Thermoneutral condition

Figure 25 Effect of temperature on A TR products from 1 mole of n-Cj at thermoneutral conditions (Calculations using HSC Chemistry 4.0 )...

At 1 090 °C reaction temperature and flow rates exceeding 3 Ndm3 min-1, thermoneutral conditions were achieved and no external energy supply was necessary to maintain the reaction temperature. No deactivation of the catalyst could be observed during 200 h TOS. However, 0.1 wt.% material was lost from the monolith, which was attributed to sublimation effects [45],... 

Evidence that p-toluenesulfonyl radicals abstract a chlorine atom from CC14 under certain conditions has also been given50 from available thermodynamic data this reaction should be nearly thermoneutral. 

We now consider heteroaromatic diamines with the condition that an amino group is not a to a heterocyclic nitrogen. The only thermochemical data we can find are for 2,8-diamino acridine for which the solid-phase enthalpy is 127 7 kJmol-1. In the absence of significant substituent and solid state effects, thermoneutrality is expected for the conproportionation reaction 40 that produces diaminoarenes from monoamine derivatives. 

The energy required to break and to form molecular bonds and to bring reactants and products to their reference states is in fact measured by the enthalpy A H, vhich thus defines the so-called thermoneutral potential AE is used only for equilibrium quantities, while AVare potential differences away from equilibrium conditions) ... 

Under standard conditions at 25 °C, AVt = 1.481V. From thermodynamic considerations, electrolysis at AV < A tn proceeds with heat absorption from the environment, whereas the opposite is the case at AV> A tn- At AV=A t > no net exchange takes place between the cell and the environment and the term thermoneutral has been coined to emphasize such a situation. 

When energy exactly equal to the enthalpy AH = AG + TAS (=285.83 kj/mol at 1 bar and 25°C) for water splitting is supplied, no heat is absorbed or evolved by the system [10]. The voltage corresponding to this condition, the thermoneutral voltage Vm is given by... 

There seems to be agreement that the interconversion of MeHSi=CH2 and Me2Si is nearly thermoneutral, with a barrier of ca 40 kcalmoC1,102. MeHSi=CH2 was found in higher concentration under conditions such that the two species could isomerize in a... 

The rate of deprotonation of an acid by a base depends on their structures [41], on the solvent and temperature, and on the difference (ApKa) between the pKa of the acid and that of the base. When acid and base have the same pfCa (ApKa=0) the change of free energy for proton transfer becomes zero and the reaction becomes thermoneutral. Under these conditions the rate of proton transfer is limited only by the so-called intrinsic barrier [34], which is particularly sensitive to structural changes in the reaction partners [39]. When ApKa increases, the rate of proton transfer also increases and approaches a limiting value, which depends on the structures of the acid and base and on the experimental conditions. For normal acids (O-H, N-H) in water the rate of proton transfer becomes diffusion-controlled (ka=10loL mol-1 s"1) when ApKa>2, but in aprotic solvents the limiting proton transfer rate can be substantially lower [42]. 

Reactions between ions and molecules occurring under the experimental conditions in a FT-ICR or FA instrument should be exothermic or thermoneutral. In both types of instrument the concentration of ions is approximately 104 times lower than the concentration of molecules. A simple pseudo-first order kinetic law is therefore observed for the ion/molecule reactions, and these proceed much faster in the gas phase than in solution. This is well demonstrated by the corresponding rate constants measured for the reaction between methyl bromide and the hydroxide ion as a function of the number of water molecules added to the OH ion and summarized in Table 2 (Bohme and Mackay, 1981). Note that the reaction in the gas phase is actually quenched by the addition of only three water molecules to OH ... 

A Wagner-Meerwein rearrangement can be part of the isomerization of an alkyl halide (Figure 14.4). For example, 1 -bromopropane isomerizes quantitatively to 2-bromopropane under Friedel-Crafts conditions. The [l,2]-shift A — B involved in this reaction again is an H atom shift. In contrast to the thermoneutral isomerization between carbenium ions A and B of Figure 14.3, in the present case an energy gain is associated with the formation of a secondary carbenium ion from a primary carbenium ion. Note, however, that the different stabilities of the carbenium ions are not responsible for the complete isomerization of 1-bromopropane into 2-bromopropane. The position of this isomerization equilibrium is determined by thermodynamic control at the level of the alkyl halides. 2-Bromopropane is more stable than 1-bromopropane and therefore formed exclusively. 

CLs), resulting in a drastic drop in cell performance [17], Figure 3.13 also shows the difference between the theoretical cell potential (1.23 V) and the thermoneutral voltage (1.4 V), which represents the energy loss under reversible conditions (the reversible loss) [18], Very often, polarization curves are converted to power density versus current density plots by multiplying the cell voltage by the current density at each point of the curve. 

Thermodynamically the insertion of an alkene into a metal-hydride bond is much more favourable than the insertion of carbon monoxide into a metal-methyl bond. The latter reaction is more or less thermoneutral and the equilibrium constant is near unity under standard conditions. The metal-hydride bond is stronger than a metal-carbon bond and the insertion of carbon monoxide into a metal hydride is thermodynamically most often uphill. Insertion of alkenes is also a reversible process, but slightly more favourable than CO insertion. Formation of new CT bonds at the cost of the loss of the ji bond of the alkene during alkene hydrogenation etc., makes the overall processes of alkenes thermodynamically exothermic, especially for early transition metals. 

As a result of coupling the exothermic oxidation reactions (l)-(3) and the endothermic CO2 (4) and steam (5) reforming reactions over the same catalyst, the overall process could be conducted in a mildly endothermic, almost thermoneutral or slightly exothermic regime by changing the reaction conditions, especially the reaction temperature, and/or CH4/O2 ratio in the feed. Thus, this process could evolve with little or no external energy requirement. 

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