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Steady-State Approximation, Reaction Mechanism and Rate Law of Chain Reactions
Published in Eli Usheunepa Yunana, Calculations in Chemical Kinetics for Undergraduates, 2022
Steady-state approximation proposes that the concentration of all intermediates remains constant and small throughout the reaction. An intermediate is any species that does not appear in the overall reaction but has been introduced into the mechanism of that reaction. In other words, an intermediate is a species that is produced and consumed during a chemical reaction. The steady-state approximation is the most common approach to the analysis of reaction mechanisms. Hence, the principle assumes that Rateofformationofintermediate=Rateofconsumptionofintermediate
Bimodal Reaction Sequences Occurring through the Active Intermediates
Published in Robert Bakhtchadjian, Bimodal Oxidation: Coupling of Heterogeneous and Homogeneous Reactions, 2019
As a rule, oxidation with dioxygen of organic and inorganic matter in the gas or liquid phases, as well as in the interphases occurs by the formation of active intermediates. They may be atoms, molecules, ions, radicals, excited species, complex compounds, conformers, active sites on the solid substances, etc. Their lifetimes vary depending on the nature of the compound and conditions under which the chemical system exists. The lifetime is the time interval in which the concentration of an intermediate decreases to 1/e. If no reaction intermediates, naturally, the reaction is an elementary chemical act and occurs as a concerted reaction. Understanding the reaction mechanism is based mainly on the detection and investigation of the key intermediates. The more active reaction intermediates, the lower is their concentration in the system, as their lifetime is short. The modern experimental methods permit detection of intermediates, whose lifetimes are about 10−15 s (femtosecond; Zewail received the Nobel Prize in Chemistry in 1999 “for his studies of the transition states of chemical reactions using femtosecond spectroscopy”10). However, application of more sophisticated spectroscopic techniques permits survey of the reactions at timescale, even 10−18 s (attosecond)!11
Internal Combustion Engines
Published in Iqbal Husain, Electric and Hybrid Vehicles, 2021
However, due to practical limitations and constraints placed on IC engine operation, complete combustion of hydrocarbons is not possible. Intermediate reaction products are formed since chemical reactions depend on pressure, temperature, concentration of species and time. Exhaust streams contain much different types of hydrocarbons. While most of these hydrocarbons are intermediate reaction products, some unburned high-molecular-weight hydrocarbon fuel may also be present. Carbon monoxide is produced when there is insufficient oxygen. Retarded ignition in gasoline engines also aid in carbon monoxide production.
An overview: recent development of semiconductor/graphene nanocomposites for photodegradation of phenol and phenolic compounds in aqueous solution
Published in Journal of Asian Ceramic Societies, 2021
Reyhaneh Kaveh, Hassan Alijani
A great number of research has mentioned that phenol can be destroyed to carbon dioxide and water [35,36]. Moreover, some of the intermediate products formed during photocatalytic degradation of phenol. The most commonly used way to discover intermediates is extraction via organic solvent, after esterification or acetylation, followed by analysis through gas chromatography or gas chromatography/mass spectrometry. The possible by products of the phenol degradation are hydroquinone, resorcinol, catechol, pyrogallol, 1,4-benzoquinone, 1,2,4-benzenetriol, salicylic acid, 2-propylphenol, 2-hydroxy-1,4-benzoquinone, 2,2-dihydroxybiphenyl, 2-hydroxybenzophenone, hydroxybenzoic acid, 2-hydroxy-benzophenone, 4,4- dihydroxybiphenyl, 2-propylphenol, hydroxybenzioc aldehyde, muconic acid, muconic aldehyde, maleic acid, 3-hydroxypropyl acid and hydroxyl-acetic acid. Scheme 2 illustrates the main phenol degradation intermediates, which these intermediates undergo more photocatalytic oxidation generate polar intermediates such as carboxylic acids and aldehydes and finally CO2 and H2O [37–39].