Does it occur or not? – A structured approach to support students in determining the spontaneity of chemical reactions

2.1 Problem analysis

Chemical reactions are processes in which substances are transformed into new substances with different properties (e.g., Brown et al., 2022; Chang & Goldsby, 2013; Tro, 2018). These transformation processes can proceed at different speed and occur either on its own without any external intervention or be forced to, e.g., by supplying energy or manipulating pressure-conditions (e.g., Brown et al., 2022; Lucas, 2008; Mortimer, 2008). The “voluntary” chemical processes are called spontaneous, although spontaneity refers to only one direction (“natural direction”). The reverse chemical reaction is nonspontaneous and must be forced, if requested. Whether a chemical reaction does or does not occur spontaneously is mainly driven by changes in two thermodynamic quantities, enthalpy and entropy (e.g., Atkins et al., 2019; Brown et al., 2022; Engel & Reid, 2021; Frenking & Shaik, 2014; Klotz & Rosenberg, 2008; Mortimer, 2008).

Reviewing the existing body of literature, two factors are broadly found to influence students’ success in thermodynamics and physical chemistry in general: mathematical proficiency and problem-solving abilities (Bain et al., 2014; Becker & Towns, 2012; Derrick & Derrick, 2002; Hahn & Polik, 2004; Nicoll & Francisco, 2001; Tsaparlis, 2007). This is not surprising as thermodynamics lessons are mostly mathematic-loaden and characterised by physical concepts, even in chemistry education (Goedhart & Kaper, 2002; Nilsson & Niedderer, 2014). However, even if thermodynamics lessons are designed more “chemical”, the topic is still highly demanding for students who need to apply a huge amount of basic chemical knowledge. Analysing chemical processes from the perspective of thermodynamics requires students to link observations at the macroscopic level with descriptions at the particle level mostly including mathematical formula and other symbolic representations (Anderson et al., 2005; Cooper et al., 2014; Goedhart & Kaper, 2002; Nilsson & Niedderer, 2014). According to Cooper et al. (2014), chemical thermodynamics and especially the concept of energy is discussed by referring to three different perspectives: macroscopic, atomic-molecular, and quantum-mechanical. However, there is often little to no connection between these perspectives, hindering students from developing a comprehensive understanding of energy changes associated with chemical processes (Cooper et al., 2014).

In the following sections, we present the chemical background the developed material relies on by integrating all perspectives, macroscopic, atomic-molecular, and quantum-mechanical. Completing the problem analysis, we present an overview of students’ conceptions and challenges in the field of chemical thermodynamics.

2.2 Design of the material

Taking up these issues, we aimed at developing “unmathematical” instructional material (as suggested by Carson & Watson, 2002) that supports students in determining the spontaneity of chemical reactions. The material should be suitable for student-centred lessons at the upper secondary level and meet the following requirements:

1. connecting thermodynamics to basic chemical knowledge,

2. relying on qualitative descriptions rather than on mathematical formulas,

3. being thoroughly structured and including supportive measures.

Considering these requirements, we expected the material to elicit students’ prior knowledge and to support them in applying it. In this way, we aimed at fostering students’ conceptual understanding and enable them to experience themselves as competent to complete tasks in the field of thermodynamics and physical chemistry (as usually mathematic loaden topics) in general. For this purpose, the instructional material was planned to consist of several, sequential parts that follow the order of the analytical process for determining the spontaneity of chemical reactions based on the analysis of the properties on reactants and products. Accordingly, after an introductory part (Task 1), the bonding properties are to be analysed, from which qualitative predictions about the enthalpy change can be made (Task 2). By analysing the particle structure of reactants and products, predictions of the entropy changes can be made (Task 3), which are to be combined with the results of Task 2 and finally be applied to the Gibbs-Helmholtz equation to determine the spontaneity of the respective chemical reaction (Task 4).

To develop material that fosters students’ conceptual understanding about chemical thermodynamics on the one hand and that is manageable at the upper secondary level at the other hand (e.g., Le Maréchal & El Bilani, 2008; NRC, 2012), the underlying chemical background needed to be reduced and simplified according the approach of educational reconstruction (Duit et al., 2012). Moreover, care was taken to incorporate all perspectives (macroscopic, atomic-molecular, and quantum-mechanical) in the material as suggested by Cooper et al. (2014). For this purpose, the three main types of chemical bonding were considered being separable, neglecting that no definite bounds exist between them.

Since qualitative estimations were aimed at, number and position of bonds broken (in molecular compounds) were ignored. Instead, the stability of the entire molecules was determined. Other disregarded aspects were intermolecular forces, physical side effects, such as solubility effects or enthalpies of solution or vaporisation, and the distinction between water formed as liquid or gaseous product. Furthermore, we did not address that Gibbs free energy is only a limited criterion for the spontaneity of chemical reactions including solutions and mixtures (due to effects of concentration/ratio), gaseous reactants (due to effects of pressure) and reactions in which both reactants and products are present in the initial state of the reaction (cf. Section 2.1.4). Concerning the concept of entropy, we decided to refrain from using the term “disorder” as this analogy has been found to cause inappropriate conceptions (Bucy et al., 2007; Bennett & Sözbilir, 2007; Lambert, 2002a). Instead, we consistently used “possible number of microstates” to characterize entropy (cf. Section 2.1.3).

The overall instructional goal for the material was set to help students understand that they can make thermodynamic predictions by analysing reactants and products of the reaction. Since several studies have identified tasks in the field of chemical thermodynamics as complex and overchallenging to many students (e.g., Nilsson & Niedderer, 2014; Tsaparlis, 2007), it was decided to provide instruction as concrete and precise as possible.

First of all, the multi-step thinking process was broken down into individual analysis steps that are formulated as several tasks and respective sub-items. To prevent students from cognitive overload, the tasks are formulated in the form of explicit instruction supported by scaffolding measures (e.g., Hammond & Gibbons, 2005; Hmelo-Silver et al., 2007; Quintana et al., 2004). Determining the spontaneity of chemical reactions by analysing the properties of reactants and products requires students to apply a huge amount of (already known) chemical concepts. Therefore, conceptual scaffolds providing essential knowledge (e.g., definitions, terminology, or relations) were developed to support students to recall their prior knowledge and strategies and to connect them with the concepts and procedures addressed in the task (Quintana et al., 2004). When implementing these scaffolding measures both implicitly (structure and instruction of the task) and explicitly (supportive material), the developed material is considered a “well-structured problem” (Jonassen, 2000) that aims at familiarising students with a possible procedure – an analysis scheme – they can then apply independently to a wide variety of reactions.

According to empirical findings, worked-out examples are appropriate to support students in working on well-structured problems that address demanding contents and complex analysis processes (Atkinson et al., 2000; Jonassen, 2011; Renkl & Atkinson, 2010). Thus, selected worked-out examples that enable students to reconstruct the problem-solving process were added to the task. Furthermore, the worked-out examples were formatted similar to reaction equations in order to provide a consistent structure throughout the entire task. In this way, the information required (e.g., type of bonding, stability of bond) can be directly assigned to reactants and products, which is expected to facilitate determining the changes in the course of the reaction.

From the perspective of practicing teachers, it was also important to us that the material is flexible for adaption and allows for differentiation. For this reason, the reactions were not directly integrated into the task, but were placed separately at the beginning so that they can be easily replaced without having to change the entire task. Varying both the number and type of reactions to be worked on and providing or withholding the scaffolds and worked-out examples allows teachers to adapt the task according to content focus and level of difficulty (differentiation by content; Taylor, 2015).

2.3 Implementation

To prove the developed material regarding its appropriateness and applicability, we implemented it for several times, as designated in DBR approaches (DBR Collective, 2003). Overall, students from more than ten upper secondary chemistry classes (grades 10/11) and four chemistry teacher education courses (6th semester of Bachelor programme) worked on the material.

During the material implementation, we collected information for revising the material in the form of lesson observation primarily (Henning et al., 2009; Knoblauch, 2019). In doing so, we documented and collected our experiences for the joint reflection and re-design by paying particular attention to the following aspects (Knoblauch, 2019):

1. Comprehensibility and clarity of the instruction: At what points in the task do students not know what to do? At what points in the task are students asking for clarification? Which terms and instructional phrases result in inquiries?

2. Application of chemical concepts and practices: Which concepts and practices cause difficulties in application? For which concepts do students need to refer to their textbook, individual notes, supportive materials or instruction by the teacher? What chemistry-specific practices are students not familiar with?

3. Progress in problem-solving: At what points in the task do students get stuck and for what reasons? At what points, for what purpose, and how often do students use the supportive material or ask for support by the teacher?

4. Suitability for students’ independent work: Do students work (collaboratively) in a task-oriented manner? How long does it take students to complete the task? Do students use the worked-out examples to assess their results and in what ways?

Complementing the lesson observation, we held informal discussions with students and integrated tasks similar to the instructional material as part of the usual exams and/or assessments. In so doing, we strived for a kind of summative assessment (Dixson & Worrell, 2016) about students’ abilities to make qualitative predictions about the spontaneity of a selected chemical reaction after having worked with the developed instructional material in both chemistry lessons and exam preparation. The outcomes of these tasks were corrected and analysed regarding common errors and problems.

The experiences gained from implementation are summarised in the following two sections, with Section 2.4 providing a general overview, before illustrating some exemplary challenges and how they were addressed when revising the material in Section 2.5.

2.4 Reflection

Having implemented the developed material for several times resulted in an increased amount of practical experience regarding the challenges of the task and the content addressed. We could identify aspects that were especially demanding for students and needed to be supported by intensive scaffolding both material-based and in person.

First of all, implementing the material revealed major differences in students’ chemical knowledge and skills they were asked to apply when working on the task. This was confirmed in each implementation cycle, regardless of whether students were in their first year of secondary chemistry education or nearly at the end of their Bachelor programme. In one upper secondary chemistry class, for example, there were some students who successfully completed tasks 1 and 2 without any further support, others were already struggling with generating the chemical formulas and determining the main types of chemical bonding. This again confirmed that immense differences in knowledge and skills can arise even after a few months in chemistry class. Thus, scaffolding measures are essential to enable students to work in their zone of proximal development (Vygotsky, 1978). To address students’ individual needs and promote their learning in the best possible way, the material-based scaffolding has shown to be indispensable. Only in this way, the teacher can allow for individual learning paths and handle students’ diverse capabilities and needs. As shown in the implementation, students were able to work independently on the material if they knew how to purposefully use supportive material, such as information cards or worked-out examples. One of the secondary chemistry students stated: “It was very difficult at the beginning. (…) But the worked-out examples helped me a lot.”

In general, working on the material was perceived as a demanding but “fruitful” experience. Many of the students – even the student teachers – were basically familiar with the required concepts, however, they did not have all the concepts ready at the same time. One student mentioned that “each task in itself was actually not that difficult” as similar tasks had been practiced in former chemistry lessons. However, it was found “challenging to remember so many different concepts and apply it all at the same time”. Another one emphasised the sequential structure of chemical subject-matter and concluded that the material “requires you to apply everything that you have learned before”. In this regard, the students gave the feedback that after a first, exhausting run, the “hardest part had been overcome” and that proceeding the tasks would become easier with each further chemical reaction to be worked on. They went on to say that “it [the material] shows you exactly what you can and cannot do of all this”, which was seen as “quite disillusioning”. However, it was also concluded that working on the material would make one not only to learn about chemical thermodynamics, but also to recall numerous basic chemical concepts. One student even stated that they were “a bit proud” when having completed the task successfully, because “then you know that you understand a large part of chemistry”.

Also, many of the student teachers gave the feedback that they would not have dealt with chemical thermodynamics in such a fundamental way until then. This was highlighted by the following statement made by one student teacher in their 6th semester of Bachelor degree: “Until just now, I didn’t know that you can really derive it [enthalpy change] from the properties of reactants and products.”

2.5 Re-design

In the course of several iterative DBR-cycles, a variety of revisions was made. While the basic structure of the material (four tasks addressing introduction, enthalpy changes, entropy changes, and changes in Gibbs free energy) essentially remained the same throughout the entire project, a number of changes were made in the structure, wording, and formatting of the tasks as well as the information included in the supportive material. In addition, the worked-out examples were even more closely aligned with the wording of the instructional sheet. In the following, we provide an insight into the reflection and re-design processes by going into some selected revisions.

It became apparent in several occasions that students had particular difficulty with tasks including a varying “number of possible answers”, e.g., gradations in bond polarity and bond stability, or energy content of reactants and products (Task 2). Therefore, in initial revisions it was explicitly stated where to differentiate into gradations. In the further course, we included the graduated options in the supportive material. Finally, the task was formulated in a way that there is one “correct answer”, which is additionally concretised by answer options in some parts. These options are directly integrated into the instruction and are visually highlighted (formatting in boxes) to be recognisable to the students (see Figure 2 in Section 3.1). Furthermore, the entire material was revised to make the wording more “relative” (e.g., the change in entropy rather promotes the spontaneity of the reaction) so that it becomes clear that the analyses provide trends and qualitative statements and do not allow any definitive statements or concrete values.

Figure 2:

Excerpt from the step-by-step instruction of task 2.

Concerning the chemical concepts and practices, students’ needs and difficulties were greatly diverse – also caused by absences in former lessons. However, some problems occurred frequently, e.g., with generating the chemical formula of element molecules (e.g., N and Cl instead of N ₂ and Cl ₂). For this reason, we added an information card (listing those elements usually occurring as diatomic molecules) to the supportive material.

Another problem arose with determining the polarities of covalent bonds. Several students conflated the polarity of single bonds with the polarity of the entire molecule. This was addressed in one hint added to the supportive material. Further, students struggled with the inverse relationship between stability and energy, which might be caused by alternative conceptions that are well known from the literature (e.g., Bain et al., 2014; see Section 2.1.5). For this reason, analysing this challenging relationship was split into three separate subtasks (bonding type, stability, energy content). Moreover, we included both information and hint cards that explicitly address this inverse relationship, e.g., by providing guiding principles, such as “The more stable the bond, the lower the energy content of the compound”.

Overall, it turned out that applying many chemical concepts and practices at the same time was completely overwhelming for some students. Since the material aims at determining the spontaneity of chemical reactions (and not, e.g., at generating chemical formulas or formulating and balancing reaction equations), providing not only conceptual, but also procedural and strategic scaffolds (e.g., Belland et al., 2017; Hmelo-Silver et al., 2007; van de Pol et al., 2010) has proven to be indispensable. For this purpose, we added references to the textbook (e.g., for chemical formulas) and an online tool (“online balancer”) for balancing chemical equations. Additionally, the supportive material was assigned to the respective tasks and categorised into two groups: information (including definitions, concepts, relationships) and hints (addressing problem solving strategies or further measures of support). After the material had become increasingly extensive, differentiating between information and hints made it easier for both students and teachers to orientate themselves through the supportive material.

3.1 Task & instruction sheet

The instructional material is structured in such a way that there is a general work order consisting of four tasks, which then have to be processed for different chemical reactions (e.g., reaction of sodium and chlorine to sodium chloride, oxyhydrogen reaction, decomposition of silver(I) oxide, burning of ethanol).

Task 1 constitutes the introduction to the respective chemical reaction. Students are asked to reflect on their general knowledge on the specific reaction and where it appears, write down names and formula of reactants and products and formulate the stoichiometrically balanced chemical equation. If students have no prior knowledge on the respective reaction, they can still proceed with the task.

In task 2, students make qualitative predictions about the enthalpy change that occurs during the reaction. For this purpose, they follow a step-by-step instruction and process the following aspects successively (see Figure 2): (a) determining bond types, (b) estimating stability of the bonds, (c) estimating the energy content of the compound, (d) applying the values to conclude ∆H _rxn , (e) deciding whether the reaction is exothermic, endothermic or no statement is possible and (f) hypothesising whether the change in enthalpy would rather promote or hinder the spontaneity of the reaction.

Task 3 deals with entropy change and is designed analogously to task 2. In the first step, students are asked to analyse in what state of matter the reactants and products exist under normal temperature (25 °C/298,15 K) and pressure (1 atm) and complete the chemical equation by adding the indices for the states of matter (s, l, g). Then, they analyse the reactants and products regarding the following two aspects: changes in lattice structures and gaseous compounds (3b) and number of gaseous molecules (3c). Considering results from these analyses, students decide whether the entropy is increasing or decreasing throughout the reaction and qualitatively estimate the value of ∆S _rxn . Analogous to task 2, students finally make a statement concerning the consequence of ∆S _rxn for the spontaneity of the chemical reaction.

In task 4, students have to combine their results from tasks 2 and 3 and consider the relationship described by the Gibbs-Helmholtz-Equation. For this purpose, they are supposed to recollect how both enthalpy change and entropy change affect the spontaneity of the reaction. Substituting the estimated values for ∆H _rxn and ∆S _rxn into the Gibbs-Helmholtz-Equation, students are asked to make a qualitative prediction about the change in Gibbs free energy (∆G _rxn ) and determine the potential impact of the temperature. Finally, it has to be concluded whether the analysed reaction would be spontaneous or nonspontaneous, or if the spontaneity is influenced significantly by the temperature.

3.2 Worked-out examples & supportive material

Each task is supported by information cards and hints for processing the task. The information cards address aspects that should have already been covered in previous lessons, such as types of chemical bonding or the definition of exothermic and endothermic reactions. Students who do not remember this information can use the cards to recall it. The hint cards include concrete solution strategies to proceed with some of the tasks or indicate further measures of support. Figure 3 shows an example of an information and hint card each.

Figure 3:

Examples for information and hint cards.

In addition, worked-out examples for each reaction are provided to the students. For one thing, these examples can be used to check the results and for another thing, they can serve as guide for working on the tasks. An excerpt from the worked-out example for reaction 3, the decomposition of silver(I) oxide, can be found in Figure 4, two complete worked-out examples (R3 – thermal decomposition of silver(I) oxide and R4 – combustion of ethanol) can be found in the Supplementary Material.

Figure 4:

Excerpt from the worked-out example of R3 – decomposition of silver(I) oxide.

4.1 Recommendations for implementation

To allow students to work through all tasks for at least one of the listed reactions, a minimum of two lessons should be assessed. Depending on content focus, the material can be processed in various ways. One possibility is to complete tasks 1 and 2 for several chemical reactions and then work on tasks 3 and 4. In so doing, emphasis is put on the basic principle of enthalpy change and exothermic and endothermic reactions (see Sections 2.1.1 and 2.1.2). Changes in entropy and Gibbs free enthalpy (see Sections 2.1.1 and 2.1.2) can then be seen as optional elaboration for the purpose of content deepening or extension. Another approach is to work through all tasks in one go, explicating the connection between the change of Gibbs free energy and the changes of enthalpy and entropy. Of course, students can work on the tasks also in different sequences depending on their prior knowledge, skills and needs and the content level they aim for. However, to understand how a statement about the spontaneity of a chemical reaction can be made from analysing properties of reactants and products, students should work through task 1 to task 4 at least once. Alternatively, they can reflect on the worked-out examples and comprehend the applied strategies.

To enable students to work successfully on the material, the underlying theoretical concepts (chemical reactions, reaction equations, models of chemical bonding, energy of bonds, bond polarity etc.) need to be addressed in preceding chemistry lessons. For one thing, students must be basically familiar with these concepts and for another thing, they also need to be used to applying their knowledge and skills independently. Only then is it possible for the teacher to support students by providing scaffolding adapted to students’ individual needs. Depending on prior experience with scaffolds, students may rely on these scaffolds independently or the teacher may provide them as needed. Some of the scaffolds (in particular hints for tasks 1 and 4) can also be applied in the sense of incremental scaffolds (Schmidt-Weigand et al., 2009).

Beyond that, it is also recommended to make clear what simplifications and limitations have been made in the material and to discuss what impact these simplifications and limitations have on the validity of the results. In any case, it must be clear that the results are to be understood only as tendencies and qualitative predictions rather than as exact results.