Deborah G.
Herrington
*a,
Shanna M.
Hilborn
b,
Elizabeth N.
Sielaff
a and
Ryan D.
Sweeder
b
aDepartment of Chemistry, Grand Valley State University, 312 Padnos Hall, Allendale, MI 49401, USA. E-mail: herringd@gvsu.edu
bLyman Briggs College, Michigan State University, 919 E. Shaw Ln, East Lansing, MI 48825, USA
First published on 18th April 2022
Equilibrium is a challenging concept for many, largely because developing a deep conceptual understanding of equilibrium requires someone to be able to connect the motions and interactions of particles that cannot be physically observed with macroscopic observations. Particle level chemistry animations and simulations can support student connections of particle motion with macroscopic observations, but for topics such as equilibrium additional visuals such as graphs are typically present which add additional complexity. Helping students make sense of such visuals requires careful scaffolding to draw their attention to important features and help them make connections between representations (e.g., particle motion and graphical representations). Further, as students enter our classrooms with varying levels of background understanding, they may require more or less time working with such simulations or animations to develop the desired level of conceptual understanding. This paper describes the development and testing of activities that use the PhET simulation “Reactions and Rates” to introduce the concept of equilibrium as a student preclass activity either in the form of directly using the simulation or guided by an instructor through a screencast. The pre-post analysis of the two most recent implementations of these activities indicates that students show improved understanding of the core ideas underlying equilibrium regardless of instructor, institution, or type of instructional environment (face to face or remote). We also observed that students were more readily able to provide particle level explanations of changes in equilibrium systems as they respond to stresses (such as changes to concentration and temperature) if they have had prior course instruction on collision theory. Lastly, we observed that student answers to explain how an equilibrium will respond to an applied stress more often focus on either initial responses or longer-term stability of concentrations, not on both key aspects.
This paper focuses on the development and evaluation of activities designed to actively engage students in the construction of a conceptual understanding of equilibrium systems and to be completed by students outside of the classroom prior to any formal in-class instruction. These activities are designed to provide students with an introduction to dynamic equilibrium systems by scaffolding their interactions with a particle-level simulation. Equilibrium is typically a challenging concept in chemistry (Bergquist and Heikkinen, 1990; Cheung, 2009) and providing students an opportunity to engage with this concept outside of the classroom allows them to work through the material at their own pace as well as provides a common experience that instructors can build on during in class instruction. However, appropriate scaffolding is critical as students interacting with complex simulations on their own can result in them missing or misinterpreting key elements of the simulation (Hegarty, 2004). Thus, an alternative to students’ independent interactions with the simulation is students viewing a screencast in which an instructor manipulates the simulation and provides some narration to highlight key elements of the simulation for students (Herrington et al., 2017; Martinez et al., 2021). As there are potential benefits and challenges to both independent simulation manipulation and watching of a screencast (Mayer and Moreno, 2003; Hegarty, 2004; Keehner et al., 2008), we aim to identify the key challenges that students have in using these activities outside of class to develop a particle-level understanding of equilibrium, as well as any differences that exist between students who use the simulation compared with those who watch the screencast. Further, as we were in the middle of this study when the COVID-19 hit, we were able to compare student learning gains using these materials to support a face-to-face class as compared to their use in a class that had transitioned to emergency remote instruction, and to determine how the necessary modifications of some questions when switching to a fully online delivery impacted information obtained from student responses.
One way to help students visualize abstract chemical concepts at the particulate level is through the use of animations and simulations (Nakhleh, 1992; Sanger et al., 2000; Kelly and Jones, 2007). Viewing particle motion and interactions involved in chemical phenomena have been successful in helping students develop a better understanding of chemical equilibrium (Akaygun and Jones, 2014; Ganasen and Shamuganathan, 2017), and are most useful when they are short, visually appealing, and cover material at the desired specificity and context (Burke et al., 1998; Suits and Sanger, 2013). As chemical equilibrium is one of the more difficult concepts to master and has applications in several other chemistry concepts (Bergquist and Heikkinen, 1990), it is critical that students develop scientifically accurate mental models, and simulations may be a powerful pedagogical tool in supporting this development.
Something else that may contribute to students’ and instructors’ difficulty with equilibrium is the overreliance on Le Chatelier's principle (LCP) to explain changes in equilibrium systems (Cheung, 2009). Using LCP to predict or justify shifts in an equilibrium system resulting from external stresses (such as changes to concentration and temperature), without an underlying understanding of how such stresses affect the concentrations and interactions of the particles, is akin to applying an algorithm to solve a problem without understanding why. Many curricula rely heavily on Le Chatelier's principle to explain how an equilibrium system responds to stresses, as opposed to collision theory or the reaction quotient, the basis for particle interactions and concentration (Cheung, 2009).
When simulations or animations are used in class it is possible for the instructor to direct student attention to the more salient aspects of the animation or simulation. However, in such cases, students are limited in the amount of time they can spend observing the particle interactions and identifying patterns that can help them construct a coherent particle level understanding of a concept. As students enter our classrooms with very different backgrounds, it is reasonable to assume that they may require differing amounts of time watching or interacting with an animation or simulation to develop the desired mental model. When using such materials outside of class, students can spend as much time as they need interacting with the simulation or re-watching parts of the screencast (Keengwe and Kidd, 2010).
We have previously reported that similar activities addressing other content areas where strong conceptual understanding requires accurate particle level models have resulted in significant learning gains with the screencast typically showing equal (Sweeder et al., 2019) or better results (Herrington et al., 2017; Martinez et al., 2021; VandenPlas et al., 2021). Since these activities are completed outside of class by the students, they also provide a common learning experience that the instructor can build upon in the classroom.
(1) What differences exist between students who use the simulation on their own as compared to those who view a narrated screencast of the simulation being used to investigate the concept of equilibrium?
(2) What are the major challenges students have in developing a particle level understanding of equilibrium?
(3) What learning differences are seen in the use of the developed simulation or screencast equilibrium materials to support in-person versus remote instruction?
(1) accurately represents aspects of particle-level interactions (a challenge for many students);
(2) focuses on common general chemistry learning objectives; and
(3) provides a connection between the particle-level and some additional level of representation (e.g., graphical or macroscopic).
Making connections between representational levels is very important in developing a deep conceptual knowledge (Talanquer, 2011; Taber, 2013); however, this additional level of complexity makes it more difficult for students to make important connections on their own, thus necessitating quality scaffolding (Vygotsky, 1980). The identified PhET simulation (Reactions & Rates, 2013) not only met these criteria, but was particularly attractive as it was used for the development of a related kinetics activity (Sweeder et al., 2019), which meant that many students had previously used this simulation to explore a different concept.
For equilibrium, the following learning objectives were identified.
Students should be able to:
(1) Identify when a reaction reaches equilibrium by finding the point when the concentration of reactants and products remain relatively constant.
(2) Explain that equilibrium is a dynamic process. That the forward and reverse reactions are constantly occurring, but that equilibrium occurs when those two rates are equal.
(3) Predict and explain how adding or removing reactants or products will influence the equilibrium system to achieve a new equilibrium state by altering the rate of the forward or reverse reactions.
(4) Describe how and why temperature affects the equilibrium position of endo- and exothermic reactions differently.
Before designing the activities, we identified suitable assessments that would allow students to demonstrate if they had achieved the learning objective. These questions were used to create a pretest and an analogous set of follow-up questions for the purpose of measuring student learning gains (Table 1). These assessment questions guided how we would want students to interact with the simulations and the observations that students would need to make to construct the desired understanding. This provided the foundation for developing the scaffolded instructions and questions embedded in the simulation assignment. We then used the simulation assignment as a script for the screencast in which an instructor provided narrated interactions with the simulation that students used to answer the same embedded assignment questions as the students who interacted with the simulation on their own. Although the instructor helped to focus student attention through their actions, they strove to avoid interpreting what the students were observing, instead leaving students to do this as they answered the questions.
Data were collected through several implementations. For all data collections, students completed a pretest and then their assignment outside of class prior to formal in-class discussion of the topic of equilibrium. However, the broad idea of equilibrium had been mentioned in previous relevant topics such as vapor pressure of liquids and colligative properties in solutions. For the initial data collection round (N ≈ 50), analysis of student responses resulted in reworking some questions to better align the pretest and follow-up questions and ensure that the assignment prompts were effectively supporting students in their learning following the revision process outlined in Fig. 1. The revised pretest and assignments were used in two subsequent semesters at the two different institutions (N = 243 students), which again led to some additional minor revisions to address student learning challenges. This third version provides the basis for the pre-COVID data analysis used in this study and was deployed in five classes at the two institutions (Table 2). This data collection was completed just before the institutions halted in-person classes as a result of the COVID-19 pandemic. All students completed the pretest in class and then completed the assignment (screencast or simulation) including the embedded assignment and follow-up questions outside of class (for full assignment see Appendix A).
Version | Semester | Implementation | N (institutions/classes/studentsa) |
---|---|---|---|
a For students the number represents those who provided consent for use of their data for research purposes and fully completed both the pretest and assignment activities. | |||
3 | Spring 2020 | Pencil and paper; Simulation and Screencast | 2/5/337 |
4 | Spring 2021 | Online data collection via Google Forms; Screencast only; Revised Question 3 (Table 1) | 1/3/215 |
The change to remote instruction for Spring 2021 provided an opportunity to directly compare the use of these materials to support in-person versus remote instruction. To do this, the screencast assignment was converted to an online-only format (Version 4) using Google Forms (2018). Although we could keep most of the questions nearly identical in this different format, one of the pretest and follow-up questions (Question 3 in Table 1) had to be modified. Since collecting images of drawings online had resulted in lower levels of participation in other studies (Sweeder and Herrington, 2020), we modified the question to have students select which graph illustrated how the concentrations of species would change over time, and then provide an explanation as to why they chose that graph. The multiple-choice options were derived from the most commonly drawn student responses for the paper and pencil version of the assessment. This implementation happened at a single institution across three classes (Table 2). Pre and posttest questions, along with how they were scored for pre-post analysis, are shown in Table 1. Full details of the scoring rubric can be found in Appendix B.
(I) Starting the simulation reaction with the numbers of reactant and product particles shown below (specific starting number of particles for each reactant and product species provided):
(a) At the beginning of the experiment, is the rate of the forward reaction (forming products) faster/slower/the same as (circle one) the rate of the reverse reaction (forming reactants)? What evidence from the simulation supports your conclusion?
(b) Use particle collisions (collision theory) to explain why the rates of the forward and reverse reaction would initially be different.
(c) In general, after the reaction has occurred for a while, is the rate of the forward reaction faster than/slower than/the same as (circle one) the rate of reverse reaction?
(d) Explain why this relationship between the forward and reverse reactions would make sense based on the idea of Collision Theory and concentrations of products and reactants.
For this set of questions, we first looked at whether students could correctly answer part a, which in all cases was at least 70% of the students. If correct, we then coded their responses to the subsequent questions. Open qualitative coding of these responses indicated answers fell into one of three general categories: (i) correct use of particle collisions to explain relative rates; (ii) used particle collisions, but did not explain relative rates; (iii) did not use particle collisions to explain relative rates.
Additionally, several questions were asked where students had to observe or predict what would happen to the concentrations of reactants and products (increase/decrease/stay the same) in an equilibrium system when a stress such as adding/removing a substance or changing temperature was applied. For these questions we identified if students provided answer sets that were internally consistent with reactions at a particulate level. For example, if students have a particle level understanding of a chemical equilibrium, then they should understand that it is impossible for the concentrations of both the reactants and products to increase if the temperature changes. For both the 2020 and 2021 data we looked at consistency for responses to question 4 on the pre/post-test (Table 1) and three of the assignment questions that required similar reasoning. Additionally, in moving to the all online administration of the assignment in 2021, we revised the question shown below from one that was more open (which side of the reaction will show an increase in the number of molecules when we decrease the temperature? – and students write in an answer) to one that forced student choices to (increased/decreased/stayed the same). This gave us an additional two questions for comparisons of consistency in the 2021 data (shown below as question II).
(II) Based on your observations from the simulation, after lowering the temperature and allowing the molecules to reach a relatively stable distribution again, note what happens to the concentration of the reactants and products:
(a) For an endothermic reaction?
(i) Reactant concentrations (increased/decreased/stayed the same)
(ii) Product concentrations (increased/decreased/stayed the same)
(b) For an exothermic reaction?
(i) Reactant concentrations (increased/decreased/stayed the same)
(ii) Product concentrations (increased/decreased/stayed the same)
(c) How does Collision Theory help explain your answers to parts a and b?
As part c to question II above also asked student for a particle level explanation, we initially tried to qualitatively code student responses to this question as we had above with question I; however, few students made the connection to activation energy and instead most attempted explanations focusing on temperature impacting the kinetic energy of the particles or the connection of energy associated with bond breaking and forming.
Graph selected | Pretest % selected (correct reasoning) | Posttest % selected (correct reasoning) |
---|---|---|
a | 20 | 22 |
b | 14 | 14 |
c (correct) | 34 (17) | 44 (28) |
d | 32 | 20 |
Institution – treatment | 1 – sim | 1 – screencast | 2 – sim | 2 – screencast | 1 – 2021 |
---|---|---|---|---|---|
Total number of students | 62 | 94 | 46 | 45 | 214 |
Correct answer to part a (%) | 89 | 70 | 78 | 71 | 81 |
(i) Correct use of collisions | 54 | 68 | 36 | 44 | 54 |
(ii) Use of collisions | 20 | 6 | 17 | 19 | 31 |
(iii) No use of collisions | 25 | 26 | 47 | 38 | 15 |
To determine if student answers maintained internal consistency at a particulate level when considering the response of an equilibrium system to stress we examined six different questions. Each question required the students to observe or predict what would happen to the concentrations of reactants and products (increase/decrease/stay the same) in a given equilibrium system when a stress such as adding/removing a substance or changing temperature was applied. If students have a particle level understanding of chemical processes, their answers should always reflect that if reactants are consumed by a reaction, that the quantity of products should increase or vice versa. Any answer where students claim both the reactants and products increase is not possible. For each of the (up to) six different question sets of this predictive format that the students encountered across the pretest, assignment, and follow up questions, we checked the set of student response for internal consistency. In any case where the question indicated that one specific compound was changed, we excluded the student response about that specific compound. For example, if compound A was removed from an equilibrium system, we ignored any response about the concentration of A. This is because it may not be clear if the students were considering the final equilibrium relative to the initial state or if the students is considering the final equilibrium relative to the moment immediately after the A was removed from the system. This meant that for some questions, students had to provide consistent answers across three question parts, whereas some questions looked at reactants or products as a set, so only two question parts were required to be consistent. These data are summarized below in Table 5.
Question (# of question parts to be consistent) | Provided chemical viable answer | |
---|---|---|
2020 (all students) N = 149 | 2021 N = 214 | |
a Questions were not asked with forced responses so data is inconsistent. | ||
Pretest 4 – endothermic rxn (3) | 68% | 57% |
Assignment – predict (add product) (3) | 78% | 68% |
Assignment – predict (remove reactant) (3) | 68% | 60% |
Assignment – observe (cool endothermic rxn) (2) | 90% | |
Assignment – observe (cool exothermic rxn) (2) | 85% | |
Posttest 4 – exothermic rxn (3) | 83% | 77% |
First, instructional content order can influence student learning. Our data suggest that meaningfully learning about collision theory prior to engaging with the activity notably increased students’ ability to explain shifts in equilibrium using particle level reasoning. Student responses to question I (Table 4), show that many of the students were able to correctly use Collision Theory and the number of collisions to explain the relative rates of the forward and reverse reactions with 36–68% of any class giving a correct explanation. However, what is striking about these data is the disparity in the rates between the two different institutions. Here there is a statistically significant difference between the two groups (58% vs. 40% correct, z = 1.74, p = 0.041). We hypothesize that this difference arises from a difference in the order of content between the courses at the two institutions. At Institution 1, kinetics and collision theory are introduced prior to this activity involving equilibrium. However, at Institution 2, kinetics and collision theory are encountered in more depth after equilibrium. Recognizing this difference and the need for students to use collision theory to explain equilibrium at the particle level, when the activity was administered at Institution 2 we included a short introductory video and a few related questions on collision theory, focusing on the fact that reactions occur as a result of particle collisions. Unsurprisingly, we see statistically higher rates of the correct use of collisions in explanations by students at Institution 1 who (presumably) had more comfort with that content having learned and used it previously in the context of explaining kinetics concepts. Ensuring that collision theory is deeply addressed prior to equilibrium may be key in addressing a reliance on Le Chatelier's Principle for explaining equilibrium (Cheung, 2009) by ensuring that they have the potential to construct a particle level explanation.
The second takeaway was that many 2nd semester general chemistry students do not inherently consider the consistency of sets of related answers when it comes to mass balance and chemical reactions. A particle level understanding of chemical reactions and the Law of Conservation of Mass necessitates that as an equilibrium system responds to a stress, either reactants are increased and products decreased, or vice versa. Yet, as is evidenced by our data, internal consistency of their answers is not yet an automatic consideration for all second semester general chemistry students. In looking at their answers to a number of different questions that involved disturbing systems at equilibrium and predicting or observing the changes to all the species present (Table 5), 10–40% of students provided chemically impossible answers such as suggesting that when one reactant was removed from a system, the concentration of one of the products increased and the other decreased (a mass balance impossibility). In general, when asked to describe what happened to reactants or products as a whole when a stress was applied (e.g., what happened to the concentration of the reactants when the temperature was increased), students provided a chemically viable answer 85–90% of the time, meaning they recognized that as one went up the other had to go down. However, when asked to predict what would happen to individual reactant and product species when a given stress was applied, this number dropped to 60–80% of students providing chemically viable answers. It should also be noted that the 2021 students performed somewhat lower than the 2020 students on all of these questions (Table 5).
The last takeaway was that it was extremely challenging for students to make the jump to use Collision Theory and particle motion to understand why a change in temperature would differentially impact rates of forward and reverse reactions (question II part c). To correctly apply Collision Theory to this phenomenon, students would have to recall and connect to previous learning about activation energy and how that differs for exo vs. endothermic reactions. Though this is something that is depicted graphically in the simulation, the activation of these prior mental resources when learning a new topic is very challenging, even with suitable prompts. Though we tried to qualitatively code the student responses to this question, too few of them gave answers that could be meaningfully grouped together. However, even if students are not yet able to make this connection on their own, the asking of the question can help students realize there should be a connection and help them identify where there might be a gap in their understanding so that instructors can more thoroughly engage with the topic during the follow-up class discussion.
As reported in the Results section under 2020 vs. 2021 Screencast Comparison we saw both groups make significant gains from pre to post with a large effect size. Though we saw a small interaction effect for year with the 2020 students making slightly larger gains, given all of the additional challenges that many students faced during the 2020–2021 academic year, similar gains could definitely be considered a success. Perhaps the largest take away from this comparative analysis is the importance of question format on what can be gleaned from student responses. Of particular note was Question 3 (Table 1). This question had to be substantially revised for use as an online assessment question. Further, although we were able to generally classify student drawing from the 2020 data into four main categories (represented by the four answer choices shown in Table 1), coding individual student graphs was often quite challenging with respect to determining which of two categories it should fit into. This may not be surprising as the ability to draw and interpret graphs is an additional significant barrier for many students (Potgieter et al., 2008). With the initial question, the implicit assumption was that if students could correctly complete the lines on the graph that indicated they had a strong understanding of equilibrium as a dynamic system and the underlying basis for Le Chatalier's Principle. In just asking students to choose the correct graph, we felt we needed to ask students to explain their choice to determine how their understanding of equilibrium was involved in their choice. Requiring this explanation was fortuitous as student responses provided some additional insights into how students may have been approaching their drawings in 2020.
A complete explanation of the correct answer would include both how the concentrations will initially change after the addition of a reactant and that a new set of stable concentrations would be formed. However, only 10% of the 2021 students on the post assessment included both of these aspects in their explanation. More tended to provide an explanation that focused on only the immediate change after the addition of the excess reactant, or on the fact that the concentrations reached a “steady state”; however, each of these reasonings individually are consistent with two different graph options. Interestingly, students who provided a pretest answer that focused on the end state (stable concentrations) were more likely on the posttest to give an answer that focused only on the changes immediately after the addition of the new compound than to add this aspect to their already solid answer. This may suggest that the assignment focuses students more on the initial changes, and not as much on the long-term equilibrium state. This might also be explained by students’ tendency to use one-reason decision making (Talanquer, 2014), or it may be the case that the prompt did not provide enough structure to help students understand that a complete explanation required addressing the initial change as well as the final state (Cooper et al., 2016).
The incorrect explanations were also quite illuminating. Students who selected Graph (a) (Table 3) correctly identified how the concentrations of compounds would shift after the disruption of the equilibrium but lacked the recognition that a new equilibrium would be reestablished. About a third of the explanations from these students suggested the students were simply applying Le Chatelier's Principle as an algorithm. Students selecting Graph (b) believed that the system at equilibrium would just remain at equilibrium or that no change would occur. This selection was generally accompanied by explanations that indicated students thought that no more reaction would occur after equilibrium was reached or that they were just focusing on the constant concentration part of the graph. Students selecting Graph (d) primarily justified their selection with the idea that the concentrations of the compounds had to be equal to be in equilibrium. However, there were a few responses which seemed to indicate a misinterpretation of the graph such as “The rates of each reaction whether forward or backward, became equal resulting in a conversion of each line.” or “all lines reach equilibrium as the concentrations and rxn rates are equivalent. This means rate of rxn for fwd and rev rxns are =”. These students seem to recognize the importance of the forward and reverse reaction rates being equal at equilibrium, but do not fully understand equal rates does not necessarily mean equal concentrations. In this case they may be applying the and associative-activation heuristic where same means identical, rather than recognizing the same rates imply not equality with respect to concentrations, but rather unchanging (Talanquer, 2014).
The results from this study also provide some broader implications. The first is that this intervention led to similar learning gains in similar populations of students regardless of instructor, institution, or course modality. The consistency of the learning gains suggest that the approach can be an effective initial introduction across a range of settings. However, as Eichler (2022) suggests, this preclass activity should not be viewed as the entirety of instruction, but rather just a foundation. If used effectively, such preclass activities can be used to inform instruction and elicit student buy-in. Student responses can provide valuable insight into the challenges that students are still having with the content so that in-class instruction can be short and targeted so as to provide ample time for students to practice applying their understanding. This is consistent with findings by Bancroft et al. (2021) showing that in addition to pre and in-class activities with accountability, another required component for flipped instruction that resulted in significant gains in course GPA was the inclusion of responsive mini-lectures. Further, we have observed that sharing a summary of the students’ responses to preclass activities at the start of class not only provides this targeted review of the content but also demonstrates to students the importance of the preclass activities for their learning and greatly increased their buy-in regarding the value of completing preclass activities.
(2) What is the current clock time?
(3) Using the reaction where the A atom is yellow and the following initial conditions
A = 20 BC = 15
AB = 1 C = 0
(a) How do the concentrations of reactants and products change initially?
(b) How do they change after the reaction has occurred for a while?
(c) The forward reaction and reverse reactions for this system are:
Forward: A + BC → AB + C
Reverse: AB + C → A + BC
At the beginning of the experiment, is the rate of the forward reaction (forming products) faster/slower/the same as (circle one) the rate of the reverse reaction (forming reactants)? What evidence from the simulation supports your conclusion?
(d) Use particle collisions (collision theory) to explain why the rates of the forward and reverse reaction would initially be different?
(e) In general, after the reaction has occurred for a while, is the rate of the forward reaction faster than/slower than/the same as (circle one) the rate of reverse reaction.
(f) Explain why this relationship between the forward and reverse reactions would make sense based on the idea of Collision Theory and concentrations of products and reactants.
(4) When 20 additional atoms of C are added:
(a) In the table, predict what you think will happen to the number of each type of particle when the simulation is restarted and equilibrium is reestablished. Use Collision Theory and rates of the forward and reverse reaction to justify your predictions.
Particle | Predict: increase/decrease | Justification of your predictions based on rates of forward and reverse reactions |
---|---|---|
A | ||
BC | ||
AB | ||
C |
(b) Was your prediction correct? Yes/No (circle one)
(c) If your prediction was not correct, explain what actually happened.
Try it Yourself
Go to the PhET simulation Reactions & Rates: https://phet.colorado.edu/en/simulation/legacy/reactions-and-rates. Note: You will need Java installed (see separate Java instructions). To start: Microsoft edge – click the image of the simulation; Chrome or other browsers you may have to download the simulation, then open it.
Set up the experiment as in the screencast with the following initial conditions:
A = 20 BC = 15
AB = 1 C = 0
• Bring up the strip chart. Zoom out on the strip chart so that you can see 20 molecules on the y axis.
• Adjust the initial temperature so that the total average energy line is halfway between the potential energy of the products and peak of the reaction coordinate diagram.
• Begin the experiment and allow it to run for a while and then pause the simulation
(d) What do you anticipate will happen to the number of each kind of particle if you decrease the number of one of the types of particles? Circle the type of particle you want to decrease the number of, and then as before, complete the table below.
Particle | Predict: increase/decrease | Justification of your predictions based on rates of forward and reverse reactions |
---|---|---|
A | ||
BC | ||
AB | ||
C |
(e) Adjust the amount of the particle you circled above, and test your prediction. Was your prediction correct? Yes/No (circle one)
(f) If your prediction was not correct, explain what actually happened.
Going further: impact of temperature
(5) Click End Experiment. When you click Begin Experiment, it should reset with the conditions from part 3.
Begin the experiment with the same settings and wait for the simulation to reach a relatively consistent number of each particle. Note the number of each particle type:
A = AB =
BC = C =
(a) Lower the temperature considerably. What happens to the total average energy as this happens?
(b) Allow the molecules to reach a relatively stable distribution again. Once this happens, are there more reactants, more products, or have each remained unchanged?
(6) Switch the exothermic reaction (4th reaction with red atom of A). Begin the Experiment using the same setting used in part 5. (Note, the Total Average Energy line may not move when you do this.) As before, allow the number of molecules to stabilized and record the numbers.
A = AB =
BC = C =
(a) Lower the temperature considerably. What do you notice about the change in reactant and product molecules?
(7) Looking at your results from parts 5 and 6, which side of the reaction will show an increase in the number of molecules when we decrease the temperature?
(a) Part 5 – endothermic?
(b) Part 6 – exothermic?
(c) How does Collision Theory help explain your answers to parts a and b?
Follow Up – Using your findings
(8) (a) Circle all of the graphs below that represent a system that comes to equilibrium
(b) What features of the graphs indicated that the system was at equilibrium?
(c) For any graph that you circled above (meaning it reaches equilibrium), put a vertical line on the graph indicating the time point at which equilibrium was achieved.
(d) Your friend Bob says a reaction reaches equilibrium because the reaction stops. You know this is not correct. How would you explain to Bob why the concentrations remain constant at equilibrium?
(e) Your friend Betty says that at equilibrium means that the concentration of the reactants and products are equal. You know that is not correct. What would you tell Betty is actually equal at equilibrium?
(9) The following reaction will reach an equilibrium.
N2(g) + 3H2(g) ⇌ 2NH3(g)
If, while this system was at equilibrium, more N2(g) were added to the system, the concentration of N2 would initially increase as illustrated on the graph below. Extend the concentration lines for each of the 3 compounds to timepoint x to illustrate how the concentrations of each would change as the reaction progressed.
(10) The reaction of 2SO2 + O2 ⇌ 2SO3 is exothermic. What would you expect to happen to the amount of each compound present if, once at equilibrium, the temperature was decreased?
SO2(g) | Increase/decrease/remain unchanged/impossible to determine |
O2(g) | Increase/decrease/remain unchanged/impossible to determine |
SO3(g) | Increase/decrease/remain unchanged/impossible to determine |
Question | Coding scheme | Value |
---|---|---|
(1a) Selecting systems at equilibrium (pre and posttest) | ¼ credit for each correctly indicate(i, ii, and iii selected, iv not selected) | 0.3333 total |
(1b) Why were 1a selected? (pre and posttest) | 1 = concentrations constant/reactants and products are constant OR rates equal/constant OR graphs level out/straight parallel lines (correct) | 0.3333 |
2 = reactants and products are equal/have equal concentrations OR Graph lines meet or end at the same place | 0 | |
3 = meet at a point OR conc of reactants and products are equal at some point OR lines cross or meet | 0 | |
4 = approach an intermediate value OR curves approach one another | 0 | |
5 = other | 0 | |
(1c) When is equilibrium established? (pre and posttest) | 1 = consistently selected where curves start to flatten out (correct) [this was still possible if iv was also selected in part 1a] | 0.3333 |
2 = consistently selected where curves cross/meet | 0 | |
3 = inconsistent or other | 0 | |
2 Who was correct (not scored for points) (pretest only) | 1 = Beth | |
2 = Betty | ||
3 = Bob | ||
4 = Other (e.g., circled more than one) | ||
2 Why was Bob wrong? (pretest and posttest) | 1 = something about reactions continuing but rates being equal | 1 |
2 = reaction doesn’t stop | 0 | |
3 = concentration remains constant | 0 | |
4 = something about the reaction not being at equilibrium if it goes to completion or reactant is used up | 0 | |
5 = misc | 0 | |
6 = reaction goes to completion | 0 | |
2 Why was Betty wrong? (pretest and posttest) | 1 = something about it is the rates that are the same, not the concentrations OR that the concentrations remain constant but don’t have to be equal | 1 |
2 = concentrations don’t have to be equal to reach equilibrium | 0 | |
3 = misc | 0 | |
2 Why was Betty wrong? (pretest only) | 1 = it is the rates that are the same, not the concentrations OR that the concentrations remain constant but don’t have to be equal | Not scored |
2 = concentrations don’t have to be equal to reach equilibrium | Not scored | |
3 = misc | Not scored | |
3 Selection (Pre and Post) | Coded by selection | |
3 Reasoning (pre and post) | 1 = indicates that initially SO2 and O2 should decrease while SO3 increases AND indicates that constant concentrations will be reestablished | 1 point if correct graph selected |
2 = indicates that constant concentrations will be reestablished (focus only on ending state) | 0.75 point if correct graph selected | |
3 = indicates that initially SO2 and O2 should decrease while SO3 increases or other Le Chatelier’ Principle inspired statement (focus only on initial change after addition of SO2) | 0.75 point if correct graph selected | |
4 = just “reaches equilibrium” without indicating what that means or supporting w prod/reactants | 0 | |
5 = other responses | 0 | |
4 (Pre and Post) | Coded by selections for N2/H2/NH3 | |
Decrease/decrease/increase | 1 (consistent) | |
Increase/increase/decrease | 0 (consistent) | |
No change/No change/no change | 0 (consistent) | |
All other combinations | 0 (inconsistent) |
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