Stability rule of reversed

More general is the rule of reversed stability (Miedema model) the more stable an intermetallic compound, the less stable is the corresponding hydride, and the other tvay around [36]. This model is based on the fact that hydrogen can only participate on a bond with a neighboring metal atom if the bonds between the metal atoms are at least partially broken (Figures 5.25 and 5.26). 

The rule of reversed stability is illustrated by Equation 15. When the stability of the intermetallic compound ABn is great, the stability of the ternary hydride is low therefore, the hydrogen dissociation pressure of the hydride is high (8). The rule of reversed stability aids in calculating approximate values... 

Considerations of enthalpy, as illustrated by the rule of reversed stability, and of configurational entropy provided insight into the factors governing the stabilities of the AB5 hydrides. Theoretical understanding to predict dissociation pressures should be developed on heat pump application, for example. Although... 

Relation (4.19) is usually called Miedema s rule of reversed stability and states that the heat of formation of a ternary hydride is the difference between the sum of the heat of formation of the elemental hydride and the alloy enthalpy of formation. Because atom A is hydride forming, the first term of the right-hand side is negative and has a large absolute value while the second term is small (or even positive) and... 

A scheme for correlating hydride stabilities, the so called rule of reversed stability see e.g., 18,19), states that for a series of analogous alloys, the more stable the alloy, the less stable (i.e., higher dissociation pressure) the corresponding hydride. Using Miedema s formula (20), the calculated heat of formation for LaNis is — II.2 kj/mol and for LaAls is — 42.1 kj/mol. Since LaAls is more stable (more negative AH) than LaNis, the rule of reversed stability predicts the LaNis- rAh hydrides to be less stable than LaNisHe contrary to observation. Similarly, Shinar et... 

One may rationalize emulsion type in terms of interfacial tensions. Bancroft [20] and later Clowes [21] proposed that the interfacial film of emulsion-stabilizing surfactant be regarded as duplex in nature, so that an inner and an outer interfacial tension could be discussed. On this basis, the type of emulsion formed (W/O vs. O/W) should be such that the inner surface is the one of higher surface tension. Thus sodium and other alkali metal soaps tend to stabilize O/W emulsions, and the explanation would be that, being more water- than oil-soluble, the film-water interfacial tension should be lower than the film-oil one. Conversely, with the relatively more oil-soluble metal soaps, the reverse should be true, and they should stabilize W/O emulsions, as in fact they do. An alternative statement, known as Bancroft s rule, is that the external phase will be that in which the emulsifying agent is the more soluble [20]. A related approach is discussed in Section XIV-5. 

Based on the reversibility of their phase transformation behavior, polymorphs can easily be classified as being either enantiotropic (interchange reversibly with temperature) or monotropic (irreversible phase transformation). Enantiotropic polymorphs are each characterized by phase stability over well-defined temperature ranges. In the monotropic system, one polymorph will be stable at all temperatures, and the other is only metastable. Ostwald formulated the rule of successive reactions, which states that the phase that will crystallize out of a melt will be the state that can be reached with the minimum loss of free... 

For the corrosion scientist it will be easy to remember that any metal for which E° is negative is liable to corrode in acid, while those having a positive value of E° will not. This rule of thumb should not be taken as being exact, since in situations of practical interest the system is rarely, if ever, under standard conditions. Pipelines rarely carry 1.0 M acid, and metal structures are not, as a rule, in contact with a one-molar solution of their ions. For any specific system of known composition and pH, the reversible potential can readily be calculated from the Nemst equation, and the thermodynamic stability with respect to corrosion can be determined. 

It is not uncommon in crystallization processes for the first crystalline phase to make its appearance to be metastable, e.g. a polymorph or hydrate (Ostwald s rule of stages - section 5.7). Some metastable phases rapidly transform to a more stable phase while others can exhibit apparent stability for an exceptionally long time. Some transformations are reversible (enantiotropic) while others are irreversible (monotropic), as explained in sections 1.8 and 4.2.1. In some cases, the metastable phase may have more desirable properties than the stable phase, e.g., a metastable pharmaceutical product may be more pharmacologically active than the stable form. If the required metastable form is first to crystallize, it is important to isolate and dry it quickly to prevent it transforming to the stable form. Once in the dry condition a metastable form can often remain unchanged indefinitely. If the stable polymorph is required, it is essential to create conditions and allow sufficient time in the crystallizer for total transformation to the more stable phase to be ensured. 

The useful life of a polymer has to be defined for conservation. Feller developed a rule of thumb for classifying materials by their photochemical stability, a useful tool that should inform aU stages in the conservation process, from planning to use (Table 2.4). In most fields, it is expected that a conservation treatment will have to be reversed in the future. For instance, picture varnishes have traditionally been replaced every 80-120 years and stained glass window installations every 2-300 years. It is likely that a material used for temporary fixing may never be entirely removed (Section 1.3). It is therefore necessary that even these have long-term stabihty. 

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