Ostwalds Rules

Ostwald proposed that when two or more new phases may form from existing phase or phases, that is, when new phases are more stable than the existing phase(s), the least stable new phase would form first and then transform into more stable phases. This is called the Ostwald rule, the Ostwald step rule, or the law of successive reactions. An alternative statement of the Ostwald rule is as follows ... 

One example of the least activation energy is when the structure of the new phase is closest to that of the existing phase(s) or when the structure is disordered so that it does not require elaborate and precise rearrangement. The Ostwald rule is especially applicable to low-temperature phase transformations because at these low temperatures it is difficult to overcome the high activation energies required to form a new phase. At high temperatures such as igneous temperatures, the Ostwald rule is less often encountered. 

Order arising through nucleation occurs both in equilibrium and nonequilibrium systems. In such a process the order that appears is not always the most stable one there are often competing processes that will lead to different structures, and the structure that appears is the one that nucleates first. For instance, in the analysis of the different possible structures in diffusion-reaction systems17-20 one can show, by analyzing the bifurcation equations, that there are several possible structures and some of them require a finite amplitude to become stable if this finite amplitude is realized through fluctuation, this structure will appear. In the formation of crystals (hydrates) the situation is similar the structure that is formed depends, according to the Ostwald rule, on the kinetics of nucleation and not on the relative stability. 

There are three general principles that accompany evolution of crystallizing PMs a) the Ostwald rule of step transitions,7 according to which systems that... 

In 1897, Friedrich Wilhelm Ostwald (1853-1932) published his now famous study of crystallization processes, which led to the Ostwald rule of stages or Ostwald step rule (Ostwald, 1897). Ostwald noticed that the course of transformation of unstable (or metastable) states into stable states normally occurs in stages,... 

This phenomenon of the increased growth of larger crystals compared to smaller ones that have a higher solubihty than the larger ones is known as the second Ostwald rule or the rule of ripening of a new phase. 

Ostwald, recalculated so as to give the equivalent conductances in ohm cm. units, instead of reciprocal Siemens units they show that the equation given above is approximately true, and hence it may be employed to determine the basicity of an acid. The method fails when applied to very weak acids whose salts are considerably hydrolyzed in solution. The results quoted in Table XX are perhaps exceptionally favorable, for the agreement with equation (27) is not always as good as these figures would imply. The Ostwald rule is, nevertheless, an expression of the facts already discussed, viz., that substances of the same valence type have approximately the same conductance ratios at equivalent concentrations and that the values diminish with increasing valence of one or both ions. The rule has been extended by Bredig (1894) to include electrolytes of various types. 

By way of example we mention an old proposal, sometimes called Stefan s rule (but which more appropriately should be called the Stefan-Ostwald rule h that in our nomenclature would read... 

These conditions are fulfilled in many cases. The sequential formation of phases having different stabilities has in principle been known for a long time, in particular in the case of precipitation processes, and is described phenomenologically as the Ostwald rule. Typical timescales on which these solid-state reactions occur are in the range from 104 to 106 s, being consequently slower by many orders of magnitude than rapid quenching from the liquid (10-6-10-3 s) or the vapor phase (10-14-10 10 s). 

When crystallizing from multicomponent systems, kinetic factors often override thermodynamic considerations (the so-called Ostwald rule of stages -section 5.7). The phase which crystallizes is not necessarily the one which is thermodynamically most stable, but the one which crystallizes the fastest. Numerous examples of this sort of behaviour are available. 

Nyvlt, J. (1995) The Ostwald Rule of Stages. Crystal Research and Technology, 30, 445 51. 

Zeolites are thermodynamically metastable phases that can be transformed at longer synthesis times into more stable (and more dense) structures [93]. This phenomenon is known as the Ostwald rule of successive phase transformations. 

Therefore, it is concluded that the first nucleating form (when the cluster size is small), according to Ostwald rule, would be the metastable a-form which has become surface stabilised due to its small size hence making it more stable than the stable p-form. hi this respect stable and metastable as used here refer to the bulk crystallographic structures. 

Feenstra TP, De Bmyn PL (1981) The Ostwald rule of stages in precipitation from highly supersaturated solutions a model and its application to the formation of the nonstoichiometric amorphous calcium phosphate precursor phase. J Colloid Interface Sci... 

Nyvlt J (1995) The Ostwald rule of stages. Cryst Res Technol 30 443 149 Helmdach L, Feth MP, Ulrich J (2012) Online analytical investigations on solvent-temperature and water vapour-induced phase transformations of citric acid. Cryst Res Technol 47 967-984... 

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