Kinetics of the thermal decomposition of alkaline earth metal hydrofluorides

et al. 1974

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Section: Enthalpies Of Nafmentioning

confidence: 99%

Thermal properties of hydrofluorides

Николаев

et al. 1974

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“…We used the method of Horowitz and Metzger [16] for calculation, the applicability of which has been shown by us on alkaline earth d. 7~ermal Anal. 2, 1970 metal hydrofluorides [17,18]. The errors in the activation energy values have been calculated as the deviation of individual values from the arithmetical mean.…”

mentioning

confidence: 99%

Thermal stability of alkali metal bifluorides

Fedotova

1970

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The thermal stability and non-isothermal kinetics of the decomposition of alkali metal bifluorides were studied using a derivatograph. The removal of hydrogen fluoride from LiF. HF and NaF 9 HF takes place before melting and their decomposition occurs in a single stage; however, potassium, rubidium and cesium bifluorides at first undergo polymorphous transformation and melting on heating, and their decomposition proceeds stepwise. The thermal stability of alkali metal bifluorides has been found to increase with increasing ionic radius of the cation, reflecting its correlation with the hydrogen bond strength in these compounds.Of the presently known hydrofluorides the alkali metal bifluorides MF 9 HF (MHF~) are the most stable thermally. The thermal stability of these compounds is important from the point of view of their practical application. However, the literature data on this point are sometimes contradictory. The variety of methods used by the authors of the works mentioned above makes the comparison of the results difficult as they often differ greatly. The insufficient purity of the studied substances, especially in the early works, also gave rise to inaccuracies. The kinetics of the decomposition of bifluorides has not been studied at all. In the present work the thermal stability and non-isothermal kinetics of the decomposition of alkali metal bifluorides were studied by thermogravimetry.Lithium bifluoride was obtained by crystallization from a saturated solution of lithium fluoride (analytical grade) in hydrofuoric acid of special purity [3]. The crystals obtained were washed several times with anhydrous hydrogen fluoride to remove water, and then excess HF was exhausted in a vacuum desiccator 1" 3., Thermal AnaL 2, 1970

Reaction of alkali and ammonium bifluorides with aluminium and silicon

Fedotova

1973

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The reactions of alkali and ammonium bifluorides with aluminium and silicon were studied by DTA. Reaction temperatures were determined and reaction products identified. The reaction mechanism of the fluorinating reaction was established. Hydrofluorides appear very promising fluorinating agents.The fluorinating actions of elemental fluorine, halogen fluorides and hydrogen fluoride are well known and these compounds are widely applied for synthetizing inorganic fluorides [1 ]. Lately, however, interest in solid fluorinating agents has continually grown, since they possess many advantages. By way of example, silver difluoride [2], cobalt and manganese trifluorides [3][4][5], xenon fluoride [6] and alkali fluorohalogenates [7] can be mentioned.Much attention has been paid to ammonium bifluoride as a potential fluorinating agent, because the melting and evaporation temperatures of this compound are relatively low. The reactivity of NH4F " HF is close to that of anhydrous hydrofluoric acid and the compound can be successfully applied as a fluorinating agent in many reactions [1,[8][9][10]. It is obvious that other bifluorides are also suited for such purposes.In the present paper we wish to report studies on the reaction of alkali and ammonium bifluoiides with aluminium and silicon. The method used was differential thermal analysis and the reaction products were identified by X-ray phase analysis and thermal analysis. In reactions of metals with bifluorides, the thermal properties of the latter are very important. The thermal stabilities and decomposition kinetics of hydrofluorides were earlier studied by thermogravimetry [I 1, 12]. It was found that lithium and sodium bifluorides are decomposed before melting, whereas potassium, rubidium and caesium bifluorides first undergo polymorphous transformation on heating, and subsequently melt without perceptible decomposition. On further heating, hydrogen fluoride is evolved from the melt [12]. Ammonium bifluoride too melts without perceptible decomposition, and on further heating decomposes, hydrogen fluoride being split off. ExperimentalAlkali and ammonium bifluorides were synthetized according to data on the HF-MF-H20 systems, and identified by methods of chemical, X-ray phase , L Thermal Anal. 5, 1973