Commentary pubs.acs.org/jchemeduc
Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX
Importance of Understanding Fundamental Chemical Mechanisms Vicente Talanquer*
J. Chem. Educ. Downloaded from pubs.acs.org by 193.9.158.197 on 09/06/18. For personal use only.
Department of Chemistry and Biochemistry, University of Arizona, Tucson, Arizona 85721, United States ABSTRACT: The ability to build arguments and explanations based on scientific models is emphasized in current educational standards as a central science practice that students should develop in their science classes. In chemistry, it is expected that students will be able to apply their understanding in the construction of mechanistic explanations using submicroscopic models of matter. The main goal of this contribution is to highlight and characterize a set of fundamental chemical mechanisms that enable professionals in different fields to build rationales about the properties and behaviors of chemical entities across a wide variety of systems and processes. Thus, they represent the types of understandings to which chemistry educators should aspire for their students to develop and transfer to other domains. These fundamental mechanisms define ways of reasoning that students should master and chemistry instructors must give priority to in their instructional and assessment efforts. KEYWORDS: First-Year Undergraduate/General, Curriculum, Analogies/Transfer, Student-Centered Learning
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scientific fields.9 I claim that such connectivity and integration could be facilitated by more actively, systematically, and explicitly engaging students in the analysis, discussion, application, and reflection of the fundamental chemical mechanisms described in this paper. Such mechanisms are commonly deployed by professionals in different fields to build explanations and support arguments about the properties and behaviors of chemical entities across a wide variety of systems and processes. The different mechanisms described in this paper are related to each other. Expert chemists are likely to think of them and deploy them in integrated ways. Demarcating and arranging them into specific groups as proposed in the following sections serves to highlight core explanatory aims each of these mechanisms helps to achieve. It makes explicit the use of chemical models of matter in different ways depending on the focus of our explanations and arguments (e.g., explaining changes in chemical composition versus explaining energy transfer during a process) and the level of description at which they are built (e.g., particulate versus electronic). However, it is possible that other chemistry educators may find the proposed categorization somewhat arbitrary. The main goal of this essay is to spark discussion and reflection on the importance of focusing our educational efforts on the development of a mechanistic perspective of chemical systems and phenomena. Research in chemistry education suggests that understanding and learning to normatively and productively apply the fundamental mechanisms described in this essay takes time and sustained educational effort. Thus, students should be offered multiple opportunities to engage in these ways of
INTRODUCTION Developing students’ ability to explain natural phenomena using scientific models is a central goal of science and chemistry education at all educational levels.1−4 Students are expected to understand central ideas in the different scientific disciplines and be able to apply them to make sense of the properties and behaviors of systems of interest. In chemistry we particularly value the development of students’ ability to use submicroscopic models of matter to rationalize the properties of chemical substances.5 These explanatory accounts tend to be mechanistic in nature as they invoke the existence of entities (e.g., atoms, molecules) whose properties, interactions, activities, and organization are responsible for the behaviors we observe.6,7 In chemistry we often associate the term “mechanism” with the sequence of steps that a chemical reaction is assumed to follow (i.e., description of what and how is happening). However, as discussed in the next section, the concepts of “mechanism” and “mechanistic reasoning” have broader meanings in studies of scientific reasoning. I adopt this more general perspective in this contribution centered on the identification and characterization of fundamental chemical mechanisms that enable us to make sense of a wide variety of phenomena and thus should be a central focus in chemistry education. Traditional chemistry curricula organize the presentation of concepts around standard disciplinary topics (e.g., atomic structure, chemical bonding, kinetics) and divisions (e.g., analytical, organic, physical). This approach often makes it difficult for students to identify and meaningfully comprehend underlying assumptions and ways of reasoning that cut across conventional content bins.8 It also poses a major challenge to any educational reform effort that seeks to help students build connections between chemistry topics and courses, and to integrate ideas within the discipline and across different © XXXX American Chemical Society and Division of Chemical Education, Inc.
Received: June 28, 2018 Revised: August 19, 2018
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DOI: 10.1021/acs.jchemed.8b00508 J. Chem. Educ. XXXX, XXX, XXX−XXX
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dissolves polar).29,30 Some students provide causal accounts, but they often do it by identifying a single cause without describing how it leads to the observed effect (e.g., ethanol dissolves in water because its molecules can form hydrogen bonds with water molecules). Students who actually build mechanistic explanations commonly rely on intuitive assumptions about what drives physical or chemical processes, such as considering that energy needs to be invested to make things happen,31 that atoms or molecules react with each other because they want to become more stable,32 or that all systems have a natural tendency to achieve equilibrium.30 Existing research findings strongly suggest that major changes are needed in the teaching of chemistry if we want students to learn to think mechanistically.
thinking within a course and across the different chemistry courses they may be required to take. Chemistry instructors at all educational levels should foster students’ ability to spontaneously and productively apply chemical mechanisms in their sense-making and meaning-making activities regardless of the context. Discussions about how to best orchestrate learning progressions in the development of mechanistic reasoning across the chemistry curricula are sorely needed.
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MECHANISTIC REASONING Accounts of scientific reasoning by philosophers of science often highlight the role that mechanisms play in scientific explanations.6 Although philosophical debate exists on the concept of mechanism,10,11 many philosophers consider that a mechanism for a phenomenon consists of entities and activities organized in such manner that they are responsible for (cause) the phenomenon. In general, mechanistic explanations (also called causal-mechanistic explanations) begin with the identification of the phenomenon to be explained, followed by the characterization of relevant entities and activities, to then describe the spatiotemporal organization of these components by which the phenomenon is produced (i.e., the mechanism provides a causal account of the phenomenon).12 Entities in the mechanism are assumed to have characteristic properties that determine how they interact with each other and the activities in which they engage. The organization of components can take place at various levels, and properties of a system at a given level often emerge from the properties, interactions, activities, and organization of subcomponents defined at a sublevel. From this perspective, reasoning in chemistry is certainly mechanistic. For example, the properties of molecular compounds are explained in terms of interactions between myriads of molecules. These interactions are in turn determined by the composition and structure of individual molecules, which emerge from dynamic processes taking place at subatomic levels involving protons and electrons.13 In recent years, some authors have advanced ideas on how to identify, assess, and support the development of mechanistic explanations in science classrooms using frameworks aligned with philosophical perspectives on mechanism.7,14 These types of works have been common in biology education.15,16 In chemistry, there is a long tradition of educational research directed at characterizing the challenges that students face to interpret, explain, and build reaction mechanisms in organic chemistry.17−20 Although not necessarily guided by the mechanistic reasoning framework described above, chemistry education researchers have uncovered many of the difficulties that students have to understand mechanisms such as energy transfer during bond breaking and formation, and chemical equilibrium.21 More recently, several authors have explicitly focused on characterizing the challenges that college students face to build mechanistic accounts of different chemical phenomena, including chemical reactions of different types,22−26 colligative properties,27 and intermolecular interactions.28 Major findings from existing research on students’ mechanistic reasoning in chemical contexts indicate that many students fail to build causal mechanistic explanations of targeted phenomena.17−28 Instead, a significant number of them tend to generate correlational accounts in which some properties or activities of the components in a system are associated with each other but not by actual causal links (e.g., ethanol dissolves in water because it is polar, and polar
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FUNDAMENTAL CHEMICAL MECHANISMS A first step in improving student learning in any area involves careful reflection on the central ideas and ways of reasoning that we want students to develop.33 If we value mechanistic reasoning in chemistry, then we should clearly identify the core understandings that we want students to master. In the following sections, I present my personal reflections on this matter. They have emerged from my knowledge, experiences, and pedagogical and philosophical stances as a practitioner and researcher in chemistry education. My goal is to highlight fundamental ways of reasoning that science and engineering majors should comprehend and be able to apply when they complete their chemistry studies. The fundamental chemical mechanisms described in the following sections have been organized into five categories labeled on the basis of the types of processes that the mechanisms help explain or bring about. They include the following: • matter transformation • energy transfer and transformation • activation • stabilization • equilibration The description of these different categories of chemical mechanisms is preceded by a section that summarizes the foundational understandings that are required to effectively and productively apply the selected mechanisms in the construction of chemical rationales in different contexts. Foundational Understandings
Chemical mechanisms are commonly built using models of matter at the submicroscopic level. Understanding the basic assumptions of these models is thus critical to properly engage in mechanistic reasoning in the discipline. Although the models of matter used in chemistry are diverse, they can be organized into three main groups: • particulate models of matter • atomic−molecular models of matter • electronic models of matter Existing educational standards2−4 and content maps34−36 list the central ideas in chemistry that students should comprehend about these three classes of models. In the following paragraphs I summarize what I identify as key understandings that enable mechanistic reasoning using these models. It is not my claim that these foundational understandings should be developed before students can engage in mechanistic reasoning in chemistry. Carefully designed B
DOI: 10.1021/acs.jchemed.8b00508 J. Chem. Educ. XXXX, XXX, XXX−XXX
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Table 1. Rationales for Fundamental Mechanisms Used To Explain Matter Transformation
Connecting the effect of these compositional and structural factors to changes in the kinetic and potential energy of electrons in the particles of matter is a critical skill in building mechanistic rationales.26 In general, the construction of the chemical mechanisms described in this commentary requires that students meaningfully understand how the properties and spatiotemporal organization of relevant model entities (e.g., molecules, atoms, electrons) affect their interactions and the amount and distribution of kinetic and potential energy among different components in a system. Learning to connect compositional and structural features to interparticle interactions and energetic and entropic effects is necessary to make sense of why and how chemical phenomena happen.41 Fundamental mechanisms used in chemistry to build these connections are described in the following sections.
learning progressions can be followed to develop key pieces of foundational knowledge and create opportunities to apply them in the construction of mechanistic accounts in a gradual manner. Existing alternative curricula for introductory chemistry show concrete examples of how this may be accomplished.37−40 Particulate Models of Matter. Students should be able to conceptualize a macroscopic sample of any substance as a collection of myriads of identical submicroscopic particles in constant random motion and interaction through forces whose nature and magnitude depend on the interparticle distance (i.e., repulsive at very short distances, attractive at distances larger than a particle’s size, and decaying in magnitude the greater the separation between particles). It is critical that students understand how interactions between particles affect their kinetic and potential energy (e.g., attractive forces between particles that bring them closer together will increase their kinetic energy and decrease their potential energy). It is also important that students realize that the kinetic and potential energy of the particles in a system can change through interactions with other particles in their surroundings (e.g., particles that make up a container). The relationship between particle interactions and energy transfer and transformation is a foundational piece of knowledge often underdiscussed in chemistry courses with negative consequences for student reasoning. Atomic−Molecular Models of Matter. Explanations of chemical properties and behaviors demand that we assume that the particles that make up different substances are composite entities that differ in atomic composition and structure. What atoms make up a particle, how they are bonded to each other, and how they are arranged in space determine relevant mechanistic properties of these particles (e.g., how they interact with each other). The construction of productive chemical mechanisms often relies on the identification of periodic atomic trends (e.g., atomic size, bonding capacity) that can be used to justify interactions, activities, and the organization of atomic components. Electronic Models of Matter. Atomic properties are explained by invoking the existence of subatomic charged particles, protons and electrons, whose interactions determine observed behaviors. The identification of relationships between the relative distribution of protons and electrons in the particles of matter (charge distribution) and the properties of such particles are key in the elaboration of chemical mechanisms. This demands the recognition of factors that affect charge distribution and their periodic variation, including the number of valence electrons in different atoms, the relative electronegativity of the atoms in a bond, or the presence of structural features that facilitate electron delocalization.
Matter Transformation
This category includes two basic mechanisms used in chemistry to explain changes in material systems that conserve (e.g., phase change) or modify (e.g., chemical reaction) the chemical identity of the substances involved. The argumentative structure of each of these mechanisms is summarized in Table 1. These mechanisms are typically introduced and discussed in foundational chemistry courses. Mechanisms for processes that conserve chemical identity are often built using the particulate model of matter, while those that lead to the formation of new substances rely on atomic−molecular and electronic models. Energy Transfer and Transformation
In this category I include mechanisms applied to make sense of energy transfer and transformation in chemical systems. These mechanisms are summarized in Table 2 and correspond to processes for the absorption and release of energy at the particulate/atomic−molecular and electronic levels. The first pair of mechanisms accounts for the transfer and transformation of thermal energy while the second pair rationalizes the transfer and transformation of light energy. Mechanisms for thermal energy transformation at the particulate level are rarely analyzed and discussed in introductory chemistry courses, which may explain students’ difficulties with understanding energy exchanges during physical and chemical processes. Mechanisms at this level require that students understand that interactions between particles affect their kinetic and potential energies, and that systems can exchange kinetic energy through collisions between components at the system’s boundaries. C
DOI: 10.1021/acs.jchemed.8b00508 J. Chem. Educ. XXXX, XXX, XXX−XXX
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Table 2. Rationales for Fundamental Mechanisms Used To Explain Energy Transfer and Transformation
Table 3. Rationales for Fundamental Mechanisms Used To Explain the Activation of Chemical Processes
Activation
Stabilization
The characterization of mechanisms in this category is based on a broader interpretation of the concept of “activation” than that traditionally presented in conventional chemistry courses. In this essay, activation refers to processes that take entities in a system from lower to higher potential energy states in which they are more likely to undergo a chemical reaction. As summarized in Table 3, I use the label “kinetic activation” for mechanisms in which reactive entities in a system are somehow excited to an unstable transition state where their probability of conversion into products is higher. On the other hand, I chose the label “thermodynamic activation” to refer to mechanisms in which molecules (e.g., ATP), through some type of grouptransfer process, “activate” other species to undergo otherwise unfavorable reactions. References to molecules such as ATP and acetyl-CoA as thermodynamically activated metabolites are now common in the biochemistry and chemical biology research literature.42 I believe the term “thermodynamic activation” is more appropriate than the conventional term of “reaction coupling” for the general mechanism described in Table 3. The reference to “reaction coupling” often generates a misconception in which the underlying mechanism is incorrectly conceived as involving a set of consecutive reactions (e.g., ATP breaks apart and the energy from this process is then used to drive a second reaction) rather than an alternative reaction path.
Different mechanisms are invoked to make sense of the relative stability of a chemical system or state compared to another. Mechanisms built to explain stability using the particulate and atomic−molecular models of matter tend to have a different argumentative structure than those used to rationalize stability using electronic models. The core characteristics of these different subcategories of mechanisms are summarized in Table 4. The mechanisms of energy minimization (energetic stability) and matter−energy dispersion (entropic stability) are often seen as competing with one another in explanations and arguments about the stability of multiparticle systems. Which mechanism is dominant in a system depends on external conditions, such as temperature and pressure. At the electronic level, the term “induction” is used to refer to mechanisms that involve the redistribution of electronic charge in a system under the influence of nearby charges (or partial charges). The term “delocalization” is used in a quite general manner for stabilization mechanisms that refer to the spreading of electrons over a larger space in a chemical system43 (e.g., spreading of electrons over more than one atom or covalent bond). Equilibration
The final set of fundamental mechanisms identified in this essay is summarized in Table 5, and they are used to explain D
DOI: 10.1021/acs.jchemed.8b00508 J. Chem. Educ. XXXX, XXX, XXX−XXX
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Table 4. Rationales for Fundamental Mechanisms Used To Explain System Stabilization
Table 5. Rationales for Fundamental Mechanisms Used To Explain Equilibration in Chemical Systems
and transfer to other domains. It is likely that most of the highlighted mechanisms are introduced and discussed in one way or another in conventional chemistry courses. However, they often get buried under the heavy weight of the vast amount of content that is typically covered. Their role in the curriculum is actually marginal as mechanistic reasoning is commonly sidestepped by students and instructors in the construction of explanations of chemical phenomena. For example, we tend to be satisfied when students explain that a reaction is exothermic because more energy is produced when forming bonds than that required to break bonds, rarely questioning student understanding of the actual mechanisms involved in energy transfer and transformation. Similarly, we are pleased when students justify that carboxylic acids are stronger acids than alcohols because of resonance stability in the conjugate base, infrequently demanding a justification based on the stabilization mechanisms at play at both the
the development, existence, and perturbation of equilibrium states in chemical systems. The kinetic equilibration mechanism is more commonly analyzed and discussed in introductory chemistry courses using the particulate model of matter. The thermodynamic equilibration mechanism can be built with or without reference to a submicroscopic model but is less frequently used in explanations of chemical equilibrium outside physical chemistry courses.
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FINAL COMMENTS Understanding and learning to apply the fundamental chemical mechanisms described in this essay and depicted in Figure 1 would enable students to make sense of a wide variety of phenomena not only in their chemistry courses but also in other disciplines where particulate, atomic−molecular, or electronic models of matter are used to build arguments and explanations. These are the types of understandings to which chemistry educators should aspire for their students to develop E
DOI: 10.1021/acs.jchemed.8b00508 J. Chem. Educ. XXXX, XXX, XXX−XXX
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Notes
The author declares no competing financial interest.
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Figure 1. Fundamental chemical mechanisms.
electronic (delocalization) and particulate (energy minimization and matter−energy dispersion) levels. The sidestepping of mechanistic reasoning in the chemistry classroom is a major educational issue somewhat ingrained in conventional approaches to the teaching of the discipline.44 Reliance on disciplinary heuristics, such as the octet rule, Le Chatelier’s principle, or the “like dissolves like” rule, to build explanations is pervasive in chemistry and difficult to overcome in educational settings.32,45 Similarly, correlations between compositional and structural variables and properties of chemical entities (e.g., periodic trends) are too often taken as satisfactory causal accounts (e.g., Br− is a weaker base than Cl− because larger anions are more stable). These issues point to the need for engaging chemistry teachers and instructors in in-depth discussion and reflection about different modes of reasoning in chemistry,29 their scope and limitations, and on how to create learning environments in which mechanistic reasoning is encouraged rather than sidestepped. Work in science education in the past 20 years has led to the identification of instructional models that support the development of students’ abilities to build mechanistic explanations.1,46 In these educational approaches, students work collaboratively in the construction, application, evaluation, and revision of models to make sense of relevant phenomena or solve authentic problems. Students’ thinking is made visible and public on an ongoing basis to enable formative assessment and to press students to construct evidence-based mechanistic explanations.47 This type of instruction demands changes in assessment practices to better explore the extent to which students can integrate central ideas in a discipline to generate mechanistic accounts. Guidelines exist for the development of assessments that better reveal students’ abilities to build explanations,29,48,49 but more specific exemplars are needed to illustrate how to effectively assess mechanistic reasoning in chemistry.
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REFERENCES
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Vicente Talanquer: 0000-0002-5737-3313 F
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