Entropy reaction efficiency

It should be born in mind, however, that the activation parameters calculated refer to the sum of several reactions, whose enthalpy and/or entropy changes may have different signs from those of the decrystalUzation proper. Specifically, the contribution to the activation parameters of the interactions that occur in the solvent system should be taken into account. Consider the energetics of association of the solvated ions with the AGU. We may employ the extra-thermodynamic quantities of transfer of single ions from aprotic to protic solvents as a model for the reaction under consideration. This use is appropriate because recent measurements (using solvatochromic indicators) have indicated that the polarity at the surface of cellulose is akin to that of aliphatic alcohols [99]. Single-ion enthalpies of transfer indicate that Li+ is more efficiently solvated by DMAc than by alcohols, hence by cellulose. That is, the equilibrium shown in Eq. 7 is endothermic ... 

AH = 17.3 4.1 kcal.mole and ASl = —15 13 cal.deg . mole Thus the efficiency of Cu(II) as a catalyst is due to a more favourable entropy of activation. In fact the enthalpy of activation for the catalysed reaction is greater than that for the uncatalysed reaction . Recently, Espenson et have measured... 

Furthermore, antibodies should be capable of efficiently catalyze reactions with unfavorable entropies of activation by acting as entropy traps the binding energy of the antibody being used to freeze out the rotational and translational degrees of freedom necessary to form the activated complex. This principle has been applied to the design of antibodies that catalyze both unimolecular and bimolecular reactions (see below). 

G and the maximum theoretical electrical energy efficiency values. We may notice the high e values for most reactions and for some even a thermal efficiency of over 100%. This rather surprising result arises due to the positive value of the entropy change of the reaction (T S term) concerned. The maximum values pertain to the situation of no load. As we may easily anticipate, the real world is much different and a real device will not achieve these terrific values under normal conditions of operation. These limitations arise from kinetic factors and we will briefly outline them next. 

AG ° and AG are expressions of the maximum amount of free energy that a given reaction can theoretically deliver—an amount of energy that could be realized only if a perfectly efficient device were available to trap or harness it. Given that no such device is possible (some free energy is always lost to entropy during any process), the amount of work done by the reaction at constant temperature and pressure is always less than the theoretical amount. 

Several attempts have been made to explain the variations in efficiency of ion exchanger catalysts for different esters and reaction media. Hammett et al. [366,479,488] suggested that the difference in efficiency for different esters arises from a difference in the magnitude of the loss in internal entropy of the ester molecule which accompanies its fixation on the resin catalyst in the formation of the transition state. It can be shown that the ratio of efficiencies for two esters, 1 and 2, is given by... 

The elevated temperature also helps to minimize the unfavorable decrease in entropy due to the heat absorbed by the reaction. For example, if a reaction absorbed 300,000 joules at 300 K (27°C) then the change in entropy is —300,000 J/300 K = —1000 J/K. If, however, the same reaction is run at 500 K (227°C), then the change in entropy is only —300,000/500 K = —600 J/K. Some exothermic reactions are so exothermic that they explode if not run at cold temperatures. The cold temperatures slow the reactive molecules down, which gives the chemist greater control. Also, the heat generated by the reaction is more efficiently dispersed under the colder conditions. This allows for a greater increase in entropy, which helps with die formation of products. 

The transition state theory indicates that the rate of a reaction is not a matter of energy alone, but also requires a favorable configuration by a change of entropy. In addition, the rate of a reaction can be speeded up through the following methods. These methods are the guiding principles in the search for the most efficient AOPs ... 

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