China
Africa
Explore chapters and articles related to this topic
Thermodynamics of Fuel Cells
Published in Xianguo Li, Principles of Fuel Cells, 2005
For a chemically reacting system, enthalpy of reaction is defined as the difference between the enthalpy for the reaction products and that for the reactants. Hence, the enthalpy of reaction can easily be calculated once the absolute enthalpy for the mixture of products and mixture of reactants is known. In laboratory measurements, enthalpy of reaction can be easily determined as the amount of heat released or absorbed during an isobaric chemical reaction. The chemical reaction can occur either in a closed system for which the final system temperature must be the same as the initial system temperature, as shown in Figure 2.1, or in a steady flow reactor where the reactants entering and products exiting the reactor have the same temperature and pressure, as shown in Figure 2.2. For either system, the product mixture and reactant mixture need to have the same temperature and pressure, although the mixture composition, as specified by the number of moles ni for each species i, is certainly different for the reactant and product because of the occurrence of the chemical reaction inside the system. Note that we have adopted the conventional notation that heat absorbed by the system is defined as positive.
Energy Metrics
Published in John Andraos, Synthesis Green Metrics, 2018
The amount of heat liberated or absorbed depends on the bond breaking and bond making processes that take place as the reaction proceeds. The enthalpy of reaction is calculated from Hess’s law using enthalpy of formation data for all reactants and all products based on a balanced chemical equation with appropriate stoichiometric coefficients. Since all of the enthalpy of formation quantities are determined under standard state conditions (298 K, 1 atm), then the resulting calculated enthalpy of reaction is also determined under the same standard state conditions.
Diatomic sulfur: a mysterious molecule
Published in Journal of Sulfur Chemistry, 2019
On the other hand, from the point of view of thermochemistry, the energy of chemical bonds is one of the main molecular constants characterizing the structural features of chemical compounds [19,29,30]. The dissociation energy of the chemical bond Do is defined as the change in the standard enthalpy of formation of the reaction products and the initial chemical compound. In reaction (1), the dissociation energy D298 of the HS−H bond in the H2S molecule under normal conditions is 92 kcal/mol (Table 2) [19,29,30], whereas the dissociation of the second S–H bond requires only 82.3 kcal/mol. In the reaction (1) of two H2S molecules, four HS−H bonds must be broken, which gives the total dissociation energy of two molecules ΣD298 = (92.0 + 82.3) × 2 = 348.6 kcal/mol. At the same time, two bonds H−H (ΣD298 = 2 × 104.2 = 208.4 kcal/mol) and one bond S−S D298 = 99.8 kcal/mol are formed (Table 2) [19,29]. This gives the enthalpy of reaction (1) as shown in Equation (7).
Catalytic Effect of Copper on Ozonation in Aqueous Solution
Published in Ozone: Science & Engineering, 2021
Naoyuki Kishimoto, Haruki Arai
The standard enthalpies of formation of chemical species shown in Eq. (11) are reported to be 72.1 kJ/mol for Cu+aq, 142.7 kJ/mol for O3g, −285.830 kJ/mol for H2Oliq, 65.78 kJ/mol for Cu2+aq, 0 kJ/mol for O2g, 38.95 kJ/mol for HO˙g, and −229.994 kJ/mol for OH−aq (Bard, Parsons, and Jordan 1985). Accordingly, the standard enthalpy of reaction of Eq. (11) is calculated to be −54.2 kJ, which means that Eq. (11) is an exothermic reaction. Thus, Eq. (11) is thermodynamically possible.
Catalytic Effect of Copper on Ozonation in Aqueous Solution
Published in Ozone: Science & Engineering, 2021
Naoyuki Kishimoto, Haruki Arai
The pKa of HO3˙ is reported to be 8.2 (Westerhoff et al. 1997). Thus, Equation (12) is a more efficient initiation step for HO˙ generation than the corrosion of Cu0 (Equations (5) and (6)). To the best of our knowledge, there is no report on the standard enthalpy of formation of O3˙−. Therefore, the standard enthalpy of reaction of Equation (12) is unknown. Equation (12) is composed of two electrode reactions as follows: