China
Africa
Explore chapters and articles related to this topic
Chemical Kinetics
Published in Himadri Roy Ghatak, Reaction Engineering Principles, 2018
Elementary reactions are exceptions rather than the rule. For the vast majority of reactions, the transformation of reactants into products is “not a single event– at the molecular level. In such reactions, though the overall outcome is the transformation of reactants into products, chemical species other than the reactants A1, A2, … An, and the products P1, P2, … Pl, are observed to be formed during the course of the reaction. However, the concentration of these chemical species is so less that the overall outcome still remains the transformation of reactants A1, A2, … An, into products P1, P2, … Pl. Such reactions are known as nonelementary reactions. The reaction rate for such reactions may or may not follow the law of mass action and it is not mandatory for the reaction rate to correspond to the balanced chemical equation as provided by the reaction stoichiometry. Nonelementary reactions can be explained as combinations of elementary reactions. Chemical species other than the original reactants and the final products are known as the reaction intermediates. Depending on the nature of the particular reacting system, these intermediates can be free radicals (moieties containing one or more unpaired electrons), ions (moieties containing one or more units of positive or negative electrical charge), or molecules with very short life spans. When a nonelementary reaction is represented as the combination of its constituent elementary reactions, such a scheme is commonly known as the reaction mechanism for the nonelementary reaction. Some illustrative examples of nonelementary reactions and their mechanisms are presented in Table 3.1.
Introduction to Inorganic Chemistry
Published in Caroline Desgranges, Jerome Delhommelle, A Mole of Chemistry, 2020
Caroline Desgranges, Jerome Delhommelle
Catalysis starts to really take off at the start of the 20th century with the discoveries by Arrhenius and Ostwald. Indeed, Arrhenius studies the rate of chemical reactions and finds that the rate constant k is related to the activation energy EA, the gas constant R and the temperature T through the following equation k = A exp(–EA/(RT)), in which A is a constant for a given reaction. This relation suggests two ways of increasing the rate of a chemical reaction. The first possible route is to increase the temperature T, which in turn, decreases the ratio EA/(RT) and thus increases k. The second route is to decrease the value of the activation energy EA. The activation energy measures the amount of energy necessary to form the reaction intermediate from the reactants. If the conditions of the reaction are modified in such a way that the formation of the reaction intermediate requires less energy, this should lead to a lower EA and thus to a greater k. Therefore, if a substance foreign to the reaction is introduced and modifies the intermediate, this substance can increase the kinetics of the reaction and become a catalyst for the reaction. Ostwald is the first to identify the importance of catalysts for a wide range of chemical reactions and their numerous applications in organic chemistry and biology as well as in industry. He also proposes a definition for a catalyst: “Based precisely on the participation of the catalyst in the reaction actually occurring, in the sum of which, however, the catalyst is not directly involved, although the partial reactions contain the catalyst as a major chemical component of the process”. He states that catalysis relies on intermediate reactions involving the catalyst and concludes: “It must be conceded that no other equally effective principle has hitherto been found in the theory of catalysis”. For instance, if we think of a reaction between two reactants X and Y resulting in the formation of a product P, the reaction can be written as X + Y → P. If we now use a catalyst C that forms a reaction intermediate with X, we then have the following two-step reaction: X + C → XC and XC + Y → P + C. During the second step, the catalyst C is released, meaning that, if we add up the two steps to obtain the overall reaction, we obtain X + Y → P, which is exactly the same equation as in the reaction using no catalyst! The difference between the two possible pathways (with or without catalyst) is the kinetics. Without a catalyst, the reaction rate is equal to v = k[X][Y]. On the other hand, when a catalyst is used, the reaction rate is given by the slowest of the two steps. This is generally the first step and thus v = k1[X][C]. Since we use a catalyst, the activation energy will be lower and thus k1 will be larger than k! Furthermore, since the catalyst is neither produced nor consumed during the reaction, [C] is constant and thus the reaction rate can be written as v = k1ʹ[X], meaning that it resembles a first-order reaction with respect to [X], with k1ʹ=k1[C].
UV/Chlorination of sulfamethazine (SMZ) and other prescription drugs: kinetics, transformation products and insights into the combined toxicological assessment
Published in Environmental Technology, 2022
Xiaoshu Yan, Han Chen, Tao Lin, Wei Chen, Hang Xu, Hui Tao
The degradations of SMZ, GEM and ANT during UV/chlorine followed the pseudo-first-order kinetics, in which the reaction rate constants (kobs) decreased with the increasing initial concentrations of drugs, indicating that the reaction intermediate products may compete for the oxidants with the target substrate. Parameters (pH value, chlorine dosage and bromide concentration) affecting to the degradation kinetics were evaluated using Box–Behnken experimental design methodology. The measured half-lives were in the range of 0.48–35.77 min, 0.28–9.99 min, and 2.35–15.27 min for SMZ, GEM and ANT, respectively. Chlorine concentration was a significant factor influencing the degradation kinetics, while bromide level was far from statistical significance. Based on the Frontier Orbital Theory, the degradation pathways of SMZ, GEM and ANT were proposed, consisting of hydroxylation, chlorine substitution and their own transformation characteristics such as SMZ desulfurization and GEM cyclization. Two, two and one newly TPs were identified in UV/chlorination of SMZ, GEM and ANT, respectively. Compared to the results of the experiments with artificial water sample, the degradation kinetics of the three prescription drugs were observed with a prolonged half-lives in both Yangtze River and Taihu Lake water, suggesting that aromatic containing TPs may also exist in UV/chlorine treated natural waters. The results of combined toxicity on E. coli showed that the antagonism effect predominated in most binary and ternary combinations. However, the synergistic toxicity of combinations at low concentrations of prescription drugs subjected to UV/chlorine should be cautioned, which was more close to the natural concentration of prescription drugs in waters.