Heat dissociation

Phosphorus pentachloride sublimes and then dissociates on heating, dissociation being complete at 600 K. It is attacked by water, yielding first phosphorus trichloride oxide, thus ... 

Alkyl fluoride boron trifluoride systems were first investigated by Olah et al. (1957). 1 1 Addition complexes were observed at low temperatures, and their specific conductivity was measured. The specific conductivity of the propyl and butyl fluoride boron trifluoride systems was found three orders of magnitude larger than those of the methyl- and ethyl-fluoride systems. The latter systems on heating dissociated into their starting materials, whereas the former gave polymer mixtures (Olah, unpublished). 

When heated above its melting point, it vaporizes first, and then on further heating dissociates to its elements ... 

The iodine vapours act as transporting agent in this modified version of the van Arkel procedure ". By replacing the resistance heated dissociation wire by an induction heated metal sphere (W or Th, respectively) large samples of Th or Pa crystals can be... 

Triammino-silver Nitrate, [Ag(NH3)3]N03.—The compound is formed when silver nitrate is exposed to dry ammonia gas much heat is developed, increase in bulk takes place, and sufficient heat may he produced to fuse the mass.3 It exhibits considerable heat of formation, is soluble in water, and loses ammonia on heating, dissociating at 63° C. under ordinary pressure, and is soluble in liquid ammonia below —10° C.4 Only the diammine exists in solution, the triammine being completely dissociated into the diammine and ammonia. 

The reaction has a radical-chain mechanism and the chains can be initiated by light or by chemicals, usually peroxides, ROOR. Chemical initiation requires an initiator with a weak bond that dissociates at temperatures between 40-80°. Peroxides are good examples. The 0-0 bond is very weak (30-50 kcal) and on heating dissociates to alkoxyl radicals, RO-, which are reactive enough to generate the chain-propagating radicals from the reactants. The exact sequence... 

Polymers exposed to air and light may contain oxidized groups such as peroxides ( OOR). The 0-0 bond is weak (30-50 kcal/mol) and, upon heating, dissociates to form free RO- radicals and radical chains. These radicals may influence the composition of the pyrolysis products. Autoxidation may take place in food, paint, rubber, etc. It is important, therefore, to consider this possibility when evaluating the composition of the pyrolysis products of a material that was exposed to air and light although the pyrolysis is performed in an inert gas. 

LiBp4, CH3CN, H2O, reflux, 62% yield. LiBF4 upon heating dissociates into LiF and BF3. 

As an example of this, we may first consider the system formed by the two components CaO and CO2, which can combine to form the compound CaCO. This substance, on being heated, dissociates into calcium oxide and carbon dioxide, and gives rise to the equilibrium CaCOa CaO + COg, In accordance with the definition given on p. 6, there are present here two solid phases, the carbonate and the quicklime, and one vapour phase the system is therefore univariant. To each temperature, therefore, there will correspond a certain definite maximum pressure of carbon dioxide (dissociation pressure) and this... 

Salts with Water of Crystallisation.—In the case of the dehydration of crystalline salts containing water of crystallisation, we meet with phenomena which are in all respects similar to those just studied. A salt hydrate on being heated dissociates into a lower hyllrate (or anhydrous salt) and water vapour. Since we are dealing with two components—salt and water —in three phases, viz. hydrate... 

The 1 1 adducts of chlorine and isothiocyanates are yellow liquids or low-melting solids which can be handled conveniently at room temperature. However, upon heating, dissociation readily occurs, with partial regeneration of the starting components. 

Aqua regia is the most known and used mixture, and is a good oxidant due to the formation of nitrosyl chloride (NOCl). Upon heating, dissociation of NOCl into CI2 occurs, which continues attacking the samples and produces an effective digestion in closed containers. This effect is the cause of the oxidation of many materials, which occurs more effectively than with HNO3 or HCl alone (8 ml to 316 W 180°C and 7 atm in 16 min). 

S = Heat of sublimation of sodium D = Dissociation energy of chlorine / = Ionization energy of sodium = Electron affinity of chlorine Uq = Lattice energy of sodium chloride AHf = Heat of formation of sodium chloride. 

Polyiodides are specific polyhalides containing species such as I3", I7", Iq". Polyhalides often dissociate on heating the solid complexes. 

Other properties of association colloids that have been studied include calorimetric measurements of the heat of micelle formation (about 6 kcal/mol for a nonionic species, see Ref. 188) and the effect of high pressure (which decreases the aggregation number [189], but may raise the CMC [190]). Fast relaxation methods (rapid flow mixing, pressure-jump, temperature-jump) tend to reveal two relaxation times t and f2, the interpretation of which has been subject to much disagreement—see Ref. 191. A fast process of fi - 1 msec may represent the rate of addition to or dissociation from a micelle of individual monomer units, and a slow process of ti < 100 msec may represent the rate of total dissociation of a micelle (192 see also Refs. 193-195). 

When a molecule adsorbs to a surface, it can remain intact or it may dissociate. Dissociative chemisorption is conmion for many types of molecules, particularly if all of the electrons in the molecule are tied up so that there are no electrons available for bonding to the surface without dissociation. Often, a molecule will dissociate upon adsorption, and then recombine and desorb intact when the sample is heated. In this case, dissociative chemisorption can be detected with TPD by employing isotopically labelled molecules. If mixing occurs during the adsorption/desorption sequence, it indicates that the mitial adsorption was dissociative. 

For many purposes, for example the estimation of approximate heats of formation (p. 63), it is sufficient to have an average value. This average of the bond dissociation energies is called the average thermochemical bond energy or (more commonly) simply the bond energy. ... 

The heats of formation of the gaseous atoms, 4, are not very different clearly, it is the change in the bond dissociation energy of HX, which falls steadily from HF to HI, which is mainly res ponsible for the changes in the heats of formation. 6. We shall see later that it is the very high H—F bond energy and thus the less easy dissoeiation of H—F into ions in water which makes HF in water a weak aeid in comparison to other hydrogen halides. 

There are many compounds in existence which have a considerable positive enthalpy of formation. They are not made by direct union of the constituent elements in their standard states, but by some process in which the necessary energy is provided indirectly. Many known covalent hydrides (Chapter 5) are made by indirect methods (for example from other hydrides) or by supplying energy (in the form of heat or an electric discharge) to the direct reaction to dissociate the hydrogen molecules and also possibly vaporise the other element. Other known endothermic compounds include nitrogen oxide and ethyne (acetylene) all these compounds have considerable kinetic stability. 

When heated above 673 K the dimer, AljCl, begins to dissociate into the monomer in which the aluminium has a regular trigonal planar configuration. 

The extent of dissociation at a given temperature can be determined by measuring the density of the vapour. Since anhydrous sulphuric acid is less volatile than hydrogen chloride, ammonium sulphate does not readily sublime on heating some ammonia is evolved to leave the hydrogensulphate ... 

As expected from the enthalpy of formation, water is thermally very stable but when steam is heated to above 1300 K slight dissociation to the elements does occur. Pure water is almost a nonconductor of electricity but slight ionic dissociation occurs ... 

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