by first-order kinetics with a rate constant of 0.15 s 1 at 353 K. (a) What is the half-life (in seconds) for the decomposition of N2Os at 353 K (b) If [N2O5]0 = 0.0567 mol-L, what will be the concentration of N2Os after 2.0 s (c) How much time (in minutes) will elapse before the N205 concentration decreases from 0.0567 mol-L 1 to 0.0135 mol-L ... 

The half-life for the first-order decomposition of A is 355 s. How much time must elapse for the concentration of A to decrease to (a) one-fourth (b) 15% of its original value (c) one-ninth of its initial concentration ... 

The first-order decomposition of compound X, a gas, is carried out and the data are represented in the following pictures. The green spheres represent the compound the decomposition products are not shown. The times at which the images were taken are shown below each flask, (a) Determine the half-life of the reaction, (b) Draw the appearance of the molecular image at 8 s. 

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