Energetic favorability thermodynamic processes

The principle of microscopic reversibility requires that the reverse process, ring closure of a butadiene to a cyclobutene, must also be a coiuotatory process. Usually, this is thermodynamically unfavorable, but a case in which the ring closure is energetically favorable is conversion of tra s,cis-2,4-cyclooctadiene (1) to bicyclo[4.2.0]oct-7-ene (2). The ring closure is favorable in this case because of the strain associated with the trans double bond. The ring closure occurs by a coiuotatory process. 

A comparison of the E°s would lead us to predict that the reduction (it) would be favored over that of (i). This is certainly the case from a purely energetic standpoint, but as was mentioned in the section on fuel cells, electrode reactions involving 02 are notoriously slow (that is, they are kinetically hindered), so the anodic process here is under kinetic rather than thermodynamic control. The reduction of water (iv) is energetically favored over that of Na+ (iii), so the net result of the electrolysis of brine is the production of Cl2 and NaOH ( caustic ), both of which are of immense industrial importance ... 

The spherical nature of the surfactant aggregates in reverse micelles is a response to a thermodynamically driven process. It basically represents a need for surfactants to reach an energetically favorable packing configuration at the interface, depending on the molecular geometry of the surfactant. The surfactant molecules can be represented as a truncated cone whose dimensions are determined by the hydrophilic and hydrophobic parts of the surfactant. Assuming that water-in-oil droplets are spherical, the radius of the sphere is expressed as... 

The carbon-carbon single bond can be broken down into either two ions (heterolytic cleavage) or into two radicals (homolytic cleavage). A bond in a molecule exists because its formation is energetically favorable. Consequently, the assemblage from either pair of precursors should be a thermodynamically allowed process. The charged species shown in Scheme 2.13 contain a large... 

To be able to judge the most favorable route, we must understand qualitatively both thermodynamics and kinetics. Therefore we need to understand the process of bond making and breaking, what makes bonds strong or weak, and how the energetics of a process makes some more favorable than others. 

Many biochemical reactions are nonspontaneous (that is, they have a positive AG° valne), yet they are essential to the maintenance of life. In living systems these reactions are conpled to an energetically favorable process, one that has a negative AG° valne. The principle of coupled reactions is based on a simple concept we can nse a thermodynamically favorable reaction to drive an nnfavorable one. Consider an in-dnstrial process. Snppose we wish to extract zinc from the ore sphalerite (ZnS). The following reaction will not work since it is highly nonspontaneons ... 

Any process that will actually take place with no outside intervention is spontaneous in the specialized sense used in thermodynamics. Spontaneous does not mean fast some spontaneous processes can take a long time to occur. In the last section, we used the term energetically favorable to indicate spontaneous... 

Thermodynamics is concerned with the overall energy change between the initial and final states for a process. If necessary, this change can result after an infinite time. Accordingly, thermodynamics does not deal with the subject of reaction rates, at least not directly. The preceding example shows that the thermodynamics of the reaction favors the production of water however, kinetically the process is unfavorable. We see here the first of several important principles of chemical kinetics. There is no necessary correlation between thermodynamics and kinetics of a chemical reaction. Some reactions that are energetically favorable take place very slowly because there is no low energy pathway by which the reaction can occur. 

The general influence of temperature on chromatographic retention can be explained as follows. In order to be retained on the stationary phase, molecules need to transform from a state of very little order while floating in the mobile phase to a much more ordered immobilized state in the stationary phase. This is a process that implies a reduction in entropy. In order to make retention still an energetically favorable process, thermodynamics defines that it must release heat. Thus, retention in chromatography is typically an exothermic process. If the temperature is increased, exothermic processes escape from this constraint by shifting the equilibrium to the original side. This implies that temperature increase is accompanied by a shift to the desorbed state of the molecules and thus a lower retention. There are some rare exemptions to this rule (as always), but those are based on secondary equilibria that overrule the effect described earlier. 

Confining a liquid between weakly attractive lyophobic surfaces (characterized by contact angles above 90 ) at a sufficiently small separation will lead to spontaneous evaporation. This thermodynamic process is controlled by competition between bulk energetics (that favors the liquid phase) and surface energetics (that favors the vapor phase). The liquid-to-vapor transition occurs when the grand potential of the confined liquid and confined vapor are comparable [28, 34, 35] ... 

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