Dissociation temperatures

Iodine is a dark-coloured solid which has a glittering crystalline appearance. It is easily sublimed to form a bluish vapour in vacuo. but in air, the vapour is brownish-violet. Since it has a small vapour pressure at ordinary temperatures, iodine slowly sublimes if left in an open vessel for the same reason, iodine is best weighed in a stoppered bottle containing some potassium iodide solution, in which the iodine dissolves to form potassium tri-iodide. The vapour of iodine is composed of I2 molecules up to about 1000 K above this temperature, dissociation into iodine atoms becomes appreciable. 

Iodine vapor is characterized by the familiar violet color and by its unusually high specific gravity, approximately nine times that of air. The vapor is made up of diatomic molecules at low temperatures at moderately elevated temperatures, dissociation becomes appreciable. The concentration of monoatomic molecules, for example, is 1.4% at 600°C and 101.3 kPa (1 atm) total pressure. Iodine is fluorescent at low pressures and rotates the plane of polarized light when placed in a magnetic field. It is also thermoluminescent, emitting visible light when heated at 500°C or higher. 

Actual temperatures in practical flames are lower than calculated values as a result of the heat losses by radiation, thermal conduction, and diffusion. At high temperatures, dissociation of products of combustion into species such as OH, O, and H reduces the theoretical flame temperature (7). Increasing the pressure tends to suppress dissociation of the products and thus generally raises the adiabatic flame temperature (4). 

H2 adsorption is weak on the anatase surfaces [8], No dissociative adsorption of H2 takes place over the smooth surfaces of Au at temperatures below 473 K [9,10]. On small Au particles, adsorption is possible at low temperature. Dissociative adsorption of H2 can be accelerated by the negatively charged molecular oxygen species at steps, edges, comers of Au particles [5]. 

On the other hand, water, itself an extremely weak electrolyte at room temperature, dissociates to a greater extent as the temperature rises (Eq. 1.2). For example, Kw increases to about 10-6 at 1000°C and a density of 1.0 g/ml. 

The p/<, of a base is actually that of its conjugate acid. As the numeric value of the dissociation constant increases (i.e., pKa decreases), the acid strength increases. Conversely, as the acid dissociation constant of a base (that of its conjugate acid) increases, the strength of the base decreases. For a more accurate definition of dissociation constants, each concentration term must be replaced by thermodynamic activity. In dilute solutions, concentration of each species is taken to be equal to activity. Activity-based dissociation constants are true equilibrium constants and depend only on temperature. Dissociation constants measured by spectroscopy are concentration dissociation constants." Most piCa values in the pharmaceutical literature are measured by ignoring activity effects and therefore are actually concentration dissociation constants or apparent dissociation constants. It is customary to report dissociation constant values at 25°C. 

Also, it may be prepared by heating beryllium sulfate at elevated temperatures. Dissociation begins at 550°C and completes at 1,000°C. 

Ammino-ferrous Bromides.—Ferrous bromide readily absorbs ammonia at low temperatures with formation of hexammino-ferrous bromide, [Fe(NH3)6]Br2. The substance is a white powder which, on rise of temperature, dissociates, giving the diammine, [Fe(NH3)2]Br2, at... 

Arsenic exhibits allotropy, which is characteristic of non-metals the usual, more stable, metallic form resembles the typical metals in appearance and in being a fairly good conductor of electricity. Under atmospheric pressure it begins to volatilise at about 450° C. and passes into a vapour containing complex molecules, As4, which at higher temperatures dissociate to As2 this complexity is not unusual in non-metals. The yellow allotrope, which is stable at low temperatures, resembles white phosphorus in being soluble in carbon disulphide—a property which emphasises the non-metallic character of this variety. The reactivity of the allotropes, as in the case of phosphorus, differs considerably. 

Cis-dihydrido-bicarbonato and -formato complexes, RhH2BL2 (B = HC03 and 02CH, L = P(iso-Pr)3) were prepared (II). The y(Rh-H) bands of the former appear at 2120 and 2140 cm-1 and the latter at 2130 cm-1. Consistent with these relatively higher i>(Rh-H) frequencies, neither compound releases H2 at an ambient temperature. Dissociation of H2 from these compounds occurs at a higher temperature (> 90°C). 

Figure B.6 shows the layout of CSMPlug for an electrical heating calculation. Inputs to the model are ambient temperature, dissociation temperature, hydrate structure, plug porosity, heat input per unit length, and the pipeline internal diameter. Selecting the default values option on the heating tab or from the defaults pull-down menu will automatically replace the dissociation temperature, ambient temperature, and porosity with the default values. These default values can be seen in Figure B.6.

This means that direct dissociation is sterically hindered at the Pt(l 1 1) surface so that it becomes a two-step process. First the molecule is trapped molecularly in the chemisorption well where it equilibrates. At sufficiently high surface temperatures dissociation will then be induced by thermal fluctuations which make the 02 molecules enter the dissociation channel. 

The cycle is closed by the high temperature dissociation of sulfuric acid to sulfur trioxide and water, and the subsequent reduction of sulfur trioxide to sulfur dioxide and oxygen, i.e.,... 

The mole fractions of AlF and AIF are determined from the relative values of the peaks in the spectrum as 0.07 and 0.3, respectively i.e., the AIF that dominates the solid at room temperature dissociates to a considerable extent in the liquid at 1270 K to the simpler complex. 

Several workers have measured the high temperature dissociation of CO2, from which data they have inferred the rate of the reverse reaction, the combination of CO and O. These calculations lead to interesting conclusions. The experimental data have been gathered and summarized in a recent review by Baulch et and... 

Dissociative adsorption of CO has been found on a variety of transition metal surfaces. Broden el al. (17) and Nieuwenhuys (14) correlated the tendency for CO, N2, and NO to dissociate with the position of the transition metal in the periodic table the tendency for dissociation increases the further to the left the metal appears in the table, and it decreases from 3d to 5d metals. Furthermore, the borderline for dissociative or molecular adsorption moves to the right in the sequence CO, N2, NO to O2, being the same order as the bond strength in the free molecules. There is sufficient evidence for the proposed correlation. For example, W and Mo surfaces dissociate CO easily al room temperature dissociative adsorption has not been reported lor Pi, Ir, and Pd(III) surfaces, and CO dissociation has been reported to occur on Ni, Co, and Ru at elevated temperatures. Ben-zinger (IS) suggested that the state of adsorption (molecular or dissociative)... 

The literature suggests that on Pd and Pt surfaces adsorption of NO is predominantly molecular al room temperature. Dissociation is observed at elevated temperatures, and the surface structure has a significant influence on the extent of dissociation. In contrast to the adsorption of CO, dissociation of NO can easily be detected on Ir, Rh, and Ru surfaces at room temperature. 

Perrot, causing considerable quantities of water vapor to pass between the multiple sparks of an induction coil, obtained a partial decomposition of water vapor into its elements. H. Sainte-Claire Deville did not hesitate to see in this experiment the ana logue of Grovers experiment. The spark, a line of fire at veiy high temperature, dissociates the water vapor as does the mass of incandescent platinum the oxygen and hydrogen liberated... 

Comparison of Infrared and Nuclear Magnetic Resonance Methods. The NMR measurements of H bonding systems are few in number but great in promise. Effective use of this technique depends rather critically upon the possibility of varying the sample temperature. At high temperatures, dissociation of the H bonded complexes can be obtained even at the rather high concentrations necessary for detection. At low temperatures the different H bonded species may be observable individually. 

Phillips D. J. and Phillips S. L. (2000) High temperature dissociation constants of HS and the standard thermodynamic values for S2. J. Chem. Eng. Data 45, 981-987. 

At low temperatures dissociation can be neglected and two exponential decays are observed. In the transition range the decay is nonexponential. This transition temperature depends on the thermodynamic parameters free energy AG, enthalpy AH, and entropy AS (9). The equilibrium constant for excimer formation can be obtained from the concentration under stationary conditions (5). 

Associative adsorption of the hydrocarbon and homolytic dissociation of hydrogen. At higher temperatures dissociative adsorption of olefins may also occur. Desorption over the range 323-773 K gave some CS2 and H2S, and coke remaining on the catalyst led to irreversible poisoning. ... 

Pseudorotation.— The methoxyphosphorane (39 X = OMe) is sufficiently stable at — 80 °C to exhibit non-equivalent methyl groups and PH coupling to the methoxy protons. However, upon raising the temperature, dissociation occurs before any pseudorotation effects can be observed. The corresponding fluorophosphorane (39 X = F) is dissociated at —85 °C. Studies of a... 

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