Endothermic step

To date there is no evidence that sodium forms any chloride other than NaCl indeed the electronic theory of valency predicts that Na" and CU, with their noble gas configurations, are likely to be the most stable ionic species. However, since some noble gas atoms can lose electrons to form cations (p. 354) we cannot rely fully on this theory. We therefore need to examine the evidence provided by energetic data. Let us consider the formation of a number of possible ionic compounds and first, the formation of sodium dichloride , NaCl2. The energy diagram for the formation of this hypothetical compound follows the pattern of that for NaCl but an additional endothermic step is added for the second ionisation energy of sodium. The lattice energy is calculated on the assumption that the compound is ionic and that Na is comparable in size with Mg ". The data are summarised below (standard enthalpies in kJ) ... 

Like tert butyloxonium ion tert butyl cation is an intermediate along the reaction pathway It is however a relatively unstable species and its formation by dissociation of the alkyloxonium ion is endothermic Step 2 is the slowest step m the mechanism and has the highest activation energy Figure 4 8 shows a potential energy diagram for this step... 

The catalytic reaction starts by bonding of the reactants A and B to the catalyst, in a spontaneous reaction. Hence, the formation of this complex is exothermic, and the free energy is lowered. There then follows the reaction between A and B while they are bound to the catalyst. This step is associated with an activation energy however, it is significantly lower than that for the uncatalyzed reaction. Finally, the product P separates from the catalyst in an endothermic step. 

F202 + M - F+F02 + M (Atf°00 23 kcal.mole-1)402 rather than the considerably more endothermic step... 

These endothermic steps from 250 to 550°C are contributed to the oxidation decomposition of organic functional groups, including decomposition of Ag-(S)-(CH2)3-Si-(OCH3)3 and the crystallization of Ag and Ag20. 

The first, endothermic step is the thermal dissociation of ZnO(s) into Zn(g) and O2 at 2300 K using concentrated solar energy as the source of process heat. The... 

The thermodynamic analysis of loannides [193] on SRE in a solid polymer fuel cell indicated that the ethanol steam reforming reaction needs to be carried out in two steps a high-temperature endothermic step (steam reforming), in which ethanol is converted to a gaseous mixtures of H2, CO, CO2, CH4 and unreacted H2O, and a subsequent, low-temperature step (WGSR) in which CO reacts with water to form H2 and CO2. 

Usually, the materials are crystalline or semicrystalline after the purification procedure, thus a melting peak occurs in the first heating curve. Upon cooling, the material is vitrified, which is indicated by a step in Cp. In the second heating curve, after the endothermal step at Tg, it can be seen whether the material has a low or high tendency toward crystallization. In the latter case, exothermal recrystallization and subsequent melting can be detected (Fig. 3.2a). If the glass is kinetically stable, no recrystallization occurs, and only the step at Tg is visible (Fig. 3.2b). 

One disadvantage of this technology is the large energy requirement of the first, highly endothermic, step. The process is also inefficient in the sense that it first transforms methane in an oxidative reaction to carbon monoxide, which, in turn, is reduced to methanol, and the latter is oxidized again to formaldehyde. The direct conversion of methane, therefore, would be a more efficient way in the production of methanol and formaldehyde. 

Step I is endothermic, step 2 is exothermic the overall reaction is endothermic Because of the largo quantities of CO- and HN s generated, the process often is undertaker in connection with urea manufacture, which permits the off-gases to lx recycled usefully. The melamine synthesis maybe carried out at low or medium pressures with the assistance of a catalyst or at higher pressures without a catalyst. 

To understand the values in Table 8.6, we can think of dissolving as a two-step process (Fig. 8.23). In the first hypothetical step, we imagine the ions separating from the solid to form a gas of ions. The change in enthalpy accompanying this highly endothermic step is the lattice enthalpy, AHL, of the solid, which was introduced in Section 6.20 (see Table 6.3 for values). The lattice enthalpy of sodium chloride (787 kj-mol-1), for instance, is the molar enthalpy change for the process... 

In a third reactor (R3, thermal support unit), the solid obtained in R2 is fully oxidized to hematite with air. This third zone is aimed at closing the thermal balance of the process the heat released is used to carry out the endothermic step. In practice, this would correspond to the burning of a part of the hydrogen produced, to supply the heat for this step. This third unit can be avoided if the endothermicity of the process is accepted, and the reoxidation with water is completed in R2, so maximizing the amount of H2 produced per unit weight of catalyst. 

The energy differences between chlorination and bromination result from the difference in the bond-dissociation enthalpies of H—Cl (431 kJ) and H—Br (368 kJ). The HBr bond is weaker, and abstraction of a hydrogen atom by Br- is endothermic. This endothermic step explains why bromination is much slower than chlorination, but it still does not explain the enhanced selectivity observed with bromination. 

Comparison of SN2 and acyl addition-elimination reactions with methoxide as the leaving group. In the concerted SN2, methoxide leaves in a slightly endothermic step, and the bond to methoxide is largely broken in the transition state. In the acyl substitution, methoxide leaves in an exothermic second step with a reactant-like transition state The bond to methoxide has just begun to break in the transition state. 

During the first endothermic step 3.7 wt.% hydrogen is released, the enthalpy of the reaction was measured to be AH = 37 kj moH H2. The second step is associated with a hydrogen release of 1.8 wt.% with AH = 47kJ moH H2. A further decomposition of NaH would increase the overall capacity to 7.4 wt.%. However, NaH is very stable and releases hydrogen only at temperatures above 450 °C. 

The mixed aluminum hydride decomposes in one endothermic step between 220 and 230 °C (Eq. (5.40)) ... 

The activation energy of an endothermic step is necessarily at least as high as the molar reaction enthalpy, AH° (see Figure 2.2 in Section 2.2), and a step with very high activation energy is apt to be quite slow. Accordingly, a pathway with one or more highly endothermic steps is suspect if there are alternatives without these. 

Unfortunately, this procedure is not conclusive because an apparently plausible mechanism that avoids highly endothermic steps may well be blocked for other reasons. The hydrogen-iodide reaction... 

If one propagation step is endothermic and the other is exothermic, the endothermic step may be reversible, the exothermic step is not. 

The likelihood of reversibility of the endothermic step is greater, the smaller the free-energy decrease accompanying the overall reaction. This is because the — AH values of the two competing exothermic steps will then be more similar, and so will be their activation energies and rates. 

The steric and frequency factors of reactions of free radicals with one another or with small molecules are rather alike, and the activation energies are very low for exothermic reactions and only barely higher than the standard-enthalpy changes AH0 for endothermic steps. In simple cases this makes it possible to use thermochemical data to identify which of the many possible steps dominate kinetics. 

As discussed above, FSEC s S-NH3 cycle also utilizes decomposition of sulfuric acid as the endothermic step for the absorption of solar thermal heat and production of oxygen. However, high temperature concentration and decomposition of sulfur acid presents daunting materials of construction issues. Like the metal sulfate based TCWSCs, it is possible to modify the S-NH3 cycle and do without the decomposition of H2SO4. There are two ways to accomplish this. The first approach is to decompose ammonium sulfate produced in the hydrogen production step of the S-NH3 cycle (Reaction (111)) to a metal sulfate in the presence of a metal oxide catalyst. The second approach is to convert ammonium sulfate to metal pyrosulfate e.g. Zto 20i)-... 

The energetics of the H abstraction reaction [reaction (27)], AG= -11.4 kcal moH, and the /3-scission [reaction (28)], AG = -1-8.1 kcal mol are from DFT calculations. The two-step mechanism was excluded because of the endothermic step in reaction (28). However, a concerted reaction [combining reactions (27) and (28)] would be spontaneous, but with only a small exothermicity, AG ncerted = -3.3 kcal mol Therefore, the source of these radicals appears to be a bimolecular homolytic substitution (Sjj2) of the acetamide, by hydrogen atoms [reaction (29)] ... 

If certain exothermic reactions with high activation energy are considered, the transfer concept may also acquire a microscopic meaning. It can indeed be applied to certain elementary endothermic steps of the reaction. 

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