Cognitive Load in Learning Chemistry

Several things swirled in my mind the last couple of days culminating in the question. Does learning chemistry impose an additional cognitive load on students, perhaps even more so than the other sciences? Is there something unique about introductory level chemistry that requires special attention from us teachers in designing our learning activities?

I meet new people on occasion, and this past week, finding out that I teach chemistry led to a personal disclosure from my new acquaintance about how chemistry was hard and didn’t make sense in school. I don’t hear this as much about biology or physics, but that might just be a sampling effect due to my being a chemist. I’ve also been reading Reactions by Peter Atkins. It is beautifully illustrated with chemical substances at the molecular level on virtually every page. I did not find the prose as engaging, and I don’t personally recommend it if I wanted to get you excited about chemistry; Periodic Tales is much better. But one thing that Reactions does well is showing you lots of molecular level pictures. This “molecular view” is something my colleagues and I stress heavily in our introductory chemistry classes.

Why is the molecular view so important for chemists? That’s where all the action is! Chemistry is about making and breaking chemical bonds, a chaotic dance where atomic partners are swapped. But the problem is that we cannot see this frenetic molecular level activity with our own eyes. We see macroscopic changes such as bubbles, a color change, perhaps a spark of light, or glimpse the appearance or disappearance of a solid. As I’ve been thinking about cognitive load and the differences between how experts and novices learn and process new information, it’s perhaps worth taking a step back to see how one might view a chemical reaction.

Consider the hydration of formaldehyde to form methylene glycol. This is a reaction of interest to chemical engineers, atmospheric chemists, and those who study the chemistry of the origin-of-life such as myself. If I asked a student to write a chemical equation for this reaction, I might get CH₂O + H₂O --> CH₄O₂ from a student in an introductory chemistry class who knew the “chemical formulae” of each of these substances. A student in organic chemistry might write the product methylene glycol as CH₂(OH)₂ to represent something about how the atoms are connected in the molecule.

Thinking molecularly, one might imagine the reaction to look like:

A student in organic chemistry might “draw” the reaction as:

The carbonyl (C=O) bond of formaldehyde is emphasized since a key chemical reaction of carbonyls is to undergo “nucleophilic addition”. Methylene glycol is drawn as a “line structure” where carbons are not explicitly labeled and hydrogens attached to carbons are not shown.

A student thinking about how the reaction proceeds would consider what chemical bonds are broken in the reactants, and what bonds are being made when the product is formed. A simple representational drawing of the “transition state” as the midpoint within the chemical reaction is shown below. Dashed lines represent the bonds being “made and broken”. Also notice the C–H bonds that do not participate in the chemical reaction are drawn with wedges that show how the CH₂O and H₂O molecules approach each other in three-dimensional space for a productive reaction.

How does this reaction happen in practice? If you were an atmospheric chemist, volatile formaldehyde could dissolve in cloud droplets. You might therefore write the reaction as: CH₂O(g) + H₂O(l) --> CH₂(OH)₂(aq) with the phases of matter of each substance specified (g = gas, l = liquid, aq = aqueous, a solution of a substance dissolved in water). If you were measuring this reaction in a lab, it would never involve just two molecules colliding to form a new molecule. The formaldehyde molecule as it dissolves into water would encounter many water molecules. In fact there is likely to exist a different transition state where the presence of other water molecules assists (or catalyzes) the chemical reaction. Below is a different transition state that is much more likely to occur than the one previously shown because it is energetically more feasible. (Energy is the currency in chemical reactions!)

In fact, there are other transition states more complex than the one above that may be even more probable. (I know this because all my new research students from the last five years do a series of computations on this very reaction as their first test case.) Formaldehyde dissolving in water, viewed at the molecular level, is likely to look like just a lot of water! The water molecules are closely spaced as shown in the water “box” below. (Such boxes are a standard part of the simulator’s toolkit.) You’d be hard-pressed to find a formaldehyde molecule even in a relatively concentrated solution, such as a 1 M (or one molar) solution. In a 1 M solution, one mole of formaldehyde is dissolved in 1 L of water. That’s approximately 2 molecules of formaldehyde per 111 molecules of water.

When we teach students to write “balanced” chemical equations, and then calculate the amount of product formed given some amount of starting reactants, the chemical reactions are written in terms of the mole (6.022 x 10²³ molecules). So while on the one hand we want students to think of “balanced” chemical reactions at the molecular level (with molecular pictures in mind), we also want them to be able to think of these same reaction at the level of moles, or the macroscopic level. One mole of water, at18 mL in volume (or just over a tablespoon) is an amount you can physically observe.

As a chemistry professor (or the “expert”), shifting back and forth between the molecular level and the macroscopic level, not to mention thinking about all the different representations shown above, poses practically no cognitive load. All these ways of thinking about the reaction have become second nature, and my mind effortlessly moves to the representation I need for the problem at hand. But this is not true of the student (or the “novice”). The cognitive load can be substantial. One reason why the introductory college chemistry course is particularly challenging is that students enter college with highly varied backgrounds. The student who has taken Advanced Placement Chemistry is much more comfortable “shifting gears” than the students who has taken no chemistry, or barely understood any of it.

When I first conceived of a Potions for Muggles textbook, I thought it should be heavily illustrated with molecular pictures (like most popular chemistry textbooks today) because that’s how I think about chemistry as the expert. But I’m starting to see that an over-emphasis of the simple (and pretty) molecular pictures sometimes obscures understanding instead of illuminating it simply by imposing an additional cognitive load that a reader may not be ready for. The transition from novice to expert requires quickly identifying the key elements of a problem. Novices are awash in a sea of information and it’s hard to prioritize which representational view is going to help them tackle the issue at hand. I still think pictures are important but they should be chosen with care.

The formaldehyde hydration reaction is actually a bit more complicated because (1) it can go in reverse, and (2) formaldehyde can react with methylene glycol to form the C₂H₆O₃ molecule (HO-CH₂-O-CH₂-OH), depending on the relative amounts of formaldehyde and water. If one simply considered one molecule of formaldehyde and one molecule of water in a highly artificial system, then (1) is easy to deal with, and (2) is irrelevant. But in a real system involving moles of molecules, things could get much more complicated. For example, a third molecule of formaldehyde could add to the C₂H₆O₃  molecule, eliminate a water molecule in the process, and form the trioxane shown below.

Phew! Maybe that’s why people keep telling me chemistry is difficult. I spent my first two years of chemistry class without understanding some of the basics I just described above. Hopefully, I can help my students overcome the barrier and present the material systematically and with the right level of cognitive load.