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The central goal of this study was to characterize the mental models of acids and acid strength expressed by advanced college chemistry students when engaged in prediction, explanation, and justification tasks that asked them to rank chemical compounds based on their relative acid strength. For that purpose we completed a qualitative research study involving students enrolled in different types of organic chemistry course sections at our university. Our analysis led to the identification of four distinct mental models, some of which resembled scientific models of acids and acid strength. However, the distinct models are better characterized as synthetic models that combined assumptions from one or more scientific models with intuitive beliefs about factors that determine the properties of chemical substances. For many students in our sample, mental models served more as tools for heuristic decision-making based on intuitively appealing, but many times mistaken, concept associations rather than as cognitive tools to generate explanations. Although many research participants used a single general mental model to complete all of the interview tasks, the presence of specific problem features or changes in the nature of the task (e.g., prediction vs. explanation) prompted several students to change their mental model or to add a different mental representation. Our study indicates that properly diversifying and sequencing the types of academic tasks in which students are asked to participate could better foster meaningful learning as different types of cognitive resources may be activated by different students, and thus shared, analyzed, and discussed. ß

Most organic chemistry reactions occur by a mechanism that includes acid–base chemistry, so it is important that students develop and learn to use correct conceptions of acids and acid strength. Recent studies have described undergraduate organic chemistry students’ cognitive resources related to the Brønsted–Lowry acid model and the Lewis acid model, providing both qualitative and quantitative analyses of these understandings. To drive changes in pedagogy and curriculum, however, faculty need to be able to quickly assess students’ conceptions of acids and acid strength. We recently reported on the development and assessment of a nine-item, multiple-tier, multiple-choice concept inventory about acid strength, named ACID I. Coefficient α for ACID I was calculated to be below 0.70. In this manuscript, we demonstrate that despite this low coefficient α, the data generated by ACID I are indeed reliable. Thus, the purpose of this paper is to (i) report two significant alternative conceptions about acid strength that persist in organic chemistry students’ minds after nearly two semesters, and (ii) discuss the meaning of reliability for concept inventories, including a description of additional measures for the reliability of data collected using ACID I. Two types of test conditions were employed within second-semester organic chemistry courses in two different regions of the United States: a course at a medium-sized, midwestern liberal arts university and a large, southeastern research university.

Gold nanoparticles (ca. 3 nm in diameter) coated with bis(diarylamino)biphenyl-based thiols with two different alkyl spacers (propyl and dodecyl) between the chromophore and the surface-anchoring thiol group have been prepared and characterized with a variety of techniques. The excited-state dynamics of the dyes in close proximity to the nanoparticle surface were studied using the time-correlated single-photon counting technique and near-IR fs transient absorption spectroscopy. The excited states of the dyes in the hybrid metal/organic systems exhibit an ultrafast (<5 ps) deactivation as evidenced by the fs transient absorption measurements. The length of the alkyl spacer between the dye and the thiol group has a profound effect on the ultrafast dynamics of the photoexcited systems. An ultrafast formation (ca. 0.5 ps) of a cation-like species has been recorded for the system incorporating the propyl spacer but not for the dodecyl-linker system. The formation of the cation-like species has been shown to be less efficient in a mixed-ligand system in which the bis(diarylamino)biphenyl-based thiol was diluted on the surface with dodecanethiol. Additionally, the ultrafast formation (ca. 1 ps) of a cation-like species with a similar spectroscopic signature has been observed in the solid state of the dye. A combination of the ultrafast dynamics and (1)H NMR spectroscopic data has been used to discuss the observed behavior in terms of dye-dye interactions in the nanoparticle systems. Due to the surface curvature of the nanoparticle, the propyl spacer imposes a closer dye-dye distance than the dodecyl spacer, thus facilitating dye-dye interactions that lead to the formation of a charge-transfer species involving two or more dye molecules.