Hess Law Applicability to Heterogeneous Combustion Reactions Resulting to Non-Monotonicity of Enthalpy Distribution in Them

Abstract

The definition of reaction irreversibility (reversibility) currently available in the literature does not allow for determining this reaction peculiarity before practice. Moreover, in mathematical terms, such a definition is not universal and leads actually to the denial of any similarity in physicochemical processes, particularly in combustion waves. We have considered the irreversibility of reactions from the mathematical viewpoint as the independence of a reaction rate on the concentration of products directly. As a result, in terms of Le_i numbers, we have formulated conditions sufficient for the enthalpy of reagents to be a complete differential (or a thermodynamic potential) and independent of the reaction path. And conversely, the conditions necessary for that enthalpy of reagents are significantly dependent on the reaction path (Le ≌ 1). A comparison of our conclusions with the data from experiments available in the literature showed good agreement and clarified the question of determining the irreversibility (reversibility) of reactions in the scientific community. We have shown that the small values of the diffusion coefficients of reagents D_i compared to the thermal diffusivity coefficient κ; and with them, the corresponding small Lewis numbers (Le_i = D_i/κ << 1) for the most heterogeneous combustion synthesis systems are sufficient conditions for the enthalpy of reactions (H) to be a complete differential of the system states. Conversely, a strong similarity appears between the enthalpy and concentration fields at Le ≌ 1.