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  • the thermal decomposition of dehydrated d lithium Potassium Tartrate monohydrate molecular modification by a homogeneous melt mechanism
    Proceedings of The Royal Society A: Mathematical Physical and Engineering Sciences, 1993
    Co-Authors: Andrew K Galwey, Genevieve M Laverty
    Abstract:

    Lithium Potassium Tartrate decomposes between 485–540 K: n LiKC 4 H 4 O 6 → n LiKCO 3 + n H 2 O + n CO 2 + (C 2 H 2 ) n . Isothermal fractional reaction ( α )-time plots are sigmoid shaped and the kinetic data for single crystal reactants obey the Avrami–Erofe’ev equation { –ln (1— α )} 1/2 = kt , 0.04 α 2 and H 2 O were identical and the activation energy for salt decomposition was relatively large, 220±20 kJ mol -1 . The study reported here was predominantly concerned with the d form of the Tartrate anion but observations included the decompositions of some related reactants including LiK salts of dl and meso tartaric acids. The reaction mechanism proposed is anion decomposition within an advancing thin layer of molten material that is formally similar (in some respects) to the reaction interface developed during decompositions of solids. Reactant melts or dissolves at one side of the active liquid zone and residual products accumulate at the outer side. Anion breakdown then occurs relatively easily in the molten region after removal from the stabilizing influence of the crystal cohesive forces. Kinetic characteristics are similar to those often found for the reactions of solids except for crushed salt samples where an increase in rate after ca . 50% reaction is ascribed to the onset of more extensive melting. This pattern of kinetic behaviour is so closely similar to that for many reactions of solids that we suggest it to be appropriate to consider the possibility of local or temporary melting in formulating detailed reaction mechanisms for all such rate processes.

  • the kinetics and mechanism of water evolution from molten dl lithium Potassium Tartrate monohydrate
    Philosophical Transactions of the Royal Society A, 1992
    Co-Authors: Sreelekha D Bhattamisra, Genevieve M Laverty, Nikolai A Baranov, Vladimir B Okhotnikov, Andrew K Galwey
    Abstract:

    A kinetic and microscopic investigation of the thermal dehydration of dl lithium Potassium Tartrate monohydrate is reported and the reaction mechanism discussed. This work forms part of a more comprehensive study concerned with the influence of reactant structure on the reactivity and the mechanism of chemical change. The other hydrated reactants with which this salt will be compared contain the d and meso forms of the Tartrate anion and crystallize with different structures. dl lithium Potassium Tartrate monohydrate lost the single molecule of water of crystallization in one predominantly deceleratory process that was studied between 350-460 K. Reaction was accompanied by melting to yield a residual glassy anhydrous product that was amorphous to X-ray diffraction. An initial, relatively rapid release of water (6%) was followed by a deceleratory process that led to a zero-order reaction (that, in crystals, extended between 18% and 80% ) before completion by an approximately first-order stage. Dehydrations of crushed powder reactant samples differed from single crystals in being relatively more rapid (an eight-fold increase); the deceleratory process was long and the zero-order process shorter (50-85%). The activation energy for dehydrations of crystal and of powder was 330 + 30 kJ mol -1 . This pattern of kinetic behaviour was not in accordance with expectation for a homogeneous reaction, the rate was not directly related to reactant concentration terms. Alternative analyses of the obedience of data to rate expressions applicable to solid state reactions were equally unsuccessful. Our mechanistic interpretation of the rate data, therefore, considered a priori the factors expected to participate in the control of water evolution from the melt. It is concluded that the vitreous or molten phase is not homogeneous and, therefore, behaviour is different from reactions in an isotropic fluid or in a solid. Two models are proposed to explain our observations. In the two phase equilibrium mechanism it is assumed that the reactant particles are composed of two phases, zones of hydrate are embedded in dehydrated material that retains a constant but small proportion of water. (These phases participate in an equilibrium analogous to that of liquid/vapour.) The surface boundary layer model envisages the initial development of a peripheral barrier zone through which the constant rate of water diffusion is rate controlling. This class of reaction, proceeding in a fluid but the absence of added solvent, has received relatively little attention. The present discussion is intended to identify the characteristic behaviour and draw attention to the necessity to consider such mechanisms in discussions of reactions of solids where there is the possibility of melt participation.

Ali Zaferanloo - One of the best experts on this subject based on the ideXlab platform.