Exothennic

Chemisorption is always an exothennic process. By convention, tire heat of adsorption, has a positive... 

Atom abstraction occurs when a dissociation reaction occurs on a surface in which one of the dissociation products sticks to the surface, while another is emitted. If the chemisorption reaction is particularly exothennic, the excess energy generated by chemical bond fomiation can be chaimelled into the kinetic energy of the desorbed dissociation fragment. An example of atom abstraction involves the reaction of molecular halogens with Si surfaces [27, 28]. In this case, one halogen atom chemisorbs while the other atom is ejected from the surface. 

The autocatalator model is in many ways closely related to the FONT system, which has a single first-order exothennic reaction step obeying an Arrhenius temperature dependence and for which the role of the autocatalyst is taken by the temperature of the system. An extension of this is tlie Sal nikov model which supports tliennokinetic oscillations in combustion-like systems [48]. This has the fonn ... 

The reactant P is again taken as a pool chemical, so the first step has a constant rate. The rate of the second step depends on the concentration of the intennediate A and on the temperature T and this step is taken as exothennic. (In the simplest case, is taken to be independent of T and the first step is thennoneutral.) Again, the steady state is found to be nnstable over a range of parameter values, with oscillations being observed. 

The above Cl reactions will occur if they are exothennic. In order for these reactions to occur with high efficiency, the pressure in the ion source must be raised to the milliTorr level. Also, the reagent species are often introduced in large excess so that they are preferentially ionized by the electron beam. 

One of the motivations for studying Van der Waals complexes and clusters is that they are floppy systems with similarities to the transition states of chemical reactions. This can be taken one stage further by studying clusters that actually are precursors for chemical reactions, and can be broken up to make more than one set of products. A good example of this is H2-OH, which can in principle dissociate to fonn either H2 + OH or H2O + H. Indeed, dissociation to H2 O -t H is energetically favoured the reaction H2 + OH—> H2 O -t H is exothennic by about 5000... 

In the case of chemisoriDtion this is the most exothennic process and the strong molecule substrate interaction results in an anchoring of the headgroup at a certain surface site via a chemical bond. This bond can be covalent, covalent with a polar part or purely ionic. As a result of the exothennic interaction between the headgroup and the substrate, the molecules try to occupy each available surface site. Molecules that are already at the surface are pushed together during this process. Therefore, even for chemisorbed species, a certain surface mobility has to be anticipated before the molecules finally anchor. Otherwise the evolution of ordered stmctures could not be explained. 

Although essentially inert in acid-base reactions, alkanes do participate in oxidation-reduction reactions as the compound that undergoes oxidation. Burning in air (combustion) is the best known and most important exanple. Combustion of hydrocarbons is exothennic and gives carbon dioxide and water as the products. 

The reactivity of the halogens decreases in the order F2 > CI2 > Br2 > I2. Fluorine is an extremely aggressive oxidizing agent, and its reaction with alkanes is strongly exothennic and difficult to control. Direct fiuorination of alkanes requires special equipment and techniques, is not a reaction of general applicability, and will not be discussed further. 

Chlorination of alkanes is less exothennic than fiuorination, and bromination less exothennic than chlorination. Iodine is unique fflnong the halogens in that its reaction with alkanes is endothennic and alkyl iodides are never prepared by iodination of alkanes. 

The main synthetic application of Grignaid reagents is their reaction with certain caibonyl-containing compounds to produce alcohols. Caibon-caibon bond fonnation is rapid and exothennic when a Grignaid reagent reacts with an aldehyde or ketone. 

The most obvious way to reduce an aldehyde or a ketone to an alcohol is by hydrogenation of the caibon-oxygen double bond. Like the hydrogenation of alkenes, the reaction is exothennic but exceedingly slow in the absence of a catalyst. Finely divided metals such as platinum, palladium, nickel, and ruthenium are effective catalysts for the hydrogenation of aldehydes and ketones. Aldehydes yield primary alcohols ... 

The process is subject to exothennic reactions witli tlie development of excessive temperatures tmd pressures... 

It has been noted by a number of workers that the presence of a-substituents which delocalize the free spin favors combination over disproportionation.127,14 175 For radicals of structure (CH )nC( )-X, kjk increases as shown in Figure 1.12. A correlation between the degree of exothennicity and the value of AU has also been found but only for the case of resonance stabilized radicals.144 176177... 

The second reaction depends on an equilibrium which shifts in the desired direction at low temperature. From a thermody namic standpoint, to obtain high hydrogen contents in the efiluent produced (raw s3mthesis gas), the operating conditions must guarantee the lowest possible service temperatures. In practice, this consideration is incompatible with the paitial oxidation operauon w hich, due to its exothennicity, creates a high reaction temperature (950 to 1250 Q. 

This is practically total above 300°C and, even at atmospheric pressure, lowers the residual CO content to less than 20 ppm and to a few ppm under pressure. It takes place in the presence of nickel base catalysts deposited on alumina and doped with chrominm oxide. The exothennicity of the reaction (Ar from 70 to 80 C/per cent CO converted) requires operation with two catalyst beds and intermediate effluent cooling. 

This conversion is conducted at moderate temperature and pressure (lOO C, IS. 10 Pa absolute), and possibly in the presence of a hydrocarbon diluent, for better control of the temperature rise in the catalyst beds, due to the high exothennicity of the reaction, which is itself related to the high diolefmic content of the initial Cj cut As a rule, the feed is introduced in a downflow stream into the reactor, which contains several beds of a noble metal catalyst on alumina. Quench by recycling and diluent injection is carried out between the beds. The diluent is recovered, by distillation in a depentanizer, after flash to eiimioate the inert compounds introduced with hydrogen gas at the same time as the feedstock. The leading licensors include FP and Shett, etc... 

Oxidatioo. This highly exothennic reacdon takes place in the absence of catalyst However, it requires the presence of basic compounds (calcium or magnesium carbonate) to neutralize the acids formed and hence to prevent the undesirable decomposidoa of the hydroperoxide, as wdl as sodium pyrophosphate or citric add, designed to counterbalance the destabilization effects exo by the metallic ions of the walls. 

Figure 24 DSC trace of N-methyl-N-butylpyrrolidinium hexafluorophosphate (PuPFe) showing the low temperature glass transition, the exothennic crystallisation and the subsequent endothermic transitions up to the melt...

The addition of large quantities of acid (>15wt% of concentrated sulfuric acid) ctm cause rapid, and liiglily exothennic decomposition of hydrogen cyanide. When sulfuric acid is involved, tlie decomposition by-products will be sulfur dioxide and carbon dioxide. 

Excess sulfur dioxide feed lo a chlorine dioxide reactor, leading to e.xcessive exothennic reaction, combined witli failure of the cooling system... 

The formation of sulphuretted hydrogen being exothennic in the conditions indicated, the law of the displacement of equi librium by variation of the temperature requires the line w/ to descend from left to right. 

Putting the first electron into a neutral atom is exothennic -energy is given out, as the attraction of the positive nucleus can still be felt at the surface of the atom. 

It is obvious that the absence of the melting point is characteristic of the DTA curve of a powdery chemical of the TD type. That is, a powdery chemical of this type decomposes prior to melting. The self-heating behavior of a powdery chemical, such as 98 % O, a -azobis(isobutyronitrile) (AIBN), which decomposes explosively prior to any remarkable exothennic decomposition reaction, is also of the TD type, so that the F-K equation is applied to calculate its 1. When confined in the closed cell and subjected to the adiabatic selfheating test started from a in the range of 65 to 74 C, 2 cm of AIBN shows such a self-healing behavior, which is typical of liquid, or powdery, chemicals of the TD type, as exemplified in Fig. 15. 

It is thus inferred that, when CuO powder coexists with the sawdust of red lauan, which is considered to be one of wood species including hcmiccllulosc rather in excess, hemicellulose in the sawdust of red lauan heats oxidatively at temperatures lower than cellulose, with the result that a large exothermic peak, which appears at about 290 C in the absence of CuO powder, divides into two peaks and in the first place, an exothermic peak afforded by the oxidatively-heating reaction of hcmiccllulosc appears at about 280 C in the meantime, cellulose remains as it is up to higher temperatures and then gives independently an exothermic peak afforded by its own oxidatively-healing reaction at about 310 C (Fig. 105). Such being the case, it is expected that, even if CuO powder coexists with the sawdust of wood species relatively hard to heat oxidatively, the first exothennic peak, which appears at about 300 in the absence of CuO powder, does not shift to the low temperature side in the DTA curve, because the quantity of hcmiccllulosc included in the sawdust is not so much (Fig. 106). 

N 20.02% OB -30.7% crysts, mp 85.0-85-1° (Desseigne gave 84.5°)- It canbe prepd by nitrating N,N -bis(2-hydroxyethoxycarbonyl)-ethyleaediamine with AcgO HNOg below 5° (Ref 4). It decompd exothennally on reaction... 

Explosion from Chlorinated Rubber-Zinc Oxide Reaction, An until-now undescribed exothennic reaction — that of chlorinated rubber with xinc oxide was responsible for an expln that leveled the manufg area of Dayton Chemical Products Laboratories,... 

Critical cooling rate, Rc, can also be obtained on the basis of an empirical correlation between the crystallization onset temperature (7 ) obtained from the exothennic peaks in the DTA curves and the cooling rate, R, employed in the DTA experiment using the relation. 

A mixture erupted v igorously one hour after preparation [1], Interaction (not vigorous) of amines and halocarbons at ambient temperatme had been recorded previously [2], The presence of 5 basic centres in the viscous amine would be expected to enhance exothennic effects. 

Frozen mixtmes of trinitromethane —2-propanol (9 1) exploded during thawing. The former (of positive oxygen balance) dissolves exothennally in the alcohol, the heat effect increasing directly with the concentration above 50% w/w. Traces of nitric acid may also have been present. 

This unstable liquid begins to decompose at 15°C and explodes exothennically at... 

The bis- or tris-complexes of phenylcopper with triphenylphosphine react violently and exothennically with carbon disulfide, even at O C. Suitable control procedures are described. 

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