A melt mechanism for the thermal decomposition of ammonium dichromate

Journal of Thermal Analysis

Laverty

Okhotnikov

et al. 1992

Meso lithium pc ssium tartrate dihydrate melted before dehydration and a kinetic study of this reaction has been completed. This system is of interest in establishing the kinetic characteristics of a homogeneous rate process in the absence of added solvent. Results are of interest in considering the mechanisms of solid or condensed state reactions where melting is a possibility.The evolution of the initial 1.2H20 from the single crystal dihydrate reactants was zero order, the rate then became deceleratory and the first order expression was obeyed to 1.6H20. The activation energy of the process was high, 230_+10 kJ mo1-1 (350-380 K). Evolution of the remaining water occurred by a slower first-order process to give the anhydrous salt. The dehydration of crushed powder reactant was initially relatively more rapid but was deceleratory throughout, obeying the first order equation. It is concluded that salt dehydration is controlled by the rate of surface release of water that is comparatively mobile within the reactant melt.

Section: John Wiley and Sons Limited Chichestermentioning

confidence: 99%

A kinetic study of water evolution from a molten hydrated salt

Journal of Thermal Analysis

Laverty

Okhotnikov

et al. 1992

“…Ammonium dichromate Decomposition of (NH 4 ) 2 Cr 2 0 7 was initiated at a number discrete local sites (previously and incorrectly reported as n + g), which microscopic examination identified as a liquid. Its froth-like texture was attributed to bubble formation [84]. Closely similar textural features appeared on heating CrO 3 (1) in an atmosphere of NH 3 + H 2 O or (2) when mixed with (NH 4 ) 2 CO 3 .…”

Section: Transition Metal Carboxylates: Copper(ii) Carboxylatesmentioning

confidence: 72%

Thermal reactions involving solids: a personal view of selected features of decompositions, thermal analysis and heterogeneous catalysis

2020

J Therm Anal Calorim

Self Cite

Convinced that some recent trends in the literature concerned with reactions involving solids have been unproductive, even discouraging interest in the subject, this reviewer analyses the reasons and charts a way forward. In particular, two topics are discussed: thermal analysis and activation energy. Thermal analysis, automated collection and interpretation of kinetic data for solid(?)-state decompositions, resulted in huge numbers of publications between late 1970s and 2010. Measurements were frequently minimalistic (few, often no, confirmatory tests complemented rate data). Kinetic data interpretations were based on the Arrhenius activation model, inapplicable to these assumed, usually unconfirmed, solid-state(?) reactions. Energy distributions within crystalline reactants differ from those of 'free-flying' gaseous reactants, and thus, mechanistic proposals are entirely speculative. Such studies yielded little more than the reaction temperature: no meaningful insights into reaction chemistry, controls, mechanisms. Despite my several highly critical articles, these inconsequential studies continued. Overall, this now sidelined topic impacted adversely on solid-state chemistry, activation energy, E. Concurrently with the above studies, L'vov published a theoretical explanation for the magnitude of E: the Congruent Dissociative Volatilisation (CDV), thermochemical approach. This was also ignored by the 'Thermoanalytical Community', possibly because it assumes an initial volatilisation step: it appears that many solid-state scientists are prejudiced against mechanisms involving a phase change. The value of this novel theory (CDV) in identifying controls and mechanisms of solid-state reactions is discussed here. This review is positive: an interesting branch of main-stream chemistry remains open for exploration, expansion, explanation and exploitation! Publisher's Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

“…2) and (NH4)2Cr2OT, (Fig. 3) [20]. D e h y d r a t i o n m e c h a n i s m s For many reactants, the reactivity within the surface layer of a crystalline solid is different from, and appreciably greater than in the material beneath.…”

Section: Unreactive Surface Layersmentioning

confidence: 99%

“…Section of partly decomposed crystal showing froth formation due to gas evolution in molten reactant. b) Decomposition of ammonium dichromate [20]. Burst bubble developed during gas generation within molten reactant…”

Section: Unreactive Surface Layersmentioning

confidence: 99%

Structures and functions of reaction interfaces developed during solid-state dehydrations

Brown

Journal of Thermal Analysis

Guarini

1997

The literature reveals that the mechanisms of some solid-state dehydrations are more complicated than has been generally accepted. Reactions at a thin advancing reactant-product interface provide the geometric models on which the most widely employed rate equations are based. For some systems, this "thin interface" model is a simplification of observed behaviour. Elimination of water from crystallographic sites may occur to a significant extent within a much thicker zone of reactant towards which the active interface is progressing. Consequently the region of chemical change may not coincide with the region of structural transformation. Limited initial dehydration may occur across all crystal faces prior to the onset of a nucleation and growth process that is usually regarded as the dominant rate process in the dehydrations of many large crystals. Experimental observations for solid-state dehydrations are discussed and reaction mechanisms with different rate controlling processes are distinguished. Studies of dehydrations have contributed substantially to the theory of solid-state reactivity, and advances in understanding may have wider application to other solid-state reactants.