Acids and Bases in Organic Chemistry

Why the model confusion?

By Curtis Bustos

Instructors and textbooks are not always clear in the use of models and how they can be applied as useful learning and thinking tools. Textbooks tend to conveniently alternate between specific models and sometimes transfer attributes from one model to another resulting in hybrids that may prove to be difficult to teach and learn. From this, it is important to take a moment and explain why a new model is introduced to prevent model-confusion.

Moreover, when students are introduced to new models that describe and explain the same phenomenon, they are instinctively inclined to combine and/or omit certain attributes of these models. The premise may be based on the students' pre-existing models that refute what seems to be an invalid inference from the new model(s). Once again, this makes it extremely important to emphasize differences in models and to explain that models have their limitations and therefore occasionally require more than one to explain and describe a single concept. 

Acid-base chemistry is taught and explained with the use of many models and has no exception to model-confusion as mentioned above. For example, research has shown that acid-base chemistry is confusing to students for several reasons related to the use of different models:

-Difficulties in understanding acids and bases as ions, Ross and Munby (1991) and Nakhleh (1994).

-Bronsted-Lowry model was confusing because students were more familiar with the  Arrhenius model, Rayner-Canham (1994) & Demerouti et al. (2004).

-Difficulties accepting water as an acid and a base, Schmidt and Volke (2003).  It was also concluded that students were confused about the attributes from different models. They did not completely understand the differences between Bronsted-Lowry and Arrhenius models. 

Finally, students were confused as to why there was not a “best model.” These students were referring to the popular concept used in physics that when several theories exist to explain the same concept or phenomenon, it is better to use the simplest one and to eliminate excessive assumptions.

So why is it difficult for students to add additional models to their thinking toolbox without being confused by them? As explained by Redish (1994), changing and adding to existing mental models requires many thoughts and predictions to occur. For example, the new mental model must be understandable, it must be plausible, there must be a strong conflict with predictions based on the existing model and the new model must be seen as useful. This can be very difficult for a person’s mind to consider and process. For practical purposes, think about how difficult it is to change an assessment you have made about someone you like or dislike????

A discussion about limitations and benefits to using different models might serve to dramatically reduce confusion. For example, suppose we wanted to explain a simple integer such as the number three. One model might suggest that 1 + 1 + 1 = 3. This model is effective and may also be extended by explaining the three points on a triangle. Nevertheless, this model has its limitations. By limitation, what happens when we do not have three integers or numerical values that = 1 to work with? Our model breaks down and is not favorable anymore. So how can we explain the number three if our model is limited to 1 + 1 + 1? This model can be replaced with a more useful one. By replace, it will not be necessary to purge our old model because it has limitations; rather, an understanding of the benefit to having additional models to reference will be appropriate.  Additional models can explain that 7 – 4 = 3 or 1 x 3 = 3 and so on. These different models all describe the integer = 3, but their process in calculating the solution is explained differently by the use of subtraction and multiplication mathematical-models.

Acid-base chemistry is no different than our number-three example mentioned above. Each unique situation will require more than one model to explain one phenomenon. When one model fails to work, use another one until an acid-base reaction can be explained with clarity.  

Now, let’s take a look back at general chemistry and review the Arrhenius model of acid-base chemistry. Remember that a strong acid completely ionizes in solution whereas a weak acid only partially ionizes. The same is true for bases. A strong base completely ionizes in solution while a weak base will partially ionize. While this model explains relative acid-base strength in reference to solutions and ions, it does not explain how H⁺ transfers are processed. This describes how the Arrhenius model has its limitations in explaining and describing acid-base reactions.

Therefore,

              Strong acid (HCl)  + Solution (water) = complete ionization

And,

Weak acid (HF) + Solution (water) = partial ionization

Next, let’s expand on our model inventory by adding two additional models. Remember, when one model fails to explain an acid-base reaction, try to use another one.

             Bronsted-Lowry acids and bases & Lewis acids and bases.

Bronsted-Lowry acids and bases are concerned with protons (H⁺). Therefore by protonation and de-protonation, Bronsted-Lowry identifies an acid as a proton-donor and a base as a proton-acceptor.     

Acids (proton donors) and bases (proton acceptors) always occur together during an acid-base reaction. Also, after a reaction occurs, bases will become acids and acids will become bases. This becomes much more clear when it is referred to as a conjugate acid-base pair.

Lewis acids and bases are concerned with electron-pairs instead of hydrogen-atoms. They are explained by:

Lewis acid = Electron pair (:) acceptor

Lewis base = Electron pair (:) donor

Example:

AlCl₃ + Cl^- à AlCl₄

The Lewis acid is the Al atom because it is receiving an electron pair.

The Lewis base is the Cl^- atom because it is “pushing” or donating its electron pair to the Al atom.

Now we can add some terminology to our acid-base toolbox of models:

Nucleophiles and Electrophiles – these terms are used to help explain reactions in organic chemistry.

First off, a nucleophile (nucleus lover) is simply a name to help identify where electrons are being “pushed” from. An electrophile (electron lover) is a name to help identify where electrons are being pushed toward. Therefore, during a chemical reaction, electrons will always flow from the nucleophile to the electrophile.

Furthermore, nucleophiles are bases (by convention of Lewis acid-base models) and tend to be negative or partially negatively-charged while electrophiles are acids and tend to be positive or partially positively-charged.

As mentioned earlier, electrons will always flow from nucleophiles to electrophiles. This automatically makes our nucleophile a Lewis base and electrophile a Lewis acid. Therefore, the Lewis acid-base model is great for explaining nucleophiles and electrophiles. 

Now we go back to our original question. “Why the model confusion?” Different models are simply used to explain a single phenomenon. When one model fails, try a different one. Perhaps it may be beneficial to compare the description of acid-base chemistry to the description of the integer 3.

Describing the integer 3:

Are we working with subtraction? If so, we need a model that describes subtraction to help explain our solution.

Are we working with multiplication? If so, we need a model that describes multiplication to help explain our solution.

Are we working with addition? If so, we need a model that describes addition to help explain our solution.

Describing acid-base chemistry:

Are we working with ionization and ions? If so, we need a model that describes ionization to help explain our reaction. We need the Arrhenius acid-base model.

Are we working with protonation, de-protonation (transfers with protons) and reactants with pK_a values? If so, we need a model that describes this. We need the Bronsted-Lowry acid-base model.

Are we working with transfers in electron-pairs instead of hydrogen atoms? If so, we need a model that describes this transfer. We need the Lewis acid-base model.  

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P411: Acid-base student models in organic chemistry

Author: Curtis Bustos, University of Colorado at Denver, USA

Co-Author: Kristina Bueter, Maile Hiu, Hunter Chase, Vanessa Fishback and Priscilla Burrow,University of Colorado at Denver, USA

Date: 8/5/14

Time: 11:30 AM – 11:50 AM

Room: LMH 176

Related Symposium: S3

Instructors and textbooks are not always clear in the use of models and how they can be useful learning and thinking tools. Instructors and textbooks tend to conveniently alternate between specific models and sometimes transfer attributes from one model to another resulting in hybrid models that may prove to be difficult to teach and learn. Acid-base chemistry is the foundation for chemical reactions in organic chemistry, but many different models are used to explain it. A learning assistant initiated research study was conducted on science and pre-health students in an organic II lecture course in order to investigate students’ understanding of acid-base models. The study consisted of answering two questions: How would you describe acid-base chemistry? How would you describe the integer 3? Acid-base chemistry answers were varied and categorized into four groups. 2% didn’t know or simply confessed to not knowing how to answer the question, 6% were able to differentiate between models and explain their application when appropriate, 33% described acids and bases as having a best model or one that describes all acid-base chemistry reactions, and 59% combined and described hybrid models. Describing the integer 3 did not prove to have any confusion. These initial findings inspired subsequent learning assistants in the organic chemistry courses to work with the organic and general chemistry instructors to develop assessment materials to track students’ acid-base model development from general chemistry I through organic chemistry II with the goal of providing timely intervention to combat misconceptions and model confusion.