Chrome alum, dehydration

These examples demonstrate conclusively that the availability of volatile product in the immediate vicinity of the site of a reversible dissociation can markedly influence the apparent kinetic characteristics of crystolysis reactions. Some systems are highly sensitive to such effects but others, such as chrome alum dehydration, are markedly less affected. The kinetics of CaC03 dissociation vary considerably with reaction conditions (83), and here the pattern of reaction rates is also influenced by heat flow during the endothermic, reversible reaction. It follows that it cannot be assumed, without examination of the influence of the procedural variables, that measured kinetic parameters are determined by a slow rate-limiting step. 

The reaction is cataly2ed by all but the weakest acids. In the dehydration of ethanol over heterogeneous catalysts, such as alumina (342—346), ether is the main product below 260°C at higher temperatures both ether and ethylene are produced. Other catalysts used include siUca—alumina (347,348), copper sulfate, tin chloride, manganous chloride, aluminum chloride, chrome alum, and chromium sulfate (349,350). 

Coalesence of neighbouring nuclei decreases the acceleratory contribution, leading to the deceleratory behaviour characteristic of the later stages of reaction. Hemispherical nuclei, indicative of equal rates of growth in all directions, are found, for example, in the decomposition of barium azide [17] and in the dehydration of chrome alum [10,19,20]. Other reactions develop nuclei of different shapes characteristic of preferred advance in certain ciysteillographic directions. 

The dehydration of chrome alum in vacuum [58] has an induction period, followed by nucleation at a constant rate. Surface nuclei were circular, but because the rate of bulk penetration was less than that of surface advance, the growth nuclei were flattened hemispheres. The rate of initial growth of each nucleus was exponential until a diameter of about 0.1 mm was attained and remained constant thereafter. Two fypes of nuclei were recognized [40], but in only one of these had the product undergone reorganization or recrystalhzation with the appearance of surface cracking. 

Kinetic studies [37,40] of the dehydration of chrome alum (260 to 270 K) yielded Arrhenius parameters of greater magnitude than predicted by the Polanyi-Wigner equation ( , = 125 kJ mol, whereas the enthalpy of dehydration is 42 kJ (mol HjO) ). This was ascribed to systematic changes in the thickness of the transition layer and thus on impedance of flie escape of water vapour. At higher temperatures (288 to 308 K) a lower value of was found (96 kJ mol" ) and the Arrhenius... 

Dehydration of Chrome Alum In contrast with the previous example, it appears that the dehydration of KCr(SC>4)2 12H20 is insensitive to the presence of water vapor, though experiments did not extend to the lowest pressures (80-82). A possible explanation is that dehydration is promoted by retained intranuclear water. The rate of interface advance at 290 K increased by about one quarter at 100 Pa water vapor compared with that in vacuum. 

Alternate methods From violet chromium (III) chloride or dehydrated chrome alum, with ethylenedlamine hydrate or ethyl-enedlamlne, respectively. 

Anhydrous or dehydrated ammonium chrome-alum was obtained by M. Traube... 

A. Recoura obtained potassium chromitetrasulphate, K2[Cr2(S04)4], by evaporating on the water-bath a mol of green chromic sulphate with a mol of potassium sulphate and the tetrahydrate, K2[Cr2(S04)4].4H20, by dehydrating chrome-alum slowly at 110°. The former compound is dark green, and it is soluble in water. 

Molecules that coordinate strongly can often displace others from the coordination sphere. Ethylenediamine does not readily replace water from the hydrated chro-mium(III) ion, but it does readily displace pyridine. Pfeiffer has described the preparation of tris (ethylenediamine) chromium(III) chloride from trichlorotripyridine-chromium and the corresponding sulfate from dehydrated chrome alum. ... 

Fig. 12. Rate of growth of dehydration patches on copper sulphate, ammonium, potassium, and chrome alums as a func< tion of water vapor pressure.

Acock et al. found that the loss of water from chrome alum when exposed to hard vacuum apparently occurs in two stages the first with a loss of 12 water molecules and the second with a loss of 4. Whether a phase change occurs in each stage is not known it is possible that the first stage is an equilibration of vacancies. The dehydration of ammonium and potassium alums may be a little more straightforward the loss of water from these salts increased from 19.8 to 20.6 of their original 24 water molecules as the temperatures was increased from 20 to 40°C. 

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