Solid-state reactions physical transformations

In both 4.1.1. and Table 4-1, S, L G refer to solid, liquid and gas, respectively. Note that we have also classified these heterogeneous mechanisms in terms of the same PHYSICAL CHANGES given above for homogeneous transformations. For the most part, the initial material will be a solid while the nature of the final product will vary according to the type of material undergoing solid state reaction. 

These authors observed that binaphthyl crystallize in two polymorphs. The one is stable at lower temperature, is centrosymmetric and is not optically active. This polymorph melts at 145 °C. The second polymorph is stable at higher temperature but is metastable at room temperature. It is optically active and melts at 158 °C. Wilson and Pincock show that as one cycles in temperature between room temperature and 150 °C a sample which is initially the optically inactive low temperature polymoiph transforms to an optically active solid. After three or four cycles one achieves the maximum optical resolution which corresponds to 56% ee. The crux of the Wilson and Pincock experiment is that at 150 °C, the reaction physically resembles a solid state reaction in which the low temperature form is melting in the near presence of high temperature polymorph crystals. These chiral crystals are therefore nucleating sites for further chiral crystal growth. As at room temperature binaphthyl retains its chirality, the resultant samples can then be dissolved with retention of stereochemistry. 

Physical transformations (usually solid-state crystallization) are more often directly linked to molecular mobility and orientation than the most common chemical reactions (oxidation and hydrolysis) thus the major stability concern for amorphous materials is with their tendency to revert to the crystalline state. As with all crystallization processes, there are the normal nucleation and propagation (crystal growth) stages to consider, and procedures that increase the barrier to nucleation or slow the rate of crystal growth can be used to physically stabilize many amorphous materials. 

Recently, we have Introduced a new concept of phonon-assisted reaction (2). In other words, a reaction In the solid state can be assisted by these low frequency cooperative molecular motions. The concept of the phonon-assisted chemical transformation bears a direct analogy to that of a phonon-assisted physical transformation In solid. Important phonon Interactions can be divided Into two categories (1) Anharmonic phonon-phonon Interactions and (11) electron-phonon Interactions. 

Mineralogical phases formed at different temperatures for each coal sample are summarized in Table III. The major mineral phases detected by XRD in LTA samples are quartz, pyrite, bassanite, kaolinite and plagioclase. The processes responsible for subsequent mineral transformations include oxidation, vaporization, sulfur fixation, dehydration and solid-state Interactions. The temperatures at which specific transformations occur are assigned on the basis of previous experimental work by Mitchell and Gluskoter (4) and published chemical data in the Handbook of Chemistry and Physics ( ). In addition to mineral-mineral interactions it is believed that reactions between minerals and exchangeable cations occur (2) ... 

The introduction of in-situ infrared spectroscopy to electrochemistry has revolutionised the study of metal/electrolyte interfaces. Modnlation or sampling techniques are applied in order to enhance sensitivity and to separate snrface species from volume species. Methods such as EMIRS (electrochemicaUy modulated IR spectroscopy) and SNIFTIRS (subtractively normalised interfacial Fonrier Transform infrared spectroscopy) have been employed to study electrocatalytic electrodes, for example. There have been surprisingly few studies of the semiconductor/electrolyte interface by infrared spectroscopy. This because up to now little emphasis has been placed on the molecnlar electrochemistry of electrode reactions at semiconductors because the description of charge transfer at semiconductor/electrolyte interfaces is derived from solid-state physics. However, the evident need to identify the chemical identity of snrface species should lead to an increase in the application of in-situ FTIR. 

Inert Gas Release from Solids. The inert gas incorporated into a solid can be evolved as a result of chemical reaction, physical transformation or of damage to the crystalline state, inert gas diffusion, or the recoil of inert gas atoms (190, 191). The gas release is dependent on the technique used for labeling, that is, if the inert gas itself or its parent nuclide was introduced into the solid. If the inert gas is a result of the former, the gas atom may escape from the solid in one of the following pathways, as shown in Figure 8.51 (190). When the parent atom lies close to the surface of the solid, the recoil energy (of the order of 100 keV) which the inert atom gains during the decay of the parent may be sufficient to eject it from the solid, or it may escape by diffusion before it decays. 

Arrhenius Law. A logarithmic relationship between reaction rate and temperature which applies to creep, diffusion and solid state phase transformations (among many other chemical and physical reactions), log V = A - q/kT... 

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