Absolute value of enthalpy in chemical reaction

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Abstract

Enthalpy is a very ancient topic, and there has been almost no one discussion on its right and wrong in recent years. However, an absolute value of enthalpy in chemical reaction remains our dream for a hundred years. From a convenient and feasible perspective, and in order to compare the enthalpy values of various reactions, a value of standard enthalpy was defined as Dr(298.15/K) in chemical thermodynamics, which is a “reference” value. The reaction enthalpy at any temperature for a reaction, Dr(T/K), can be calculated from theory by Kirchhoff's law 1,2 . However, many reactions do not generate thermal effects at 298.15/K so that Dr(298.15/K) is still a "virtual" value with no real chemical or physical connotation. Secondly, in the face of this series of enthalpy values at any temperature, Dr(T/K) the chemical thermodynamics is unable to pick out the absolute value of the reaction enthalpy for a reaction. For nearly a hundred years, we have been asking: what is the absolute value of the reaction enthalpy? Our work suggests that the lowest temperature at which a chemical reaction produces a thermal effect is defined as the enthalpy temperature (T H ) and a 111enthalpy theory, which states that a reaction has only one unique value of enthalpy temperature (T H ) and one unique value of reaction enthalpy Dr(T/K), an absolute Value of reaction enthalpy. Namely, when T<T H , Dr(T)=0; when TT H ,Dr(T)=Dr(T H ). In addition, according to 111 theory, the meaning of Qp value in DSC may been redefined as: Qp=Dr(T H ), not Qp= which is a very fuzzy concept or expression for thermodynamics 1,6 . By using the DSC method, the T H and Qp values of many reactions can be discovered and determined, thus Dr(T H ) of the reaction can be calculated and evaluated. The rationality of 111 theory has be mutually verified by the experimental data in DSC and the enthalpy value of thermodynamic calculation for dehydration reaction of calcium oxalate monohydrate (COM). ΔH, ΔG and ΔS are all state functions and ΔG= ΔH−TΔS, and then will the appearance of T H and Dr(T H ) cause changes in free energy and entropy in chemical reaction? Can the 111 theory induce a new idea and method for exploring the absolute internal energy of matter?

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