Highly Endothermic Reactions

So cold, so icy, that one bums one s finger at the touch of him Every hand that lays hold of him shrinks back — And for that very reason many think him red-hot. 

200-mL beaker, thermometer (covering the range from -50°C to +50°C), wooden board 10 x 10 x 2 cm, glass rod, protective gloves, safety glasses. 

15 g of barium hydroxide are mixed well with 5 g of ammonium thiocyanate in the beaker and the mixture placed on a wooden board, the surface of which is damp. Within a few seconds the smell of ammonia is clearly noticeable. The mixture of solids becomes liquid and its temperature falls within one to two minutes from about +20°C to - 25 °C or even below, as the thermometer shows even after 10 minutes the temperature is still about -20°C. If the beaker is picked up, it is found to be frozen solid to the wood.  

The solid reaction takes place according to equation (1)  

This highly endothermic reaction can only occur because it is clearly entropy-determined. The disorder of the system increases because of the formation of a large number of single molecules, to that the entropy increases dramatically. The free enthalpy AG is thus negative, since the value of the product TA5 is larger than the relatively high positive enthalpy this leads to the decrease in temperature (eqn. (2))  

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Back strain effects are most important for the homolysis of hydrocarbons (4), a highly endothermic reaction, which does not produce a stable molecule byproduct, as do diazenes (N2) and peresters (CO2). Destabilization of the reactants in reaction 4 back strain is essential in lowering the energy of activation of reaction. The results of this study suggest that only reaction 4 requires the use of A values to obtain a good correlation between reaction temperatures and calculated product radical stabilities. 

Finally, we can see that, neutral meolecules, either in nitro-type or in aci-nitro-type, are more stable than acid-dissociated anions the anion formation is a high endothermic reaction. The energy difference between neutral molecules and acid-dissociated anions calculated at the MP2/6311+-I-G level is 1539 kJ/mol for nitro-type species, and 1683 kJ/mol for aci-nitro-type species. It is clear that, in these conditions, the acid dissociation of the neutral molecules can hardly occur.in pure nitromethane solutions. It provides another theoretieal support for nitromethane as an ideal model of aprotic solvents. 

The acid dissociation of neutral molecules is such a highly endothermic reaction that the acid dissociation of nitromethane can hardly take place. The results of the calculations presented here provide a theoretical support for nitromethane as an ideal model of aprotic solvent in the acid-base theory of organic molecules. 

The activation energies for highly endothermic reactions are known to be virtually equal to the enthalpy of the reaction. According to IPM, each group of reactions is characterized by the critical value of the enthalpy of the reaction A//cm ix. When the reaction enthalpy AHe > AWemax, the activation energy E=AH+0.5RT, whereas A//emax depends on parameters a and bre [115]. 

JThis is not necessarily true for highly endothermic reactions. 

In highly endothermic reaction, the transition-state (TS,) resembles product B. Case II. Highly Exothermic Reaction... 

In an endothermic reaction, the reactant temperature will fall as reaction proceeds unless heat is supplied from an external source. With a highly endothermic reaction, it may be necessary to supply a considerable amount of heat to maintain a temperature high enough to provide a rate of reaction and equilibrium conversion which are large enough for the process to be operated economically. Under these circumstances, the rate of heat transfer may effectively determine the rate of reaction and so dominate the problems involved in the reactor design. 

The suitability of a cycle for hydrogen production depends upon the overall thermal efficiency and operational feasibility. A highly endothermic reaction step is required in a cycle to achieve effective heat-to-chemical energy conversion. For efficient mass and momentum transfer a fluid based system is preferred [96] and, ultimately, for large-scale hydrogen production other factors such as environmental effects and cost effectiveness must also be considered. 

Unimolecular decomposition of ClsSnOO is highly endothermic (reactions Eq. 61 and Eq. 62 for example), but exothermic decomposition pathways involving H-atoms exist (Eq. 64 and Eq. 65). Reaction with Cl atoms is en-... 

An important question regarding SN2 reactions in the gas phase concerns the stereochemistry and the extent to which a Walden inversion occurs at the reaction site. Since the experimental techniques monitor exclusively ion concentration, the actual nature of the neutrals produced in the reaction is subject to some doubt. An indirect method to ascertain the nature of the products is to assess the thermochemistry of other possible reaction channels. In the case of methyl derivatives, the alternatives are few and result in highly endothermic reactions, as exemplified in (22) and (23). For more complicated systems, this argument may not be satisfactory or may not yield an unequivocal answer. 

The energetics of various types of alkane ion molecule reactions are displayed in Table III. These heats of reaction should be reasonably accurate within less than 10 kcal./mole. They give an approximate idea of the type of reactions that are energetically allowed. Highly endothermic reactions will probably be so inefficient as to be unimportant. Slightly endothermic reactions (less than 1 e.v. endothermic) may be quite efff-... 

The conversion of cyclohexanes to aromatics is a highly endothermic reaction (AH 50 kcal./mole) and occurs very readily over platinum-alumina catalyst at temperatures above about 350°C. At temperatures in the range 450-500°C., common in catalytic reforming, it is extremely difficult to avoid diffusional limitations and to maintain isothermal conditions. The importance of pore diffusion effects in the dehydrogenation of cyclohexane to benzene at temperatures above about 372°C. has been shown by Barnett et al. (B2). However, at temperatures below 372°C. these investigators concluded that pore diffusion did not limit the rate when using in, catalyst pellets. 

In the example of partial oxidation of methane the highly endothermic reaction of steam reforming of natural gas was combined with the exothermic combustion of methane. Reverse flow operation makes it possible to achieve high temperature in the catalyst bed at a low average difference between outlet and inlet temperatures, thus decreasing the methane consumption for exothermic combustion. Pilot plant tests [32] have demonstrated the feasibility of this concept. 

Fuel Requirement. Most of the fuel used in a steam reforming plant is required to sustain the following highly endothermic reaction in the primary reforming furnace ... 

The conversion of ethylbenzene to styrene is a highly. endothermic reaction which obeys the following overall mechanism ... 

FTocesses for the production of tertiary amyl methyl ether (TAME) Brockwell et ah, Hyd. Proc., 70(9), 133 (1991)]. Highly endothermic reactions may require intermediate reboilers. None of these heat management issues preclude the use of reactive distillation, but must be taken into account during the design phase. Comparison of heat of reaction and average heat of vaporization data for a system, as in Fig. 13-97, gives some indication of potential heat imbalances [Sundmacher, Rihko, and Hoffmann, Chem. Eng. Comm., 127, 151 (1994)]. The heat-neutral systems [-AH (avg)]... 

Possibility to supply heat directly to the catalyst by using the strings onto which the catalyst material is fixed as electrical heating elements. Clearly this feature can be exploited for quick startup of reactors and for highly endothermic reactions. This enables, for example, the steam reforming of methane in a very compact, electrically heated BSR. 

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