Browsing by Subject "reaction kinetics"

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  • Cáceres-Jensen, Lizethly; Rodríguez-Becerra, Jorge; Jorquera-Moreno, Bárbara; Escudey, Mauricio; Druker-Ibañez, Sofía; Hernández-Ramos, José; Díaz-Arce, Tatiana; Pernaa, Johannes; Aksela, Maija (2021)
    Teaching the fundamentals of chemical kinetics on the college level is challenging to teachers and students alike due to its abstract nature of concepts and limited connection with real context applications. This study consisted of two phases starting with designing a chemistry education for the sustainable development-based learning environment of reaction kinetics, followed by a case study in which students' perceptions toward learning chemistry by solving a real environmental problem using digital resources, spreadsheets, and an active learning environment, were explored. First, we designed a Socio-Scientific Environmental Chemistry module centered on the sorption kinetic processes of herbicides in volcanic ash derived soils (VADS) and their potential to pollute groundwater. The objective of the learning module was to contribute to the development of sustainability skills, to promote learning of contextualized chemistry knowledge, and to develop scientific skills. This module employs spreadsheets as computational tools in chemistry to model real sorption kinetic data of herbicides in VADS. The learning module was designed for one section of two Analytical Chemistry courses and one Physical Chemistry course of an undergraduate chemistry teacher-training program. After the design phase, the learning module was implemented in each course, and students' perceptions were gathered using the focus group technique. The sample was of 22 students distributed into three focus groups. The data collected were analyzed and categorized through qualitative content analysis using the Technological Pedagogical Science Knowledge (TPASK) framework. On the basis of our findings, the students acquired contextualized chemistry knowledge and develop skills and knowledge related to using digital resources and spreadsheets in a scientific context. Besides, the preservice chemistry teachers' knowledge of pedagogy allowed them to develop some elements of their pedagogical science knowledge and TPASK. This case study shows that the problem-based learning approach offers great potential in supporting a learning environment suitable to working with spreadsheets to solve real-environment problems in chemistry education.
  • Lattouf, Elie; Anttalainen, Osmo Antero; Kotiaho, Tapio; Hakulinen, Hanna Idamaria; Vanninen, Paula; Eiceman, Gary Alan (2021)
    Gas phase reactions between hydrated protons H+(H2O)(n) and a substance M, as seen in atmospheric pressure chemical ionization (APCI) with mass spectrometry (MS) and ion mobility spectrometry (IMS), were modeled computationally using initial amounts of [M] and [H+(H2O)(n)], rate constants k(1) to form protonated monomer (MH+(H2O)(x)) and k(2) to form proton bound dimer (M2H+(H2O)(z)), and diffusion constants. At 1 x 10(10) cm(-3) (0.4 ppb) for [H+(H2O)(n)] and vapor concentrations for M from 10 ppb to 10 ppm, a maximum signal was reached at 4.5 mu s to 4.6 ms for MH+(H2O)(x) and 7.8 mu s to 46 ms for M2H+(H2O)(z). Maximum yield for protonated monomer for a reaction time of 1 ms was similar to 40% for k(1) from 10(-11) to 10(-8) cm(3).s(-1), for k(2)/k(1) = 0.8, and specific values of [M]. This model demonstrates that ion distributions could be shifted from [M2H+(H2O)(z)] to [MH+(H2O)(x)] using excessive levels of [H+(H2O)(n)], even for [M] > 10 ppb, as commonly found in APCI MS and IMS measurements. Ion losses by collisions on surfaces were insignificant with losses of
  • Keshavarz, Fatemeh; Thornton, Joel A.; Vehkamaki, Hanna; Kurten, Theo (2021)
    Alkyl nitrates (ANs) are both sinks and sources of nitrogen oxide radicals (NOx = NO + NO2) in the atmosphere. Their reactions affect both the nitrogen cycle and ozone formation and therefore air quality and climate. ANs can be emitted to the atmosphere or produced in the gas phase. In either case, they can partition into aqueous aerosols, where they might undergo hydrolysis, producing highly soluble nitrate products, and act as a permanent sink for NOx. The kinetics of AN hydrolysis partly determines the extent of AN contribution to the nitrogen cycle. However, kinetics of many ANs in various aerosols is unknown, and there are conflicting arguments about the effect of acidity and basicity on the hydrolysis process. Using computational methods, this study proposes a mechanism for the reactions of methyl, ethyl, propyl, and butyl nitrates with OH- (hydroxyl ion; basic hydrolysis), water (neutral hydrolysis), and H3O+ (hydronium ion; acidic hydrolysis). Using quantum chemical data and transition state theory, we follow the effect of pH on the contribution of the basic, neutral, and acidic hydrolysis channels, and the rate coefficients of AN hydrolysis over a wide range of pH. Our results show that basic hydrolysis (i.e., AN reaction with OH-) is the most kinetically and thermodynamically favorable reaction among our evaluated reaction schemes. Furthermore, comparison of our kinetics results with experimental data suggests that there is an as yet unknown acidic mechanism responsible for acidic catalysis of AN hydrolysis.