Acids and bases: Why the confusion?

It never fails. As I begin a class saying we will be covering the chemistry of acids and bases, either with science or engineering students, I get a general groan from the whole group. They all tell me they’ve covered this before, but when presented with a few questions, it becomes clear that I will need to do more than a simple review. They tell me it seems different and harder now. That in itself is not necessarily what I find concerning, but questions such as: “How can I memorize which are weak acids?” make me pause and reflect on how we collectively teach this material.

I am not familiar with current curriculums nor with the pedagogical approaches used in high schools when it comes to acids and bases. I don’t know how much time is spent on this topic on average nor even whether my students come from schools well represented by this average. I only see a problem. It is the same every year, and I have found it at several institutions across the country where I have taught this material. I have tried to bridge the obvious gap that exists between when students first hear about acids and bases and when I approach this topic in a first year chemistry class. What I describe here is the process that reflects how I currently try to address the student confusion. This is an idea for those who are looking to modify their approach and establish links between the chemistry covered in different high school grades, and just as importantly, an opportunity to continue the dialogue between high school and university instructors, as, while we may have different challenges, we are trying to reach many similar goals.

Knowing how to interpret what you draw

In this approach, structure and reactivity concepts are continuously intermeshed. The introduction to BrØnsted-Lowry reactions is linked to the drawing of Lewis structures as both are important first steps in trying to address how students understand acidic or basic behavior. One topic gives you the description of what to expect as a reaction proceeds, and the other tells you where to look inside your molecule to be able to predict such behavior.

My introductory lectures on Lewis structures quickly incorporate identification of acidic protons or reactive lone pairs in the molecular structure. Which protons can you never dissociate in water, and which are the first to come off? And where did they go? The reactivity of compounds will be driven by their structure. This is the beginning of functional group recognition. If you focus on the important section of the molecule, you can make pretty good guesses as to what will happen. This analysis of structure returns when predicting pH changes in solution, and again later when dealing with equilibrium constants. Therefore, you don’t memorize the list of acids, you learn how to “read” your structure.

Recognizing your compound

A great deal of confusion then arises when compounds are presented as salts. At the stage of identifying acids and bases, many of my students can correctly identify the fluoride ion as a weak base in water, but when it is listed in a variety of fluoride salts, significant numbers of students are confused by the other ion. I specifically ask them to identify compounds as they would be in a bottle, and you can’t buy a bottle of only fluoride ions.
I really want them to understand the chemical behaviour in the context of what they might read on a label.

I make a point of addressing acidic or basic behaviour of salts when discussing ionic solids. I see little value in only identifying an ionic solid, and not discussing what that solid might do in terms of reactions. Then when predicting pH changes from salts, a little review of ionic solids is generally required. Recognizing that there is no N-N bond in ammonium nitrate is the first step. Drawing the Lewis structure of ammonium ion and finding a cationic nitrogen center is next, followed by identifying any acidic hydrogen atoms in their Lewis structure. I find students seem more confident and seem to remember how to predict behaviour effectively, though this observation is primarily anecdotal.

Predicting the outcome

The properties of molecules mentioned above are very useful to review when undertaking acid/base equilibrium problems, where the goal is the quantitative analysis of reactions in aqueous solutions. Since the students can identify the properties of their compound, they can also predict the reaction outcome. I have found this reduces the initial stress associated with starting difficult questions, and we all know that it is the starting of questions that students find difficult.

Building the bridge

Maybe part of the confusion lies in the compartmentalization of the unit dealing with acids and bases early on, and in some introductory courses as well. In the above approach, acidity or basicity is presented as the reactivity in aqueous solution, where reactivity is predicted by structure and quantified using equilibrium calculations. I have found that many students find difficulty in problems where they need to connect with material covered either a long time ago or in a different context.

I was one of those students. As a result I not only try to continuously review, but also try to tie these ideas into other units of the course, for example, when teaching intermolecular forces. At that point there is no longer one unit that covers acid/base behaviour. This is a work in progress, and the students seem to appreciate how they’ve come full circle with a better understanding of how many of our topics are connected.