Spontaneity, conditions for

The flash point of an oil is the temperature at which it gives off, under specified conditions, sufficient vapor to form a flammable mixture with air. This is very different from the temperature of spontaneous combustion. The test is an empirical one and the result depends upon the instm-ment used and the prescribed conditions. For example, the flash point may be closed or open , depending on whether the test apparatus has a lid or not. As far as lubricating oils are concerned, the test is of limited significance, although it can be indicative of contamination (for example, the dilution of crankcase oil by fuel). 

AG° is negative, the reaction is spontaneous at standard conditions. For example, the following reaction is spontaneous at 25°C ... 

The condition for spontaneity is obtained by repeating the derivation using the inequality instead of the equality in equation (5.59). The result is... 

In Chapter 5, we considered systems in which composition becomes a variable, and defined and described the chemical potential. We showed that the chemical potential provides the condition for spontaneity or equilibrium. It is the potential that drives the flow of mass in a chemical process, A useful quantity related to the chemical potential is the fugacity. It can also be thought of as a measure of the flow of mass in a chemical process, and can be used to determine the point of equilibrium. It is often known as the escaping tendency since it can be used to describe the ease with which mass flows from one phase to another, particularly the flow from a solid or liquid phase to a gas phase. 

Besides the spontaneous, complete wetting for some areas of application, e.g., washing and dishwashing, the rewetting of a hydrophobic component on a solid surface by an aqueous surfactant solution is of great importance. The oil film is thereby compressed to droplets which are released from the surface. Hydrophobic components on low-energy surfaces (e.g., most plastics) are only re wetted under critical conditions. For a complete re wetting of a hydrophobic oil on polytetrafluoroethylene (PTFE) by an aqueous solution, the aqueous solution-oil interface tension must be less than the PTFE-oil interface tension... 

A note on good practice There are limits to this conclusion. By under standard conditions we mean that there is a spontaneous tendency for pure reactants, each at 1 bar, to change completely into pure products, also each at 1 bar (at the stated temperature). 

The so-called bioactive ceramics have been attractive because they spontaneously bond to living bone, however, they are much more brittle and much less flexible than natural bone. Previous studies reported that the essential condition for ceramics to show bioactivity is formation of a biologically active carbonate-containing apatite on their surfaces after exposure to the body fluid [337]. Calciiun sulfate was also used [338]. 

Schematic view of the spontaneous process for a water-and-ice mixture on a table. The energy-absorbing process, melting, is spontaneous under these conditions.

The values of these functions change when thermodynamic processes take place. Processes in which the Gibbs energy decreases (i.e., for which AG<0), will take place spontaneously without specific external action. The Gibbs energy is minimal in the state of equilibrium, and the condition for equilibrium are given as... 

The above form of the thermodynamic representation of the condition for the occurrence of a spontaneous process or of equilibrium is frequently used. 

The condition for a spontaneous reaction is that the change in the Gibbs free energy must be negative (dG < 0) - a so-called exoergic reaction. 

The addition of allenyl ether-derived anions to Weinreb [4] or to morpholino amides [5] follows a slightly different pathway (Eq. 13.2). For example, the addition of lithioallene 6 to Weinreb amide 7 at -78 °C, followed by quenching the reaction with aqueous NaH2P04 and allowing the mixture to warm to room temperature leads to cyclopentenone 9 in 80% yield [6]. The presumed intermediate of this reaction, allenyl vinyl ketone 8, was not isolated, as it underwent cyclization to 9 spontaneously [7]. These are exceptionally mild conditions for a Nazarov reaction and are probably a reflection of the strain that is present in the allene function, and also the low barrier for approach of the sp and sp2 carbon atoms. What is also noteworthy is the marked kinetic preference for the formation of the Z-isomer of the exocyclic double bond in 9. Had the Nazarov cyclization of 8 been conducted with catalysis by strong acid, it is unlikely that the kinetic product would have been observed. 

Pawlik A, Kirstein S, De Rossi U, Daehne S (1997) Structural conditions for spontaneous generation of optical activity in J-aggregates. J Phys Chem B 101 5646-5651... 

In most of the early studies 9> of H20(as) the vapor was condensed on metal surfaces in the temperature range 77 K diffraction data, supplemented by new experimental studies, convinced Olander and Rice that most deposits obtained at or above 77 K are likely contaminated with crystalline ice. They established conditions for the deposition of pure H20(as) on a variety of substrates 10>. Briefly put, the temperature of the substrate should be low, preferably below 55 K, and the rate of deposition very small (a few mg/hour). There is evidence that H20(as) can be deposited on a substrate at 77 K if the deposition is slow enough. The use of high deposition rates at 77 K leads to polycrystalline ice Ic mixed with H20(as). A sample of pure H20(as) is stable indefinitely long (at least several months) if maintained below 20 K. At about 135 K, with some variation from sample to sample, the amorphous solid transforms spontaneously and irreversibly to ice Ic. 

The equilibrium constraint of Equation (13.3) can be met only if = P, which is the condition for mechanical equilibrium. (We will discuss several special cases to which this requirement does not apply.) Or, to put the argument differently, if the pressures of two phases are different, the phase with the higher pressure will spontaneously expand and the phase with the lower pressure will spontaneously contract, with a decrease in A, until the pressures are equal. Thus, for p phases, p — independent relationships among the pressures of the phases can be written as follows ... 

It should be noted that the 1 1 adducts of benzyne with pyrrylmag-nesium iodide, and A -methyl- and iV-benzylpyrrole were isolated and characterized only as the hydrobromide (113), the methiodide (115), and the picrate of 102, respectively. The low yields of all these derivatives are due in part to further reactions of thenaphthalen-l,4-imines with benzyne (see Section III, F and G). Yields are better where the starting pjnrole has an electron-withdrawing N-substituent. Some analogous naphthalen-l,4-imines expected from 1,2,5-trisubstituted pyrroles apparently rearrange spontaneously to j3-naphthylamine derivatives under the conditions for their formation (see Section III, F). The related adducts 107 and 108 are formed from tetrahalobenzynes and N-methy Ipyrrole. 

The condition for localized electrons with spontaneous atomic moments is U > w that for itinerant electrons with no spontaneous moment is Un < w. The intermediate case Un = w is of considerable theoretical interest. In the case of iron oxides, a U5 = 3 eV ensures a localized 3 d majority-spin configuration at both Fe " and Fe " ions since the cubic-field splitting Ag < Ag, is small enough to leave the ions in the high-spin state. However, localization of the minority-spin electron, particularly in the mixed Fe / Fe " state, does not necessarily follow. Similarly, a much smaller U4 will be seen to make U4 = w for Fe " in the perovskites A Fe03. 

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