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Reactions and Electrolytes
Published in Marc J. Assael, Geoffrey C. Maitland, Thomas Maskow, Urs von Stockar, William A. Wakeham, Stefan Will, Commonly Asked Questions in Thermodynamics, 2022
Marc J. Assael, Geoffrey C. Maitland, Thomas Maskow, Urs von Stockar, William A. Wakeham, Stefan Will
Some solids, for example metals or metal oxides, can act as a heterogeneous catalyst to promote a chemical reaction (see Question 5.7.3), such as hydrogenation, oxidation, polymerization, hydrocarbon cracking and many of the synthesis gas conversion processes described in Questions 7.3.2–7.3.5. This occurs because under selected reaction conditions, the Gibbs free energy of adsorption ΔadG is favorable to significant adsorption and the chemistry of the gaseous reactants and the solid surface are such that a gas molecule undergoing chemisorption can be dissociated or rearranged owing to the unsatisfied valency requirements of the surface atoms. This produces molecular fragments that can more easily react with other gas molecules as they adsorb or other dissociated molecular species as they migrate across the solid surface. After completion of reaction on the surface, product molecules are likely to desorb because their partial pressure in the gas phase is low. Processes such as these provide more favorable pathways for the reaction, via lower energy molecular transition states, which lower the activation energy ΔrG*, and hence increase the rate of reaction at a given temperature or enable lower temperatures to be used to reach the required equilibrium product yield. However, in some cases, strongly adsorbed product molecules, or unwanted by-products, can hinder the reaction by occupying potential adsorption sites and poisoning the catalyst surface, so decreasing the catalyst lifetime and requiring its replenishment.
An investigation of natural nano-particles for cleaning
Published in Matthew Laudon, Bart Romanowicz, 2007 Cleantech Conference and Trade Show Cleantech 2007, 2019
A catalyst decreases the activation energy of a chemical reaction. Catalysts participate in reactions but are neither reactants nor products of the reaction they catalyze. Catalysts work by providing an (alternative) mechanism involving a different transition state and lower activation energy. The effect of this is that more molecular collisions have the energy needed to reach the transition state. Hence, catalysts can perform reactions that, albeit thermodynamically feasible, would not run without the presence of a catalyst, or perform them much faster, more specific, or at lower temperatures. Catalysts cannot make energetically unfavorable reactions possible — they have no effect on the chemical equilibrium of a reaction because the rate of both the forward and the reverse reaction are equally affected. The net free energy change of a reaction is the same whether a catalyst is used or not; the catalyst just makes it easier to activate.eaction helps to accelerate the same reaction. They work by providing an alternative pathway for the reaction to occur, thus reducing the activation energy and increasing the reaction rate. Catalysts generally react with one or more reactants to form a chemical intermediate that subsequently
Enzyme Catalysis
Published in Harvey W. Blanch, Douglas S. Clark, Biochemical Engineering, 1997
Harvey W. Blanch, Douglas S. Clark
(5) From Fersht, A. "Enzyme Structure and Mechanism", Freeman & Co. 2nd Edition, (1985). temperatures. A rate constant (k) is assumed to be proportional to the concentration of the transition state species. Classical transition state theory 6 leads to the following expression for k : k=kBThK±
The gas phase oxidation of HCOOH by Cl and NH2 radicals. Proton coupled electron transfer versus hydrogen atom transfer*
Published in Molecular Physics, 2019
Josep M. Anglada, Ramon Crehuet, Albert Solé
In this study we have considered the reaction of both conformers of formic acid with Cl and NH2 radicals according reactions 3 to 6. The relative energies of the different stationary points computed for these reactions are contained in Tables 1 and 2, and Figures 1 and 3 show a schematic picture of the potential energy surfaces of these reactions. Figure 2 contains the most relevant electronic features describing the processes investigated in this work. In all elementary reactions, the transition states are mediated by the formation of a pre-reactive complex in the entry channel, which is formed by interaction of the two reactants, and by a post-reactive complex in the exit channel before the release of the corresponding products. Along the text, we have named the pre-reactive complexes by the letters CR, the transition states by TS and the post-reactive complexes by CP, followed by a number to distinguish the different reaction paths. In order to differentiate the reaction of formic acid with the two radicals investigated. The acronym of each stationary point is preceded by the letter A for the reactions involving Cl radical and by the letter B for the reactions with NH2 radical.
Enzymatic synthesis of Isopropyl stearate, a cosmetic emollient: optimisation and kinetic approach
Published in Indian Chemical Engineer, 2023
Sarita D. Gawas, Prasanna Joshi, Virendra K. Rathod
Activation energy (Ea) is the energy required to raise the initial reactant to the transition state. The activation energy (Ea) for most reaction limits is 40–400 kJ/mol [23]. The reaction will accomplish rapidly if Ea value is lesser than 40 kJ/mol. From the Arrhenius plot (Figure 3), the activation energy was found to be 10.59 kJ/mol for isopropyl stearate synthesis. From the values of activation energy, it is concluded that the enzymatic synthesis of isopropyl stearate could occur rapidly. R.N. Vadgama et al. obtained the activation energy as 37.13 kJ/mol for isopropyl myristate synthesis [9].
Mechanisms of N2 Formation from Armchair Configurations with Different Dinitrogen Active Sites During Coal Pyrolysis
Published in Combustion Science and Technology, 2023
Tingting Jiao, Pengzheng Shi, Wenguang Du, Shoujun Liu, Ju Shangguan
According to the transition state theory, the elementary reaction with the highest activation energy (rate-determining step) determines the overall rate of the reaction. The rate-determining steps of R1 and R2 during pyrolysis are IM4 → IM5(ΔE = 273.26 kJ·mol −1)and IM6 → IM7(ΔE = 273.13 kJ·mol −1). Both of which are the conversion of the dinitrogen six-membered ring to the dinitrogen four-membered ring and the energy barriers of the two reactions are comparable.