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Theoretical Models for Chemical Kinetic and Arrhenius Equations
Published in Eli Usheunepa Yunana, Calculations in Chemical Kinetics for Undergraduates, 2022
In the transition state theory, an activated complex is proposed to exist between reactants and products in chemical reactions. This implies that reactants do not directly become products until an amount of energy is acquired to reach the transition state or activated complex. The activated complex is a short-lived specie because it is highly reactive (i.e. may break apart to form products or revert to reactants). The minimum energy required to get a reaction started is known as the activation energy Ea. Even if reactant molecules have the right orientation when colliding and do not have enough energy to reach the transition state, then the reaction will not proceed. ΔH=Ea forward−Ea reverse
Chemical Kinetics of Combustion
Published in Kenneth M. Bryden, Kenneth W. Ragland, Song-Charng Kong, Combustion Engineering, 2022
Kenneth M. Bryden, Kenneth W. Ragland, Song-Charng Kong
It is observed from both the kinetic theory of gases and experiment that the rate constant for an elementary reaction is an exponential function of temperature and is of the Arrhenius form k=k0e−E/R^TWhere k0 is the pre-exponential factor, and E is the activation energy for the reaction. The activation energy is the energy required to bring the reactants to a reactive state referred to as an activated complex such that the chemical bonds can be rearranged to form products. Both E and k0 have been determined from experiment for many of the elementary reactions that occur in combustion.
General Princlpes
Published in Martin B., S.Z., of Industrial Hygiene, 2018
Activation energy is the amount of energy needed for two colliding molecules to form an activated complex. If the molecules do not have enough energy to form an activated complex at collision, they would just bounce off each other. Different molecules have different activation energies. For example, if white phosphorus comes into contact with air, the white phosphorus will immediately react with the oxygen to produce a very hot flame. However, the violent reaction between oxygen and hydrogen, which is used to power spacecraft must occur at a temperature of 400°C. Therefore, the activation energy is higher for the oxygen/hydrogen reaction than the phosphorus/oxygen reaction. For the industrial hygiene professional, this means that certain compounds must be segregated from each other if they have low activation energies.
Catalytic pyrolysis of algae: kinetics and thermodynamic analysis
Published in Chemical Engineering Communications, 2023
Anjana P. Anantharaman, Chalamala Jaya Prakash, Osipalli Bangarraju, Tamilmani Jayabalan
Figures 12–14 shows the changes in enthalpy (ΔH), Gibbs free energy (ΔG), and entropy (ΔS) for the catalytic pyrolysis of algae based on kinetic properties calculated by different isocoversional methods at different conversion, respectively (see Table 4) (Fu et al. 2023). The enthalpy is the amount of energy needed to transform biomass into different products. The potential energy barrier that reactants must pass through in order to form an activated complex relates to the difference between enthalpy and activation energy. A slight difference in enthalpy and activation energy (∼4-8 kJ/mol) at different conversions corresponds to the feasibility of product formation with energy efficiency (Shahid et al. 2019). ΔH value ranges between 130 − 320 kJ/mol while using FWO, KAS, and SCR methods. The obtained data agrees with the various models that were employed. The ΔH value remain almost constant in stage I, increases with an increase in conversion in stage II, and further, the value drops in stage III. The endothermic nature of pyrolysis reaction is definite based on the positive readings of ΔH. The enthalpy increases with the increase in conversion and remains constant at higher conversion. The enthalpy value is highest for the conversion of 60 − 70% where the activation energy is also highest. Small energy differences between Ea and ΔH represent the ease of product formation and improve the activated complex formation (Açıkalın and Gözke 2021).