Dissociation, thermal

The thermal dissociation of phosgene into carbon monoxide and elemental chlorine. Equation (8.2), is the reverse of the reaction used widely, either thermally, catalytically or photochemlcally promoted, for the synthesis of phosgene (see Section 5.1). Thus, much of the chemistry relevant to this Section has already been covered in Chapters 5 and 6. Much of the early work, both kinetic and thermodynamic, was marred by a lack of appreciation of the importance of high purity, and the fact that the system takes a very long time to come 

A modern study of this system by Lord and Pritchard [1275] confirmed that the data of Bodenstein and Plaut [218] were of the highest accuracy when the data from Bodenstein and Plaut [218] and the modern study [1275] are combined (see Fig. 8.2(B)), a value of AH of +(112.5 1.5) kJ mol is obtained for reaction (8.2) from the van t Hoff plot [1275]. However, as the slight curvature of the plot betrays, AH varies slightly with temperature. The best data treatment yields AHjgg as +(108.6 i 0.4) kJ mol [1275]. The equilibrium constants for Equation (8.2) are given by [1275]  

The enthalpy of reaction (8.2) at 25 C was also determined as +(113.4 1.3) kJ mol, directly, by studying the photochemical formation of phosgene from CO and Clj in a microcalorimeter [1998], The data from Kuznetsov et al. [1187] should be disregarded. 

Although [COCl]- has been universally recognized as the key intermediate in the thermal dissociation of phosgene, and there is much evidence for its existence in 

The thermal dissociation of phosgene is too slow at temperatures below 500 C to act as a useful source of [COCl]- radicals [2027]. However, at 14(X)-2(XX) K, under shock tube conditions, [COCl]- is the primary decompostion product [1253a]. The bond dissociation 

When high-temperature products are in an equilibrium state, many of the constituent molecules dissociate thermally. For example, the rotational and vibrational modes of carbon dioxide are excited and their motions become very intense. As the temperature is increased, the chemical bonds between the carbon and oxygen atoms are broken. This kind of bond breakage is called thermal dissociation. The dissociation of H2O becomes evident at about 2000 K and produces H2, OH, O2, H, and O at 0.1 MPa. About 50% of H2O is dissociated at 3200 K, rising to 90% at 3700 K. The products H2, O2, and OH dissociate to H and O as the temperature is increased further. The fraction of thermally dissociated molecules is suppressed as the pressure is increased at constant temperature. 

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Seeley J V, Morris R A, Viggiano A A, Wang FI and Flase W L 1997 Temperature dependencies of the rate constants and branching ratios for the reactions of Cr(Fl20)g 3 with CFIjBr and thermal dissociation rates for CI (CFl3Br) J. Am. Chem. Soc. 119 577-84... 

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). 

Tetrasubstituted and some hindered trisubstituted alkenes react rapidly only to the monoalkylborane stage. Rarely, when the tetrasubstituted double bond is incorporated in a cycHc stmcture, does hydroboration under normal conditions fail (25—27). However, such double bonds may react under conditions of greater force (25,28—31). Generally, trialkylboranes are stable at normal temperatures, undergoing thermal dissociation at temperatures above 100°C (32—34). In the presence of B—H bonds, trialkylboranes undergo a redistribution reaction (35—38). 

Pyrolysis approaches can also be used to prepare substituted isocyanates which caimot be prepared using other methods. For example, A[,A[(A[ -trichlorocyanuric acid [87-90-1] thermally dissociates to yield chloroisocyanate [13858-09-8] and carbonyl diisocyanate [6498-10-8]. The carbonyl isocyanate is unstable and polymerizes (8,94). Table 3 Hsts specialty isocyanates. 

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. 

Reduction. BrezeHus attempted the first reduction of zirconium in 1824 by the reaction of sodium with potassium fiuorozirconate. However, the first pure ductile metal was made in 1925 by the iodide thermal-dissociation method. The successfiil commercial production of pure ductile zirconium via the magnesium reduction of zirconium tetrachloride vapor in an inert gas atmosphere was the result of the intense research efforts of KroU and... 

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. 

Naphthoquinone 1-methide thermal dissociation, 3, 785 N aphtho[2,3 -d ][2,1,3]selenadiazole... 

Chain reactions such as those described above, in which atomic species or radicals play a rate-determining part in a series of sequential reactions, are nearly always present in processes for the preparation of thin films by die decomposition of gaseous molecules. This may be achieved by thermal dissociation, by radiation decomposition (photochemical decomposition), or by electron bombardment, either by beams of elecuons or in plasmas. The molecules involved cover a wide range from simple diatomic molecules which dissociate to atoms, to organometallic species with complex dissociation patterns. The... 

The basic thermodynamic data for the design of such reactions can be used to assess the dissociation energies for various degrees of dissociation, and to calculate, approximately, tire relevant equilibrium constants. One important source of dissociation is by heating molecules to elevated temperamres. The data below show the general trend in the thermal dissociation energies of a number of important gaseous molecules. 

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 reason for the poor conversion efficiency to synthetic fuels is the high energy cost in liberating hydrogen from water (thermal dissociation, electroly-... 

On the other hand, both reduction and thermal-dissociation reactions will result in an increase in weight (equivalent to the solute deposited) and a slight increase in dimensions which will depend on the average composition of the diffused layer. 

Some interesting conclusions can be drawn by plotting log against temperature for thermal dissociation reactions of the type M + CI2 MClj (Mis any given metal). (See Fig. 12.18) . 

H2 Is equal to 1 if hydrogen is at atmospheric pressure. If no interchange occurs, Qg 1. The reduction reaction will proceed effectively if log Kp g is not more than 1 or 2 units greater than log Kp Thermal dissociation is not feasible, except for iodides. 

It should be noted that the hydride generation method may also be applied to the determination of other elements forming volatile covalent hydrides that are easily thermally dissociated. Thus, the hydride generation method has also been used for the determination of lead, bismuth, tin, and germanium. 

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]. 

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