Endothermic reactions 528 table

This is also an endothermic reaction, and the equilibrium production of aromatics is favored at higher temperatures and lower pressures. However, the relative rate of this reaction is much lower than the dehydrogenation of cyclohexanes. Table 3-6 shows the effect of temperature on the selectivity to benzene when reforming n-hexane using a platinum catalyst. 

Although olefins are intermediates in this reaction, the final product contains a very low olefin concentration. The overall reaction is endothermic due to the predominance of dehydrogenation and cracking. Methane and ethane are by-products from the cracking reaction. Table 6-1 shows the product yields obtained from the Cyclar process developed jointly by British Petroleum and UOP. ° A simplified flow scheme for the Cyclar process is shown in Figure 6-6. 

We can see from Table 9.2 that the equilibrium constant depends on the temperature. For an exothermic reaction, the formation of products is found experimentally to be favored by lowering the temperature. Conversely, for an endothermic reaction, the products are favored by an increase in temperature. 

Reaction 27 is exothermic by 38 kcal./mole, Reaction 28 by 60 kcal./ mole, and Reaction 29 by 16 kcal./mole while C2H3+ appears to be somewhat endothermic (8). Table II shows the spectrum of these ions at two different kinetic energies. The more extensive decomposition at higher impacting ion velocity is clear evidence for the conversion of kinetic energy to internal energy. 

TABLE 6.3 Comparison of Ideal Reactors for Consecutive, Endothermic Reactions... 

Heat flow from any external thermo-source into the dehydrogenation reactor should take the role of affording the endothermic reaction heat and the evaporation heat of both reactant and product in addition to the apparent heat for raising their temperatures from the ambient up to the external heating one. Under assumptions of the sufficient amounts of active catalyst and the adequate feed rates of organic chemical hydride, the minimum required heat is obtained as shown in the example of methylcyclohexane at 285°C on the basis of 100% conversion of methylcyclohexane to toluene and hydrogen (Table 13.5). 

A change in temperature, however, does force a change in the equilibrium constant. Most chemical reactions exchange heat with the surroundings. A reaction that gives offbeat is classified as exothermic, whereas a reaction that requires the input of heat is said to be endothermic. (See Table 13-2.) A simple example of an endothermic reaction is the vaporization of water ... 

These values of A Hr are standard state enthalpies of reaction (aU gases in ideal-gas states) evaluated at 1 atm and 298 K. 7VU values of A are in kilojoules per mole of the first species in the equation. When A Hr is negative, the reaction hberates heat, and we say it is exothermic, while, when A Hr is positive, the reaction absorbs heat, and we say it is endothermic. Tks Table 2-2 indicates, some reactions such as isomerizations do not absorb or liberate much heat, while dehydrogenation reactions are fairly endothermic and oxidation reactions are fairly exothermic. Note, for example, that combustion or total oxidation of ethane is highly exothermic, while partial oxidation of methane to synthesis gas (CO + H2) or ethylene (C2H4) are only slightly exothermic. 

In either event, the products are ZnCl 2, CO, and A12O 3 The zinc oxide cools and whitens the smoke by consuming atomic carbon in an endothermic reaction that occurs spontaneously above 1000°C (equation 8.6). The reaction with aluminum (equation 8.4 or 8.8) is quite exothermic, and this heat evolution controls the burning rate of the smoke mixture. A minimum amount of aluminum metal will yield the best white smoke. Several "HC" smoke compositions are listed in Table 8.4. 

The oxidation of a heteroatom bearing a lone pair of electrons (e.g. amine, sulfide or phosphine) with increased nucleophilicity of the heteroatom typically results in an early TS, especially with phosphines. Since the N—O bond in an N-oxide is particularly weak, this can sometimes be an endothermic reaction, in contrast to the highly exothermic oxidation of a phosphine with its associated very strong P—O bond (Table 13). 

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-... 

Hydroperoxides can be formed via hydrogen-atom abstraction by R02. The RO2—H bond strength is about 90 kcal. per mole consequently, many such abstractions will be endothermic. Benson (3) has estimated an activation energy of 6 kcal. per mole for exothermic abstractions and 6 + AH for endothermic reactions. The pre-exponential factors should be about 108 0 M"1 sec."1—i.e., about a factor of 2 lower than hydrogen-atom abstraction by CH3 radicals, Some typical abstraction reactions are listed in Table II. 

NICS, HOMA, and Bird indices were also calculated for the transition states of the reactions of 61H-0 and 61H-S with a series of carbanions. The results are reported in Table 23. The trends in these parameters show a clear increase as the transition state becomes more product-like with increasing endothermicity, indicating an increase in transition state aromaticity. Even more revealing is the % progress at the transition state which indicates that this progress is >50% not only for the endothermic reactions (product-like transition states) but even for most of the exothermic reactions (reactant-like transition states) except those with strongly negative AH° values. 

Obviously, the heat of the reaction depends on the concentration of reactant (C). By convention, exothermal reactions have negative enthalpies, whereas endothermic reactions have positive enthalpies.11 Some typical values of reaction enthalpies are given in Table 2.1 [1],... 

TABLE 1 Heat Exchange Employed for High-Temperature Endothermic Reactions... 

In the first reaction, 393.51 kJ are liberated (exothermic reaction) when 1 mol of gaseous C02 is formed from graphite and oxygen. When 2 mol HI are formed from gaseous hydrogen and solid iodine, there are 52.72 kJ absorbed (endothermic reaction). In the case of the second reaction, the standard heat of formation is +26.36 kJ/mol HI formed the total amount of energy involved in the reaction as written is twice the standard heat of formation because there were two moles of product formed. The reason why AH is the symbol instead of AHf is that the reaction does not address the formation of one mole of product therefore, AHf, which is calculated on a per-mole basis, is not an appropriate symbol for the reaction. Further, notice that the 0 is used in AHj and with other factors (S°, AGp or AE°) to indicate the standard condition of pressure, 1 atm (1 bar), usually 25°C and, for dissolved substances, of concentration 1 molal (refer to Chapter 12). For easy reference, selected standard heats of formation for selected substances are located in Table 7-1 however, notice that there are no elements listed in the table. 

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