Thermal dissociation of

The validity of Johnston s interpretation of the experimental facts in terms of the simple unimolecular dissociation (1) has been questioned by Lindars and Hinshelwood120 and by Reuben and Linnett121. These workers maintain that isothermal plots of k versus p are not smooth curves, but consist of a number of straight lines linked by markedly curved portions. To explain such behaviour they incorporate into their mechanism a collision-induced crossover of vibrationally excited N20 (XS) to repulsive 3II and 3E states. While we incline towards the simpler view held by Johnston105 and others106-116, we feel that this feature of the decomposition kinetics merits further investigation. 

Rate coefficient Rate coefficient expression Temp. (°K) Total cone. (mole.X 1 ) Method Ref.  

The low pre-exponential factor associated with fc°° is an indication of a non-adiabatic reaction. Since there is good evidence122 against a significant extent of thermal dissociation of N20 to N+NO, the alternative dissociation, reaction (1), has been universally accepted as the initial step in the decomposition. The subsequent fate of the ground-state O atoms is unquestionably reaction with N20, viz. 

A dispute123,125 126 over the magnitude of k2h (Table 8) seems to have been settled in favour of Fenimore s value by recent observations107 109, 127. The finding of Gutman et al,107 of a stationary state in O atoms for the decomposition of 4 % N20 in Ar at temperatures between 1800° and 2500 °K certainly supports Fenimore s rate coefficient. Olschewski et al.109 have clearly demonstrated in their shock-tube study that departure from a steady state in O atoms only occurs at very low N20 concentrations, viz for T 2000 °K at 0.02 % in Ar. This observation suggests, in fact, that Fenimore s value of k2b represents a lower limit for the 

Reaction Rate coefficient Rate coefficient expression (l.mole-1.sec-1) Temp(°K) Ref. 

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Kiefer J H, Mudipalli P S, Wagner A F and Harding L 1996 Importance of hindered rotations in the thermal dissociation of small unsaturated molecules classical formulation and application to hen and hcch J. Chem. Phys. 105 1-22... 

An excess of acetic acid is usually added before heating in order to repress the hydrolysis (and also the thermal dissociation) of the ammonium acetate, thus preventing the escape of ammonia. The excess of acetic acid, together with the water, is removed by slow fractional distillation. The method is rarely used except for the preparation of acetamide. 

Water-soluble peroxide salts, such as ammonium or sodium persulfate, are the usual initiators. The initiating species is the sulfate radical anion generated from either the thermal or redox cleavage of the persulfate anion. The thermal dissociation of the persulfate anion, which is a first-order process at constant temperature (106), can be greatly accelerated by the addition of certain reducing agents or small amounts of polyvalent metal salts, or both (87). By using redox initiator systems, rapid polymerizations are possible at much lower temperatures (25—60°C) than are practical with a thermally initiated system (75—90°C). 

Tantalum. Numerous methods developed to extract tantalum metal from compounds included the reduction of the oxide with carbon or calcium the reduction of the pentachloride with magnesium, sodium, or hydrogen and the thermal dissociation of the pentachloride (30). The only processes that ever achieved commercial significance are the electrochemical reduction of tantalum pentoxide in molten K TaF /KF/KCl mixtures and the reduction of K TaF with sodium. 

Qua.driva.Ient, Zirconium tetrafluoride is prepared by fluorination of zirconium metal, but this is hampered by the low volatility of the tetrafluoride which coats the surface of the metal. An effective method is the halogen exchange between flowing hydrogen fluoride gas and zirconium tetrachloride at 300°C. Large volumes are produced by the addition of concentrated hydrofluoric acid to a concentrated nitric acid solution of zirconium zirconium tetrafluoride monohydrate [14956-11-3] precipitates (69). The recovered crystals ate dried and treated with hydrogen fluoride gas at 450°C in a fluid-bed reactor. The thermal dissociation of fluorozirconates also yields zirconium tetrafluoride. 

Chlorination of Methane. Methane can be chlorinated thermally, photochemicaHy, or catalyticaHy. Thermal chlorination, the most difficult method, may be carried out in the absence of light or catalysts. It is a free-radical chain reaction limited by the presence of oxygen and other free-radical inhibitors. The first step in the reaction is the thermal dissociation of the chlorine molecules for which the activation energy is about 84 kj/mol (20 kcal/mol), which is 33 kJ (8 kcal) higher than for catalytic chlorination. This dissociation occurs sufficiendy rapidly in the 400 to 500°C temperature range. The chlorine atoms react with methane to form hydrogen chloride and a methyl radical. The methyl radical in turn reacts with a chlorine molecule to form methyl chloride and another chlorine atom that can continue the reaction. The methane raw material may be natural gas, coke oven gas, or gas from petroleum refining. 

From the foregoing it is clear that BH3 is a fugitive reaction species it exists only at exceedingly low concentrations but can be isolated and studied using matrix isolation techniques. Thus it can be generated by thermal dissociation of loosely bound 1 1 adducts with Lewis bases, such as PF3.BH3, and its reactions studied. 1 The relative stability of the adducts L.BH3 has been determined from thermochemical and spectroscopic data and leads to the following unusual sequence ... 

Reactivity is enhanced in conditions which promote the generation of halogen atoms, though this does not imply that all reactions proceed via the intermediacy of X atoms. The reversible thermal dissociation of gaseous I2 v 21 was... 

The pentafluorides of Rh and Ir may be prepared by the deliberate thermal dissociation of the hexafluorides. They also are highly reactive and are respectively dark-red and yellow solids, with the same tetrameric structure as [Rup5]4 and [Osp5]4 (p. 1083). 

The thermal dissociation of chelating agents and chelates of analytical interest. W. W. Wendlandt, Chelates Anal. Chem., 1967,1,107-143 (65). 

New sources of RaSn- radicals that have been developed include the reversible thermal dissociation of bis(trialkylstannyl)pinacols (290-292), the )3-scission of /3-stannylalkyl radicals (293), and the photolysis of cyclopentadienyltin compounds (294). 

Naiijj Na apor exists. Above this T, the sodium pressure is no longer sufficient to prevent the thermal dissociation of NaB, and syntheses lead to a second phase with a lower Na content, Na Bu. However, Na,(B 5 can be prepared at < 1100°C, as long as the sodium pressure in the vapor phase is kept low by having a cold wall in the reactor or, e.g., by substituting a Na-K alloy for sodium. ... 

The initiation step. Reaction (I), represents the thermal dissociation of bromine, which is brought about by collision with any other molecule, denoted by M. 

The reaction takes place probably by a kind of inverse Wittig reaction , corresponding to the thermal dissociation of an oxaphosphetene resulting from a [2+2] cycloaddition between the phosphine oxide and the activated acetylenic compounds (Scheme 2) [11,12]. 

The dissociation into silylenes observed in photoelectron spectroscopy for 3 and 25 has been described in Section VII. In addition, thermal dissociation of 25 has recently been noted.63 This requires a somewhat higher temperature than that for dissociation of 27. At 100°C, 25 shows no reactivity toward Et3SiH or (remarkably) toward EtOH, but at I20°C it reacts with both substrates to give silylene trapping products (Scheme... 

These recent observations suggest that thermal dissociation of disilenes into silylenes may be a more general phenomenon than has previously been thought. Additional examples are likely to be reported in the future. 

Dahl, J. et al., Solar thermal dissociation of methane in a fluid-wall aerosol flow reactor, Int. J. Hydrogen Energ., 29, 725, 2004. 

Zambelli T, Barth JV, Wintterlin J (2002) Thermal dissociation of chemisorbed oxygen molecules on Ag(110) an investigation by scanning tunneling microscopy. J Phys Condens Matter 14 4241... 

The temperature dependence of luminescence from the sample irradiated at 1 x 1013 cm-2 with 28Si+ indicates, above —110 K, an activation energy of 90 meV for the competing nonradiative recombination process— this competing process may be the thermal dissociation of geminate pairs or bound excitons at donorlike or acceptorlike centers. The 0.09-eV value of activation energy is consistent with the results of Troxell and Watkins (1979). 

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