Negative entropy change

Here we have the formation of the activated complex from five molecules of nitric acid, previously free, with a high negative entropy change. The concentration of molecular aggregates needed might increase with a fall in temperature in agreement with the characteristics of the reaction already described. It should be noticed that nitration in nitromethane shows the more common type of temperature-dependence (fig. 3.1). 

If the entropy production in the stationary state falls, a negative fluctuation occurs, and the system becomes unstable the stable stationary state of the system is disturbed or destroyed. The system reacts by changing its composition until a new stable state is reached. In this new state, the system is characterised by a lower entropy content than that present prior to the fluctuation, since only a negative entropy change can occur. However, the lower entropy corresponds to a higher degree of order in the system. 

The addition reaction has a substantial negative entropy change => At low temperatures, the TAS° term in AG° = AH° - TAS°, is not large enough to offset the favorable AH° term. 

Which of the following reactions have a negative entropy change ... 

The association reactions of Si+" (2P) with acetylene and benzene have been measured by Glosik and coworkers96, who found that the rate coefficients for these reactions have strong negative-temperature dependencies. These observations were rationalized in terms of a negative entropy change in the reactions. 

The principal structures into which organolithium compounds assemble are unsolvated octahedral hexamers 6, and cubic tetramers, and solvated cubic tetramers 7, bridged dimers 8 and monomers, 9. In common among the solvated species, lithium is always tetracoor-dinate so that the dissociating direction is exothermic with negative entropy change due to the increase in coordination of lithium to a ligand ether or a tertiary amine. 

The negative entropy changes observed in all solvents are a result of an ordering of solvent molecules in the environment of the zwitterionic form. Since polar solvents are per se more structured than apolar solvents, proportionally less negative entropy changes are obtained in more polar solvents such as ethanol. The rate of the spiropyran/ merocyanine interconversion is also solvent-dependent, as is the position of the visible absorption band of (27a), which, as is typical for a merocyanine, exhibits a pronounced negative solvatochromism (see Section 6.2) [99c, 99d]. 

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