A specimen sample of coral skeleton (Pavona, Okinawa, Japan) was selected to study the mechanistic relationship between aragonite−calcite transformation and the thermal dehydration of the included water in biogenic aragonite. The sample exhibited three mass loss steps attributed to the thermal dehydration of the included water (2.8% of the sample mass) before the thermal decomposition of CaCO 3 . The aragonite− calcite transformation simultaneously occurred during the second dehydration step of the included water in a temperature region lower by approximately 100 K than in synthetic and geological aragonites. During the thermal dehydration of the included water and the aragonite−calcite transformation, the root structure of polygonal plate stacking in the biogenic aragonite was preserved. A possible mechanistic relationship between the thermal dehydration of the included water and the aragonite−calcite transformation is explained by the three successive dehydration steps: (a) the diffusional removal of the included water from the interstitial spaces between the polygonal aragonite plates, (b) the dehydration step accompanying the aragonite−calcite transformation in the polygonal aragonite plate, and (c) the diffusional removal of residual water accompanying the growth of as-produced calcite crystals.
Thermal oxidation of carbon/carbon composites in an oxidizing atmosphere is a multistep process regulated by the intrinsic heterogeneity of the solid-gas reaction, the additional heterogeneity of the compositional and structural characteristics of the composite, and how these two properties change as the reaction progresses. By focusing on the overlapping features of the component reaction steps, the kinetic characterization of the multistep kinetic process was studied to reveal the correlation between the thermal oxidation behavior and the compositional and structural characteristics of carbon/carbon composites. Using commercially available mechanical pencil leads as a typical model system for a carbon/carbon composite, the thermal behaviors of two different leads manufactured by different companies were investigated comparatively via thermoanalytical techniques and morphological observations. On the basis of a reaction model considering the different reactivities of the main (graphite) and secondary (carbonized polymer) carbon components, the kinetic features of two partially overlapping reaction steps were revealed via a kinetic deconvolution analysis of the thermoanalytical data for the thermal oxidation process. The kinetic results were correlated with the compositional and structural characteristics of carbon/carbon composites using morphological observations of the partially reacted samples. Herein, the practical usefulness of the kinetic analysis in characterizing carbon/carbon composites is discussed.
This study focuses on the design of a learning program to introduce complexometric titration as a method for determining water hardness in a high school chemistry laboratory. Students are introduced to the different properties and reactions of hard water in a stepwise manner so that they gain the necessary chemical knowledge and conceptual understanding of the basic principles of complexometric titration. This approach involves investigating the performance of soap and household laundry detergent in hard water and using a colorimetric method to semiquantitatively determine the concentration of calcium ions in hard water by a test kit. The stepwise inquiry and learning are promoted using coordinated experimental work, logical thinking, and discussion with the aid of demonstrations and explanations. As each inquiry and learning step is completed, students develop models that describe the observed chemical properties and reactions of hard water. Using the simple models that they develop, students finally propose the basic principles of complexometric titration for determining water hardness. Based on their experimental principles, practical titration experiments are performed and the experimental data are analyzed to determine water hardness. Throughout the learning program, students actively apply preliminary knowledge and acquire new chemical knowledge and conceptual understanding from the laboratory exercises. Therefore, the students experience the process of scientific inquiry accompanied by the development of their understanding of chemical concepts. This paper reports that the developed learning program may be introduced as a suitable laboratory learning exercise in high school chemistry courses.
This article demonstrates a kinetic approach to partially overlapping multistep chemical reactions in solid-gas systems as exemplified by the thermal decomposition of granular sodium perborate tetrahydrate. This reaction proceeds via successive thermal dehydration and decomposition occurring at different temperatures to form sodium metaborate. Each reaction process comprises several kinetic steps originating from different physicochemical and physico-geometric phenomena. The partially overlapping multistep processes were characterized using available thermoanalytical techniques and microscopic observations. Conventional isoconversional kinetic analysis and empirical mathematical deconvolution were applied to each reaction process as preliminary kinetic approaches to extracting provable kinetic information. Then, each reaction process was analyzed kinetically based on a cumulative kinetic equation, i.e., kinetic deconvolution analysis. The results of the kinetic deconvolution analysis were further examined by comparison with other kinetic information for the specific kinetic steps obtained from different thermoanalytical measurements. From the results of this comprehensive kinetic approach, the kinetic features of the thermal dehydration and decomposition processes were revealed by identifying their contributing physicochemical and physico-geometric phenomena and evaluating their influences on the overall multistep processes.
A laboratory exercise for the education of students about thermal runaway reactions based on the reaction between aluminum and hydrochloric acid as a model reaction is proposed. In the introductory part of the exercise, the induction period and subsequent thermal runaway behavior are evaluated via a simple observation of hydrogen gas evolution and measurement of the temperature, which also provide basic information on the mechanistic features of the thermal runaway reaction. The exercise also includes PC aided thermometric measurements of the reaction and kinetic analysis of the induction period. The initiation time of the thermal runaway behavior under certain reaction conditions is calculated using the kinetic parameters determined experimentally by the students. The laboratory exercise provides a fundamental understanding of the mechanistic features of a thermal runaway reaction, recognition of the need for adopting safety measures when performing chemical reactions, and experience of using kinetic analysis for safety assessment.
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