Temperature of dissociation

A bond dissociation energy (Section 2.14) is strictly the energy required to break the bond at T = 0 a bond enthalpy is the change in enthalpy at the temperature of dissociation (typically 298 K). The two quantities differ by a few kilojoules per mole. 

At temperatures between 120° and 300° C. a brownish-red crystalline substance, of composition 3SnCl2.2NH3, is formed. This is the most stable of the compounds, and is only slowly decomposed by water.2, 3 On examination of the temperature of dissociation of the addition products of stannous chloride and ammonia, the highest ammine obtained has composition SnCl2.9NH3 this is formed by the action of liquid ammonia on anhydrous stannous chloride at —78° C. The chloride increases greatly in volume during addition, and the temperature, at which the dissociation pressure is 100 mm., is —55° C. This compound and tetrammino-stannous chloride, [Sn(NH3)4]Cl2, with temperature — 15° C., at which the dissociation pressure is 100 mm., are the only ammines of stannous chloride and ammonia which exist with certainty.4... 

One of the interests of a spectroscopic study of hydrogenated CZ silicon was a search for electronic spectra associated with partially passivated TDD°s similar to those observed for sulphur in silicon. DLTS measurements have proved the passivation of the electrical activity of TDDs in silicon after exposure to a hydrogen plasma at relatively low temperature (100-150°C) [42,120]. The reduction of the TDDs concentration indicates that in the region closest to the surface, full passivation is achieved. The thermal stability of the (TDD,H) complexes thus created has been found to be moderate, and full electrical recovery takes place at about 200°C. The passivation efficiency depends on the TDDi considered, but this is not discussed here. It has been suggested that some of the STD(H) centers could be H-passivated TDDi [187]. However, the temperature difference between the reactivation of the H-passivated TDDi ( ---200 ( ) and the temperature of dissociation of STD(H) centres (—500°C) has led to abandon this assumption [216]. 

The optimal temperature of dissociation is about 4000 K. At higher temperatures closer to 10" K, which are reached in the Ar plasma, AlO is unstable and dissociates (Rains Kadlek, 1970) ... 

In aqueous solution the two homopolynucleotides poly I and poly C readily associate to give a double hehcal complex poly I poly C (Davies and Rich, 1958). The stoichiometry of this complex has been estabUshed by a variety of techniques and at pH 7 only the double-stranded complex is obtained. No triple-stranded structure has been demonstrated under these conditions. The stabihty of the complex is a function of salt concentration and in 0.15 MNa+, pH 7.0, the temperature of dissociation of the two strands is about 60°. The complex is a right-handed hehx and appears to have a geometry similar to that of the A form of RNA. 

These molecules exist in the solid halides, explaining the low melting points of these halides, and also in the vapour phase at temperatures not too far above the boiling point. At higher temperatures, however, dissociation into trigonal planar monomers, analogous to the boron halides, occurs. 

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 ... 

Dinitrogen trioxide, the anhydride of nitrous acid, is very unstable. At low temperature it dissociates thus ... 

Gaseous Hydrogen Chloride. Cast Hon (qv), mild steel, and steel alloys are resistant to attack by dry, pure HCl at ambient conditions and can be used at temperatures up to the dissociation temperature of HCl. The corrosion rate at 300°C is reported to be 0.25 cm/yr and no ignition point has been found for mild steel at 760°C, at which temperature HCl is dissociated to the extent of 0.2%. 

There are three essential factors in the thermal decomposition of limestone (/) the stone must be heated to the dissociation temperature of the carbonates (2) this minimum temperature (but in practice a higher temperature) must be maintained for a certain duration and (J) the carbon dioxide evolved must be removed rapidly. 

AH ammonium haUdes exhibit high vapor pressures at elevated temperatures, and thus, sublime readily. The vapor formed on sublimation consists not of discrete ammonium haUde molecules, but is composed primarily of equal volumes of ammonia and hydrogen haUde. The vapor densities are essentiaHy half that expected for the vaporous ammonium haUdes. Vapor pressures at various temperatures are given in Table 2 (11). Latent heats of sublimation, assuming complete dissociation of vapors and including heats of dissociation are 165.7, 184.1, and 176.6 kJ /mol (39.6, 44.0, and 42.2... 

First Carbonation. The process stream OH is raised to 3.0 with carbon dioxide. Juice is recycled either internally or in a separate vessel to provide seed for calcium carbonate growth. Retention time is 15—20 min at 80—85°C. OH of the juice purification process streams is more descriptive than pH for two reasons first, all of the important solution chemistry depends on reactions of the hydroxyl ion rather than of the hydrogen ion and second, the nature of the C0 2 U20-Ca " equiUbria results in a OH which is independent of the temperature of the solution. AH of the temperature effects on the dissociation constant of water are reflected by the pH. 

This equihbrium favors COS up to ca 500°C. At higher temperatures, COS dissociates increasingly, eg, to 64% at 900°C. The reaction may be mn at 65—200°C to produce carbonyl sulfide if an alkaline catalyst is used (31). A Rhc ne-Poulenc patent describes the manufacture of carbonyl sulfide by the reaction of methanol with sulfur at 500—800°C (32). 

Data on chemical properties such as self-dissociation constants for sulfuric and dideuterosulfuric acid (60,65,70,71), as well as an excellent graphical representation of physical property data of 100% H2SO4 (72), are available in the Hterature. Critical temperatures of sulfuric acid solutions are presented in Figure 10 (73). 

Sihcon carbide is comparatively stable. The only violent reaction occurs when SiC is heated with a mixture of potassium dichromate and lead chromate. Chemical reactions do, however, take place between sihcon carbide and a variety of compounds at relatively high temperatures. Sodium sihcate attacks SiC above 1300°C, and SiC reacts with calcium and magnesium oxides above 1000°C and with copper oxide at 800°C to form the metal sihcide. Sihcon carbide decomposes in fused alkahes such as potassium chromate or sodium chromate and in fused borax or cryohte, and reacts with carbon dioxide, hydrogen, ak, and steam. Sihcon carbide, resistant to chlorine below 700°C, reacts to form carbon and sihcon tetrachloride at high temperature. SiC dissociates in molten kon and the sihcon reacts with oxides present in the melt, a reaction of use in the metallurgy of kon and steel (qv). The dense, self-bonded type of SiC has good resistance to aluminum up to about 800°C, to bismuth and zinc at 600°C, and to tin up to 400°C a new sihcon nitride-bonded type exhibits improved resistance to cryohte. 

Sulfur vapor is an equiUbrium mixture of several molecular species, including Sg, S, and S2. The equiUbrium shifts toward S2 at higher temperatures and lower pressures. The overall reaction is endothermic and theoretically consumes 1950 kj/kg (466 kcal/kg) of carbon disulfide when the reactants are at 25°C and the products are at 750°C. Most of the heat input goes into dissociation of sulfur vapor to the reactive species, S2. Equation 25 is slightly exothermic when the reactants are at a constant temperature of 750°C. 

Flame Temperature. The adiabatic flame temperature, or theoretical flame temperature, is the maximum temperature attained by the products when the reaction goes to completion and the heat fiberated during the reaction is used to raise the temperature of the products. Flame temperatures, as a function of the equivalence ratio, are usually calculated from thermodynamic data when a fuel is burned adiabaticaHy with air. To calculate the adiabatic flame temperature (AFT) without dissociation, for lean to stoichiometric mixtures, complete combustion is assumed. This implies that the products of combustion contain only carbon dioxide, water, nitrogen, oxygen, and sulfur dioxide. 

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