Ostwald’s law

The question as to this is answered by Ostwald s law of dissociation for the weak acids and bases, whose electrolytic dissociation is slight for ordinary concentrations, and which may therefore be spoken of as half-electrolytes. Assuming that the electrolytic conductivity is effected by the free ions present in the solution only, then the conductivity calculated for a fixed mass of dissolved substance is a measure of the fraction of it dissociated into ions, and that is the case too with the so-called molecular conductivity referred to the normal solution. The latter increases with the dilution, on account of increasing dissociation, and reaches the limiting value which corresponds to complete dissociation, so that the fraction —... 

Suppose a and h gram-molecules respectively of two weak acids Z H and Z2H, which therefore obey Ostwald s law of dissociation (first simplification) and to them added c gram-molecules of a strong baseMOH, insufficient to saturate both acids. Let the volume of the entire solution be V htres, and as second simplification let it be assumed that the salts formed are completely dissociated. Without regard to dissociation we have... 

The equation in this form is known as Ostwald s law of dilution. 

There are situations where kinetics determines the course of crystallization, and thermodynamics becomes of secondary consideration. For example, Ostwald s Law of Stages states that, when leaving an unstable state, a system does not seek out the most stable state, rather the nearest metastable state which can be reached with loss of free energy. This form then... 

However, Ostwald s law of stages is not universally valid because the appearance and evolution of solid phases are determined by the kinetics of nucleation and growth under the specific experimental conditions (Bernstein et ak, 1999 Davey et ak, 1997 Threlfall, 2000) and by the link between molecular assemblies and crystal structure (Davey et ak, 2002 Blagden et ak, 1998 Gu et ak, 2001). 

Although a number of at least apparent exceptions to Ostwald s law have been found, it may nevertheless be accepted as a very useful generalisation which sums up very frequently observed phenomena. Of these factors, the most important, according to Tammann, is the readiness with which spontaneous formation of nuclei of the metastable and stable phases takes place in the supercooled system. 

Polymorph discovery by solvent recrystallization is based on the principle that the less stable forms recrystallize before the more stable ones. This is often referred to as Ostwald s law of stages [13]. As the system progresses from the liquid to the solid state, it will go to the state nearest in free energy to that of the liquid state. Frequently a metastable crystal form will appear and then convert to... 

In some cases crystallisation sequences are observed where a first formed phase transforms after extended reaction times to other phases. This kind of behaviour is in accord with Ostwald s law, which states that under kinetically controlled conditions, the first phases to form will be those with higher entropy, and these may transform towards the thermodynamically most stable phase via phases with progressively lower entropy and lower free energy. The sequential synthesis of zeolite Na-Y and then the denser zeolite Na-P from the same... 

Ostwald s law of successive transformation is very important in the consideration of zeolite s)mthesis. According to the law, the first phase formed during a reaction is thermodynamically the least stable, and is replaced by ever more stable phases until the most stable product is formed. However, in some instances, thermodynamically unfavourable products may persist if there is a significant activation energy barrier to the more stable phase. All zeolite structures are metastable with respect to more dense phases. 

Isolating the less thermodynamically stable of two interconvertible forms of the same composition (whether it be coordination compounds, allotropes of elements, or any other chemical composition) can be done only if the rate of reaching equilibrium is so slow as to render the conversion of the metastable isomer to the stable one very protracted. This is a form of Ostwald s law of metastable intermediates. Such rates are slow (minutes < ti/2 < years) for the equilibrations of coordinated cobalt(III), chromium(III), a few (spin-paired) d ferrous compounds, such as salts of the ferroin 22a, tris-l,10-phenanthrolineiron(II) cation, and a few octahedral nickel(II) species with strong field ligands (3d, e.g., the tris-l,10-phenanthrolinenickel(II) ion 22b). 

Figure 2.29 Visualization of Ostwald s law of or oil. As unstable solid-state forms are stages. If a multitude of solid-state forms is prone to a (solution-mediated) phase possible, the most unstable form will transformation, the unstable form might...

If a system can crystallize in more than one crystalline form and if the substance can form polymorphs (cf Chapter 5), the instable modifications will develop first -Ostwald s law of stages does say that the most unstable modification will nucleate first (cf. Figure 2.29). 

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