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Chemical Thermodynamics and Thermochemistry
Published in Armen S. Casparian, Gergely Sirokman, Ann O. Omollo, Rapid Review of Chemistry for the Life Sciences and Engineering, 2021
Armen S. Casparian, Gergely Sirokman, Ann O. Omollo
Thermodynamics is the study of energy transformations. Thermochemistry is the study of the thermal energy changes that accompany chemical and physical changes. Heat is one form of energy; sound and light are two others that may accompany a chemical reaction. Heat is the energy transferred between objects or systems, which is measured by temperature changes. The total energy is a property of a substance or system; it cannot be measured directly.
A multicomponent multitemperature model for simulating laminar deflagration waves in mixtures of air and hydrogen
Published in Numerical Heat Transfer, Part B: Fundamentals, 2023
The well-known one-step reaction for hydrogen oxidation shadows a far more complex chain of chemical reactions that produce and consume intermediate reaction-products [11, 12]. This complex thermochemistry strongly depends on the initial temperature and on the mass fractions of the gases in the mixture. It can be mandatory to account for this complete thermochemistry to numerically reproduce some very fine physical phenomenon which cannot be predicted with the sole one-step reaction. This is the case for instance for all the situations involving autoignition processes of hydrogen [13]. To account for that complex chain of reactions, specific kinetic/chemical solvers, as Cantera [14] for instance, are used in the numerical codes dedicated to the fine simulation of combustion. Nevertheless, for the simulation of the ignition of gas clouds at an industrial scale, a one-step reaction may be sufficient. This choice has been made for instance in FLACS [15, 16] or P2REMICS [17]. In the present work, the same assumption is made and the thermochemistry of the sole one-step reaction will be considered. Moreover, in the proposed model the reaction rate is modeled by an Arrhenius law. This feature is also classical and its interaction with thermal diffusion is essential for the model. In particular, the balance between the heat release due to the reaction and the heat conduction in the gases is a key point.
Active Thermochemical Tables: the thermophysical and thermochemical properties of methyl, CH3, and methylene, CH2, corrected for nonrigid rotor and anharmonic oscillator effects
Published in Molecular Physics, 2021
In order to fairly compare the NRRAO and RRHO thermochemistry, each examined reaction has initially the same 298.15 K reaction enthalpy within both models, fixed by the ATcT 298.15 K enthalpies of formation of the reactants and the product, but then the two models use differing thermophysical properties to develop the thermochemistry at other temperatures. Being the reverse of bond dissociation processes, both examined reactions are exothermic throughout the explored temperature range. They both also start as exergonic at the low temperature end, but become endergonic at the high temperature end (meaning that at sufficiently high temperature the reverse reaction, which corresponds to thermal dissociation, becomes spontaneous). Within the RRHO thermochemistry, becomes endergonic at ∼3110 K. Within the NRRAO thermochemistry, this point is pushed upward by more than 100 K, and the reaction switches from being exergonic to becoming endergonic at ∼3235 K. Similarly, the becomes endergonic at ∼2265 K within the RRHO thermochemistry, but at ∼2320 K within the NRRAO thermochemistry. What this means is that NRRAO thermochemistry results in both cases in slightly lower free energies at the high temperature end than their RRHO counterparts. Thus, in both cases at combustion temperatures the NRRAO thermochemistry favours association (as opposed to dissociation) slightly more than the RRHO thermochemistry.
Ring-opening pathway of 2, 4, 6-trichlorophenol initiated by OH radical: an insight from first principle study
Published in Molecular Physics, 2020
Subrata Paul, Ramesh Chandra Deka, Nand Kishor Gour, Abhishek Singh
During frequency calculation, we are obtained the total energy (Eo), enthalpy (H) and Gibbs free energy (G) values of all reaction species and these values are provided in Table S2. From Table S2, thermochemistry parameters such as enthalpy change and Gibbs free energy change are determined for each step in the ring-opening pathways of 2, 4, 6-TCP + •OH reaction and given in Table 1. As we are interested to get more refined thermochemistry parameters, So we have further performed energetic calculations of all reaction species at higher level of CCSD(T) method along with the same basis set and obtained more accurate electronic energies. So the electronic energies (at CCSD(T) method) added to zero-point corrected energy, enthalpy and Gibbs free energy (obtained at M06-2X functional) to get more refined values of E0, H and G which are given in Table S3. The value of standard reaction enthalpy change (ΔrHo) and Gibbs free energy change (ΔrGo) for the reaction channels also given in Table 1 at CCSD(T)//M06-2X/6-311++G(d, p) level of theory.