Prediction of spontaneity

The value of the Ising model lies therein that it is the only model of disorder to have produced valid theoretical predictions of spontaneous phase changes. To understand the role of symmetry it is noted that spontaneous magnetization, starting from a random distribution of spins, amounts to a process of ordering that destroys an existing isotropic arrangement. 

It can be shown that the actual AS value for the reaction shown in Eq. (4.29) is only 0.066 kJ K-1, so we are correct in our assumption that AG is approximately the same as AH, and our prediction of spontaneity based on AH is valid in this case. 

Bourke, J.P, et al.. Routine programmed electrical stimulation in survivors of acute myocardial infarction for prediction of spontaneous ventricular tachyarrhythmias during follow-up results, optimal stimulation protocol and cost-effective screening. J Am CoU Cardiol, 1991. 18(3) p. 780-8. 

Summarizing the Prediction of Spontaneous Direction for Redox Reactions ... 

Clearly, the direction of a spontaneous change is not always determined by the tendency for a system to go to a state of lower energy. There is another natural tendency that must be taken into account to predict the direction of spontaneity. Nature tends to move spontaneously from a state of lower probability to one of higher probability. On as G. N. Lewis put it,... 

The negative voltage shows that the state of equilibrium favors the reactants more than the products for the reaction as written. For standard conditions, the reaction will not tend to occur spontaneously. However, if we place Ag(s) in 1 M H+, the Ag+ concentration is not 1 M— it is zero. By Le Chatelier s Principle, this increases the tendency to form products, in opposition to our prediction of no reaction. Some silver will dissolve, though only a minute amount because silver metal releases electrons so reluctantly compared with H2. It is such a small amount, in fact, that no silver chloride precipitate forms, even though silver chloride has a very low solubility. 

Chemical processes are central to the study of chemistry. The thermodynamic principles and relationships we have developed provide powerful tools for describing these processes, especially in predicting the spontaneity of the process and the equilibrium conditions that apply. 

We have seen that the value of an equilibrium constant tells us whether we can expect a high or low concentration of product at equilibrium. The constant also allows us to predict the spontaneous direction of reaction in a reaction mixture of any composition. In the following three sections, we see how to express the equilibrium constant in terms of molar concentrations of gases as well as partial pressures and how to predict the equilibrium composition of a reaction mixture, given the value of the equilibrium constant for the reaction. Such information is critical to the success of many industrial processes and is fundamental to the discussion of acids and bases in the following chapters. 

EXAMPLE 12.7 Predicting the spontaneous direction of a redox reaction under standard conditions... 

The question arises as to how useful atomistic models may be in predicting the phase behaviour of real liquid crystal molecules. There is some evidence that atomistic models may be quite promising in this respect. For instance, in constant pressure simulations of CCH5 [25, 26] stable nematic and isotropic phases are seen at the right temperatures, even though the simulations of up to 700 ps are too short to observe spontaneous formation of the nematic phase from the isotropic liquid. However, at the present time one must conclude that atomistic models can only be expected to provide qualitative data about individual systems rather than quantitative predictions of phase transition temperatures. Such predictions must await simulations on larger systems, where the system size dependency has been eliminated, and where constant... 

All such animal procedures suffer from the obvious and basic problem that laboratory animals do not behave like humans and that humans cannot reliably interpret their reactions and behaviour. Thus we know that Parkinson s disease is caused by a degeneration of the dopaminergic nigrostriatal tract but its lesion in animals does not produce any condition which resembles human Parkinsonism, except in primates, even though there are functional tests (e.g. rotational movements) which readily establish that loss of dopamine function and also respond to its augmentation (Chapter 15). By contrast, there are many ways, e.g. electrical stimulation and the administration of certain chemicals, to induce convulsions in animals and a number of effective antiepileptic drugs have been introduced as a result of their ability to control such activity. Indeed there are some tests, as well as animals with varied spontaneous seizures, that are even predictive of particular forms of epilepsy. But then convulsions are a very basic form of activity common to most species and epileptic seizures that are characterised by behavioural rather than motor symptoms are more difficult to reproduce in animals. 

Genetically predisposed animals or induced animal models may also be used to study and predict chemical-induced autoimmunity. In induced models, a susceptible animal strain is immunized with a mixture of an adjuvant and an autoantigen isolated from the target organ. Examples are adjuvant arthritis (AA), experimental allergic encephalomyelitis (EAE) and experimental uveitis in the Lewis strain rat. Examples of spontaneous models... 

One of the goals of chemists is to be able to predict if a reaction will be spontaneous. A reaction may be spontaneous if its AH is negative or if its AS is positive, but neither one is a reliable predictor by itself about whether or not a reaction will be spontaneous. Temperature also plays a part. A thermodynamic factor that takes into account the entropy, enthalpy, and temperature of the reaction would be the best indicator of spontaneity. This factor is the Gibbs free energy. 

The alkyllithium-initiated, anionic polymerization of vinyl and diene monomers can often be performed without the incursion of spontaneous termination or chain transfer reactions (1). The non-terminating nature of these reactions has provided methods for the synthesis of polymers with predictable molecular weights and narrow molecular weight distributions (2). In addition, these polymerizations generate polymer chains with stable, carbanionic chain ends which, in principle, can be converted into a diverse array of functional end groups using the rich and varied chemistry of organolithium compounds (3). 

By Equation (5.17), we see that the probability of spontaneous emission is proportional to I/LXIso we can use the same selection rules as previously established for absorption and stimulated emission to predict the allowance of spontaneous emission. Moreover, it can be noted that A is proportional to ci>l and so, for a small energy separation between the two levels of our system, the radiative emission rate A should also be small. In this case, noimadiative processes, described by A r (see Equation (1.17)), can be dominant so that no emitted light is observed. 

Tantisira KG, Hwang ES, Raby BA, et al TBX21 a functional variant predicts improvement in asthma with the use of inhaled corticosteroids. Proc Natl Acad Sci USA 2004 28 101 18099-18104. Finotto S, Neurath MF, Glickman JN, et al Development of spontaneous airway changes consistent with human asthma in mice lacking T-bet. Science 2002 295 336-338. 

For a scientist, the primary interest in thermodynamics is in predicting the spontaneous direction of natural processes, chemical or physical, in which by spontaneous we mean those changes that occur irreversibly in the absence of restraining forces—for example, the free expansion of a gas or the vaporization of a hquid above its boiling point. The first law of thermodynamics, which is useful in keeping account of heat and energy balances, makes no distinction between reversible and irreversible processes and makes no statement about the natural direction of a chemical or physical transformation. 

Ionization of 1,5-hexadiene in fluorochloroalkane matrix (Scheme 2.43) represents cation-radical monomolecular reactions. The initially formed cation-radical collapses to the cyclohexane cation-radical, that is, spontaneous cyclization takes place (Williams 1994). Zhu et al. (1998) pointed out that the ring formation from the excited valence isomer in the center of Scheme 2.43 is easier than in the corresponding ground-state dienes. Notably, tandem mass spectrometry revealed the same transformation of 1,5-hexadiene in the gas phase too. This provides ns with a hint that mass spectrometry can serve as a method to express predictions of monomolecnlar transformation of cation-radicals in the condensed phase. A review by Lobodin and Lebedev (2005) discnsses this possibility in more detail. 

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