Spontaneity, thermodynamic

Any change taking place which results in an increase in entropy has a positive entropy change (AS). Most spontaneous thermodynamic processes are accompanied by an increase in entropy. Entropy has units of Joules per degree K per mole. For representative values see table on p. 393. 

From intermediate 43, the path to monensin would seemingly be straightforward. A significant task which would remain would be the construction of the l,6-dioxaspiro[4.5]decane substructure of monensin. You will note that the oxygen atoms affixed to carbons 5 and 12 in 43 reside in proximity to the ketone carbonyl at C-9. In such a favorable setting, it is conceivable that the action of acid on 43 could induce cleavage of both triethylsilyl ethers to give a keto triol which could then participate in a spontaneous, thermodynamically controlled spiroketalization reaction. Saponification of the C-l methyl ester would then complete the synthesis of monensin. 

Equation (3) defines the equilibrium condition under the constraint that temperature and pressure are constant. A related consequence of the Second Law is that if AG < 0 the reaction of the reactant to product is thermodynamically spontaneous. Thermodynamic spontaneity means that... 

In an open system, the entropy may change due to either increases caused by spontaneous thermodynamically irreversible internal processes in the system, djS, or exchanges between the system and the surrounding, dgS. In chemically reactive systems, djS may change as a result, for example, of spontaneous reactions inside the system, while dgS may change as a result of supply or extraction of heat and/or some reactants. 

We can find out whether a proposed reaction is possible by determining whether it is a spontaneous thermodynamic process. In this context, spontaneous has a precise technical meaning (see later for clarification) that should not be confused with its conversational meaning, such as describing the spontaneous behavior of people in social situations. Thermodynamics can tell us whether a proposed reaction is possible under particular conditions even before we attempt the reaction. If the reaction is spontaneous, thermodynamics can also predict the ratio of products and reactants at equilibrium. But, we cannot use thermodynamics to predict the rate of a spontaneous reaction or how long it will take to reach equilibrium. These questions are the subject of chemical kinetics. To obtain a large amount of product from a spontaneous reaction in a short time, we need a reaction that is spontaneous and fast. 

I See the Saunders Interactive General Chemistry CD-ROM, Screen 6.3, Thermodynamics and Kinetics, and Screen 20.2, Reaction Spontaneity (Thermodynamics and Kinetics). 

As mentioned earlier, a supersaturated solution is not in the equilibrium condition. Crystallization moves the solution toward equilibrium by relieving its supersaturation. A supersaturated solution is thus not stable. There is a maximum degree of supersaturation for a solution before it becomes unstable. The region between this unstable boundary and the equilibrium (binodal) curve is termed the metastable zone, and it is here that the crystallization process occurs. The absolute limit of the metastable zone, known as the spinodal curve (8), is given by the locus of the maximum limit of supersaturation at which nucleation occurs spontaneously. Thermodynamically, the spinodal curve within the two-phase region is defined by the criterion... 

Knowledge of thermodynamic functions such as enthalpy, entropy, and free energy enables biochemists to predict whether a process is spontaneous (thermodynamically favorable). This does not indicate that a reaction will occur (is kinetically favorable), but that it can occur under the right set of conditions. Reactions are kinetically favorable only if there is sufficient energy available to the system undergoing change. 

Structure formation that takes place in disperse systems is the result of spontaneous thermodynamically favorable processes of particle aggregation that lead to a decrease in free energy of the system. These processes include coagulation of dispersed phase or condensation of substance in the zones of direct particle contact. The development of spacial networks (disperse structures) of different kinds defines the ability of disperse system to be converted into a material with particular mechanical properties. Such system becomes qualitatively different from its initial, unstructured state. 

Thermodynamic measurements are possible only when both the initial state and the final state are essentially at equilibrium, i.e. internally and with respect to the surroundings. Consequently, for a spontaneous thermodynamic change to take place, some constraint—internal or external—must be changed or released. 

The experimental investigations prove the validity of the reciprocal relations for several types of irreversible processes moreover, as we show below the linear and generalized reciprocal relations are clear phenomenological consequence of the principle of minimal entropy production. Consequently, the generalized reciprocal relations are reasonably well-established thermodynamic law, however of course it has no such general validity as the basic laws of thermodynamics, (e.g. the energy conservation or the direction of the spontaneous thermodynamic processes). 

Amphiphilic molecules [11-14] consist of mutually incompatible components. Since these components are chemically joined, complete segregation is impossible. It is replaced by various forms of microphase separation. These involve formation of segregated domains such that at least one of their dimensions is comparable to the molecular size. The domains are formed by spontaneous, thermodynamically driven aggregation of the amphiphiles. The process is thus often referred to as self-assembly. The resulting structures, micelles, lamellae, etc. can also form ordered mesophases. The microphase separation can take place in a solvent that selectively solubilizes one component or in a melt of neat amphiphiles. These characteristics are common to both polymeric and monomeric, low molecular weight amphiphiles. For the purposes of our discussion monomeric amphiphiles are defined, somewhat arbitrarily, as those consisting of 10 atoms. Polymeric amphiphiles, on the other hand, can incorporate 10 -10 atoms. The consequences of this difference are the topic of this article. 

The absorbed light may act as calalv. i for a spontaneous reaction, but in other cases it may supply energy to make possible a reaction which, without light, would be thermodynamically impossible. In some cases, such a reaction reverses itselfby thermal reaction (e.g. if left in the dark) and, hence, during irradiation a phoiostationary state is reached. 

It is quite clear, first of all, that since emulsions present a large interfacial area, any reduction in interfacial tension must reduce the driving force toward coalescence and should promote stability. We have here, then, a simple thermodynamic basis for the role of emulsifying agents. Harkins [17] mentions, as an example, the case of the system paraffin oil-water. With pure liquids, the inter-facial tension was 41 dyn/cm, and this was reduced to 31 dyn/cm on making the aqueous phase 0.00 IM in oleic acid, under which conditions a reasonably stable emulsion could be formed. On neutralization by 0.001 M sodium hydroxide, the interfacial tension fell to 7.2 dyn/cm, and if also made O.OOIM in sodium chloride, it became less than 0.01 dyn/cm. With olive oil in place of the paraffin oil, the final interfacial tension was 0.002 dyn/cm. These last systems emulsified spontaneously—that is, on combining the oil and water phases, no agitation was needed for emulsification to occur. 

If there are no reactions, the conservation of the total quantity of each species dictates that the time dependence of is given by minus the divergence of the flux ps vs), where (vs) is the drift velocity of the species s. The latter is proportional to the average force acting locally on species s, which is the thermodynamic force, equal to minus the gradient of the thermodynamic potential. In the local coupling approximation the mobility appears as a proportionality constant M. For spontaneous processes near equilibrium it is important that a noise term T] t) is retained [146]. Thus dynamic equations of the form... 

We now turn specifically to the thermodynamics and kinetics of reactions (5. EE) and (5.FF). The criterion for spontaneity in thermodynamics is AG <0 with AG = AH - T AS for an isothermal process. Thus it is both the sign and magnitude of AH and AS and the magnitude of T that determine whether a reaction is thermodynamically favored or not. As usual in thermodynamics, the A s are taken as products minus reactants, so the conclusions apply to the reactions as written. If a reaction is reversed, products and reactants are interchanged and the sign of the AG is reversed also. 

Microemulsion Polymerization. Polyacrylamide microemulsions are low viscosity, non settling, clear, thermodynamically stable water-in-od emulsions with particle sizes less than about 100 nm (98—100). They were developed to try to overcome the inherent settling problems of the larger particle size, conventional inverse emulsion polyacrylamides. To achieve the smaller microemulsion particle size, increased surfactant levels are required, making this system more expensive than inverse emulsions. Acrylamide microemulsions form spontaneously when the correct combinations and types of oils, surfactants, and aqueous monomer solutions are combined. Consequendy, no homogenization is required. Polymerization of acrylamide microemulsions is conducted similarly to conventional acrylamide inverse emulsions. To date, polyacrylamide microemulsions have not been commercialized, although work has continued in an effort to exploit the unique features of this technology (100). 

The most important materials among nonlinear dielectrics are ferroelectrics which can exhibit a spontaneous polarization PI in the absence of an external electric field and which can spHt into spontaneously polarized regions known as domains (5). It is evident that in the ferroelectric the domain states differ in orientation of spontaneous electric polarization, which are in equiUbrium thermodynamically, and that the ferroelectric character is estabUshed when one domain state can be transformed to another by a suitably directed external electric field (6). It is the reorientabiUty of the domain state polarizations that distinguishes ferroelectrics as a subgroup of materials from the 10-polar-point symmetry group of pyroelectric crystals (7—9). 

Polymorphism. Many crystalline polyolefins, particularly polymers of a-olefins with linear alkyl groups, can exist in several polymorphic modifications. The type of polymorph depends on crystallisa tion conditions. Isotactic PB can exist in five crystal forms form I (twinned hexagonal), form II (tetragonal), form III (orthorhombic), form P (untwinned hexagonal), and form IP (37—39). The crystal stmctures and thermal parameters of the first three forms are given in Table 3. Form II is formed when a PB resin crystallises from the melt. Over time, it is spontaneously transformed into the thermodynamically stable form I at room temperature, the transition takes about one week to complete. Forms P, IP, and III of PB are rare they can be formed when the polymer crystallises from solution at low temperature or under pressure (38). Syndiotactic PB exists in two crystalline forms, I and II (35). Form I comes into shape during crystallisation from the melt (very slow process) and form II is produced by stretching form-1 crystalline specimens (35). 

The potential of the reaction is given as = (cathodic — anodic reaction) = 0.337 — (—0.440) = +0.777 V. The positive value of the standard cell potential indicates that the reaction is spontaneous as written (see Electrochemical processing). In other words, at thermodynamic equihbrium the concentration of copper ion in the solution is very small. The standard cell potentials are, of course, only guides to be used in practice, as rarely are conditions sufftciendy controlled to be called standard. Other factors may alter the driving force of the reaction, eg, cementation using aluminum metal is usually quite anomalous. Aluminum tends to form a relatively inert oxide coating that can reduce actual cell potential. 

Complete wetting, i.e. spontaneous spreading should always be sought to maximize adhesion. This condition occurs when, with reference to Fig. 4, it is not possible to satisfy the horizontal force balance, i.e. ys > Vl + Ysl- The thermodynamic driving force for the spreading process is the spreading coefficient. 

The properties of a system at equilibrium do not change with time, and time therefore is not a thermodynamic variable. An unconstrained system not in its equilibrium state spontaneously changes with time, so experimental and theoretical studies of these changes involve time as a variable. The presence of time as a factor in chemical kinetics adds both interest and difficulty to this branch of chemistry. 

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