Why do bases react with acids

Although these definitions were useful, they were entirely descriptive. The first person to define acids and bases in detail was the Swedish chemist Svante Arrhenius —; Nobel Prize in Chemistry, Because of the limitations of the Arrhenius definition, a more general definition of acids and bases was needed.

One was proposed independently in by the Danish chemist J. Acids differ in the number of protons they can donate.

For example, monoprotic acids a compound that is capable of donating one proton per molecule are compounds that are capable of donating a single proton per molecule. A compound that can donate more than one proton per molecule is known as a polyprotic acid. In chemical equations such as these, a double arrow is used to indicate that both the forward and reverse reactions occur simultaneously, so the forward reaction does not go to completion.

Instead, the solution contains significant amounts of both reactants and products. Over time, the reaction reaches a state in which the concentration of each species in solution remains constant.

The reaction is then said to be in equilibrium the point at which the rates of the forward and reverse reactions become the same, so that the net composition of the system no longer changes with time. We will not discuss the strengths of acids and bases quantitatively until next semester. Strong acids and strong bases are both strong electrolytes. In contrast, only a fraction of the molecules of weak acids and weak bases react with water to produce ions, so weak acids and weak bases are also weak electrolytes.

The ionization reaction of acetic acid is as follows:. All other polyprotic acids, such as H 3 PO 4 , are weak acids. Arrow pushing is a formalized shorthand to show how interactions begin and how electron density shifts over the course of a reaction, but if students fail to recognize this, it is unlikely that they will be able to use arrows with any degree of success. For example, many students use arrows to indicate the movement of the hydrogen atom itself in acid—base reactions rather than the attraction of electron-rich sites to electron-poor sites via movement of electrons.

As we have noted earlier, 40 there is a great deal of evidence to support the idea that asking students to explain why a phenomenon occurs leads to deeper learning. That is, having students approach mechanistic reasoning in organic chemistry systems by helping students articulate why reactions occur may help them construct a framework with which to reason.

However, there is considerable evidence that most students do not consider the underlying reasoning for why mechanistic arrows are drawn this way. That is, students do not seem to associate arrow drawing with the causal mechanism by which the reaction occurs.

Constructing a causal mechanistic explanation about an acid—base reaction would require that students do more than describe the course of the reaction whether it be by proton transfer or movement of electrons , rather it would require that students discuss the cause of the reaction as beginning with an interaction between the lone pair on an electronegative atom e.

That is, students should understand that acid—base reactions start with an electrostatic interaction between moieties of opposite partial charge.

Understanding the mechanism how the reaction occurs is not the same as understanding the causal mechanism why the reaction occurs. Similarly, it is possible to provide causal reasoning without providing a mechanism as discussed below. Assessment of Student Reasoning. Our goals are to characterize the ways in which students reason about acid—base reactions and to develop an assessment protocol for acid—base mechanistic reasoning.

To capture such reasoning requires the design of tasks that can elicit evidence of student understanding. That is, assessments must elicit evidence which can be used to make an argument about student understanding.

Using this approach typically involves several steps:. In this study, the construct is how students reason about acid—base reactions. Note that the construct is more than knowledge; we are interested not only in student knowledge about acid—base reactions but also the ways in which students reason about why such reactions occur.

Evidence of student reasoning about acid—base reactions can be elicited by asking students to provide causal mechanistic explanations for why these reactions occur: by which we mean that students should be able to identify an acid—base reaction, describe the scientific principles or evidence that supports their identification, and provide reasoning about why this reaction is occurring.

There is a great deal of evidence to support the use of tasks that involve having students provide causal mechanistic explanations for phenomena.

We have designed open-ended tasks because we believe they will provide stronger evidence about student reasoning than forced choice items. As discussed in the Methods section, the structure of the prompt to which students respond is crucial. In a similar vein, it has been reported that multiple choice tests tend to overestimate the level of student understanding when compared to open-ended responses. Evidence from student-constructed responses can be analyzed in a number of ways. For example, much of the prior work on student understanding has focused on the identification of types of misconceptions or the model of acid—base behavior used.

However, we are eventually interested in comparing student responses across courses and over time, and for this, we have developed an approach based on the type of reasoning of the student response see discussion below. The development of assessment tasks to elicit student acid—base reasoning included several groups of students enrolled in college level general chemistry and organic chemistry at two different universities Table S1 in the Supporting Information.

All administrations of the assessment tasks were conducted at the end of the semester indicated except where noted. In addition, all students involved in this research agreed to participate in this study and signed informed consent forms.

These two groups of students were selected to illustrate how, by changing the task prompt, we were able to elicit stronger evidence about the ways in which students reason about acid—base reactions. Both of these groups of students were enrolled in transformed second-semester general chemistry courses at these two institutions. Even though these students attended different universities, both of the groups of students used the same two-semester general chemistry curriculum Chemistry, Life, the Universe and Everything CLUE.

At the same time as the Lewis model, students are also taught to draw simple reaction mechanisms i. In addition, acid—base chemistry is threaded throughout the second semester as the focus of sections on equilibrium reactions and networked biological reactions.

The SP12 and SP15 students were compared to determine if the two groups were similar according to the available demographic information and assessment measurements. Although the SP12 students had slightly higher average ACT scores, we believe that the two groups are similar enough for our comparison purposes here particularly since the SP15 students constructed more sophisticated explanations for the assessment item as discussed below. It should also be noted that, in our earlier work, students from University 2 SP15 performed at the same level as those from University 1 SP12 on a different assessment task involving understanding intermolecular forces.

The assessment tasks were designed to elicit molecular level mechanistic reasoning about acid—base reactions. The initial iteration of the assessment Figure 1 was developed as part of a series of studies on student understanding of structure—property relationships. In the preliminary iterations of the assessment tasks, we explored how presenting students with representations of condensed structures without any structural cues compared to providing Lewis structures Table S1 in Supporting Information , early spring We found that students were less likely to mention lone pairs during their discussion of the molecular level explanations when presented with the condensed structural representations.

Therefore, in the study, a common acid—base reaction was presented to students reaction of hydrochloric acid HCl with water H 2 O. Figure 1. Initial iteration of the assessment tasks SP High Resolution Image. Since this iteration required only written responses from students, it was administered using SurveyMonkey , an online survey program, and was composed of questions about the reaction of hydrochloric acid HCl with water H 2 O Figure 1.

That is, many students merely stated what atoms were rearranging instead of explaining why these atoms rearrange. As Jin and Anderson 43 noted, the structure of the prompt is crucial in designing assessments that are intended to provide evidence of student reasoning. It must be accessible so that all levels of students can understand what is intended, and it should also provide enough structure so that students understand what is required to answer the question.

We want students to provide as much relevant information as they can, but they must understand what is needed. On the other hand, overstructuring the prompt may provide students with enough information to answer the question in ways that they may not have thought of otherwise.

By providing too much information in the prompt, we may encounter the problem found with multiple choice questions that have been shown to overestimate student understanding, 44 or it may even send students off in an unproductive direction. This iteration of the survey was administered using beSocratic , an online system where students can construct written and drawn free-response answers. In addition, another question that asked students to draw mechanistic arrows for the reaction was added.

Figure 2. Final iteration of the assessment tasks SP To make sure that students understood what was meant by the task, we interviewed five students toward the end of the spring general chemistry course before the final iteration SP15 was administered. We found that students were consistently able to provide explanations that were aligned with the provided prompts, and therefore no further interviews were necessary.

Data Analysis. The student responses for both what was happening on the molecular level Figure 2 b and why the reaction occurs Figure 2 c were combined for analysis purposes since some students responded to the why prompt in the what response space and vice versa.

Since we were interested in identifying the type of reasoning in the student response rather than right or wrong, misconception, or model used , we placed these responses into categories that were aligned with student reasoning, as shown in Table 1. Table 1. The partial negative oxygen in water is attracted to the partial positive hydrogen in hydrochloric acid.

H 2 O and HCl are attracted to each other because of their partial charges. The H—Cl bond is broken and forms a new bond with oxygen. The reaction occurs because the partial negative charge on the oxygen attracts the partial positive charge on the hydrogen. Students who used the Lewis model to discuss how the reaction occurred were Lewis Mechanistic, and if they added causal reasoning, they were designated as Lewis Causal.

These characterizations are further discussed in the Results and Discussion section. The three authors further discussed the coding scheme, which resulted in a Kappa value of 0. The arrow pushing mechanisms that students constructed for this reaction were also inspected and coded for 1 whether the first arrow was shown in the correct direction, from the lone pair on the oxygen in water to the hydrogen in HCl and 2 whether the complete mechanism was correct, as shown in Figure 3.

Results and Discussion. Other responses included redox, displacement, hydrolysis, and substitution reactions. That is, most students recognized without prompting that the given reaction was an acid—base reaction.

Examples of each type of student reasoning are shown in Table 1. And Cl — becomes conjugate base. On the other hand, responses that invoked a causal mechanism, for example, by including terms such as attraction , pulling off , and interacting partial charges were all considered as a demonstration of causal accounts. Responses that described the involvement of the lone pair on the base were assigned to the Lewis category.

It is a proton acceptor. It will accept the proton from the HCl. The HCl will donate its proton to the H 2 O. The polarity of the H 2 O is what first attracts the partially negative oxygen to the partially positive H in the HCl. Since O is highly electronegative, it will pull off the H from the HCl and make a bond with one of its lone pair of electrons. The electrons from the bond of the HCl will stay on the Cl since it is more electronegative than H.

To determine how the change in prompt impacts student reasoning, we compared the responses from SP12 and SP As shown in Figure 4 , it is clear that the SP15 iteration has elicited a larger proportion of Lewis causal mechanistic responses. There is a clear and significant difference between the two iterations of the assessment tasks. The largest group of students described, in varying detail, what they saw happening in the reaction scheme provided.

In retrospect, this is not surprising. The nature of the prompt did not appear to provide students with enough structure to elicit an explanation for how or why the reaction is happening. In contrast, the SP15 assessment task required two separate responses from students. Students were asked to describe what is happening on the molecular level during the reaction and, then in a separate prompt, asked why the reaction occurs Figure 2.

Inherent in the structure of the prompts was the idea that merely describing what happens is not sufficient. It should be re-emphasized that we actually coded these responses together as one response since many students answered both tasks in the same response box.

In this implementation, we see a significant shift in the pattern of responses. We believe that the difference between the two iterations is primarily due to the change in prompt.

By providing students with more scaffolding, we were able to help them structure their response to provide more evidence of understanding. While the two groups of students were from different universities, there were no major differences in the demographic data that we compared, as discussed in the Methods section also shown in the Supporting Information Tables S2 and S3 , except that students at University 1 SP12 had a slightly higher incoming average ACT composite score.

Both groups of students were enrolled in general chemistry courses that used the same curriculum materials, 45, 55 had approximately the same time on task, similar homework assignments and examinations, and the same set of learning objectives.

Therefore, though it was not feasible to administer the items in an identical manner to both cohorts, we believe that the difference in responses stemmed from a refinement of the task prompt. In SP15 assessment, students were also asked to draw mechanistic arrows for the acid—base reaction. The responses were coded simply for 1 is the first arrow correct? While the tasks in our earlier studies were more difficult e. The students in this study are enrolled in a general chemistry course. A similar but less pronounced pattern is seen for students who provide causal reasoning.

The ratio for Lewis Causal is 4. Table 2. It is interesting to note that the ratio of incorrect to correct responses increases across Table 2 , with students who provide Lewis Causal explanations being most likely to draw correct mechanisms and to provide the most scientifically sophisticated explanations. We believe that this provides support for the idea that our classifications actually correspond to levels of sophistication of student reasoning, and that they may be used to monitor trajectories for student understanding over time.

Conclusions and Implications. The findings from this study indicate that students use a variety of ways to reason about acid—base reactions that can be characterized using a framework which allows responses to be classified according to the model used and whether the students invoke a causal explanation.

In essence, by looking for how students describe what is happening, how it is happening, and why it is happening, we have also shown:. Here, we have provided evidence that students who use Lewis models are indeed more likely to construct appropriate mechanisms.

However, as noted earlier, students in this study were enrolled in a transformed general chemistry course designed to emphasize causal mechanistic reasoning. As students progress through the course they are asked to articulate what , how , and why chemical phenomena occur and are expected to provide both written and drawn responses.

This approach we recommend is important, not only to help students in learning about mechanisms but also to reinforce the idea that the use of appropriate models is part of the scientific enterprise, and the choice of model can influence subsequent learning.

That is, students should be asked to articulate both what the arrows mean and why they are drawn from electron source to sink. As students become more expert in this process, the explanation step will be omitted, but if students are never asked to explain as they learn, they will tend to draw arrows without understanding their meaning.

It is quite common to ask students to explain their thinking, but if we do not provide students with some structure about what we expect them to include in their response, it can be confusing. As noted earlier, there is a fine balance between providing enough scaffolding to indicate to students what is expected and providing too much scaffolding so that students do not really have to construct their own response.

Over-prompting can result in the same kind of overestimate of student understanding that has been reported with some multiple choice questions. We have found that it is particularly helpful to separate the prompts so that students understand that they should both describe what is happening and why it is happening.

Future Work. Since we believe that our characterizations correspond to levels of sophistication in student reasoning, it is possible that by administering the tasks to students at different places in the curriculum we may be able to identify student trajectories over time.

We intend to monitor student responses by following cohorts of students throughout the curriculum. It may also be possible to use the task to evaluate the effects of interventions within a particular course, such as investigating the effect of introducing a Lewis acid—base module into a more traditional course structure.

In addition, we plan to expand this type of assessment to other classes of reactions to investigate whether similar approaches can improve student mechanistic reasoning for other more complex reactions.

We have a great deal of past data on how students, who have not been asked to provide mechanistic and or causal reasoning, draw mechanisms, and it will be interesting to investigate whether changes improvements can be made for future cadres of students by helping them articulate their understanding in words. Limitations of the Study. A limitation of this study is that the student participants were enrolled in a transformed general chemistry curriculum, and it may well be that students in a more traditional curriculum do not respond to the prompts in the same way.

In the transformed curriculum, mechanistic reasoning is emphasized and students are asked to draw and write responses to open-ended homework tasks on a regular basis. Acid—base chemistry also plays a prominent role.

For example, the topic of equilibrium is introduced in the context of acid—base chemistry rather than in the context of gas-phase reactions, and the course ends with discussions of how networked acid—base reactions regulate blood pH. Students from traditional general chemistry programs who are not expected to support their assertions with reasoning may have trouble providing normative explanations for chemical phenomena.

Lastly, students in the CLUE general chemistry curriculum are also taught to draw simple reaction mechanisms for acid—base reactions using curved arrows. Therefore, our results are almost certainly not typical. Supporting Information. Author Information. Melanie M. Sonia M. The authors declare no competing financial interest. Acid-Base Equilibria, Part I. Upper secondary students' misconceptions and difficulties.

Chemical Educator. Although several aspects of acid-base chem. The misconceptions of a sample of twelfth-grade Greek students on the subject of acid-base equil. Students' misconceptions and difficulties in understanding and applying the relevant concepts were categorized into seven categories: a dissocn.

Royal Society of Chemistry. The effects on students' achievement and misconceptions of new teaching material developed for the unit 'acids and bases' were studied and the students' attitudes towards chem.

The new material included worksheets based on the conceptual conflict strategy. The sample consisted of eighty-eight students. The research was carried out with an exptl. Attitude Scale' were used to collect data before and after the study as pre-tests and post-tests. The results from the post-tests indicated that the students in the exptl. In addn. This shows that the implementation of the new material produced better results both in terms of achievement and attitude.

The students' misconceptions in exptl. Effect of developmental level and disembedding ability on students' conceptual understanding and problem-solving ability. The effect of two psychometric variables: developmental level i.

Disembedding ability clearly had a larger effect. Multiple-regression anal. On the other hand, disembedding ability was involved both in situations that required conceptual understanding alone, esp.

Acid-base titrns. Using a series of qual. The findings indicated that students had considerable difficulty with acid-base chem.

Further, most students could not relate the concepts to actual solns. Student difficulties stemmed from a lack of understanding of some underlying chem. The conceptual d. Model Confusion in Chemistry Res. Most org. Recent studies have described undergraduate org. To drive changes in pedagogy and curriculum, however, faculty need to be able to quickly assess students' conceptions of acids and acid strength. We recently reported on the development and assessment of a nine-item, multiple-tier, multiple-choice concept inventory about acid strength, named ACID I.

In this manuscript, we demonstrate that despite this low coeff. Thus, the purpose of this paper is to i report two significant alternative conceptions about acid strength that persist in org.

Two types of test conditions were employed within second-semester org. Understanding key foundational principles is vital to learning chem. In the general chem. This study reports the outcomes of an investigation of org.

The results indicated that most students maintain declarative knowledge rooted in general chem. This could be because it is a hydroxide salt, like NaOH, or because it takes hydrogen ions from water, leaving hydroxide behind. A good example of this is ammonia, NH 3 , which is sometimes used in house cleaning products. In general, bases react with hydrogen ions.

This is how neutralization happens. The acid produces hydrogen ions, and the base produces hydroxide ions. These react together to make water. The anion that came from the acid and the cation are left, so if you evaporate the water, you would get a salt. The general reaction looks like this:. Thus, the hydrogen ions, which makes acids acidic, are consumed, and the hydroxide which makes bases basic is also consumed, and if the moles of acid and base are equal, only neutral water and a salt is left.

Actually, it is a little bit more complicated than this if the acid or base is weak. The solution will only really become neutral when the moles are equal if both are strong. A: Usually cakes include an acidic ingredient this varies and sodium bicarbonate, a base. When they react, the proton from the acid is transferred to the bicarbonate, making the weak acid carbonic acid. Carbonic acid is the product of an acid anhydride reaction between carbon dioxide and water.

This reaction can be reversed, or carbonic acid can decompose into water and carbon dioxide. Especially at the high temperatures inside a baking cake, this decomposition will happen, and produce carbon dioxide gas. The pressure of the hot gas will form bubbles inside the cake, making it fluffy. In the previous section Precipitation , instead of having hydroxide react with hydrogen ions to form water, the acid base reaction made carbonic acid from protons and bicarbonate.