Experienced Teachers’ Pedagogical Content Knowledge of Teaching Acid–base Chemistry

Abstract

We investigated the pedagogical content knowledge (PCK) of nine experienced chemistry teachers. The teachers took part in a teacher training course on students’ difficulties and the use of models in teaching acid–base chemistry, electrochemistry, and redox reactions. Two years after the course, the teachers were interviewed about their PCK of (1) students’ difficulties in understanding acid–base chemistry and (2) models of acids and bases in their teaching practice. In the interviews, the teachers were asked to comment on authentic student responses collected in a previous study that included student interviews about their understanding of acids and bases. Further, the teachers drew story-lines representing their level of satisfaction with their acid–base teaching. The results show that, although all teachers recognised some of the students’ difficulties as confusion between models, only a few chose to emphasise the different models of acids and bases. Most of the teachers thought it was sufficient to distinguish clearly between the phenomenological level and the particle level. The ways the teachers reflected on their teaching, in order to improve it, also differed. Some teachers reflected more on students’ difficulties; others were more concerned about their own performance. Implications for chemistry (teacher) education are discussed.

Appendix

Excerpt from Interviews with Students

Acid–base Reactions

• 1. Students were asked to write a reaction equation between an acid and a base.

  $${\text{HAc}} + {\text{NaOH}} \to {\text{NaAc}} + {\text{H}}_2 {\text{O}},$$

  • Since water is formed, it becomes neutral.

    $${\text{HCl}} + {\text{NaOH}} \to {\text{NaCl}} + {\text{H}}_2 {\text{O}},$$

  • In an acid–base reaction, it becomes neutral.

    $${\text{HCl}} + {\text{NaOH}} \to {\text{NaH}}_2 {\text{O}} + {\text{Cl}}^ - $$

  • The base NaOH takes a proton from the acid HCl.

• 2. Students were asked what an acid–base reaction is.

  • In a reaction there is always one [particle] acting as a base. Even if you have two acids, for example HCl and H₂SO_4, one is a base.

  • Acids and bases are very difficult for me...Acids donate protons and bases are substances that accept these protons. It is pretty tricky, because, if you look at water, it is both an acid and a base. If a water molecule is first a base, it takes a proton from another water molecule, so that the water molecule that was first a base suddenly becomes an acid. It is quite difficult to keep up with that, how it turns around. In an acid–base reaction the result will be neutral... They consume each other depending on their concentrations. Hydrogen ions or oxonium ions and hydroxide ions form water in a neutralisation.

Water is an Acid

• 3. Students were asked to identify acids and bases in the equation:

  $$\text{NH}_3 + \text{H}_2 \text{O} \rightleftarrows \text{NH}_4^ + + \text{OH}^ - $$

  When they hesitated to define water as an acid, they were asked why.

  • It is difficult to think about water as sour...

  • Because water is not really...it is both an acid and a base, as I recall it...but... I do not know...water is water, right? It makes me think of something neutral, something that is not extreme in any way.

  • You can’t imagine drinking an acid, but you drink water.

Model Confusion

• 4. The students were asked to explain the differences between the following two equations.

  $$\begin{array}{*{20}l} {{\text{acid}} + {\text{base}} \to {\text{salt}} + {\text{water}}} \hfill \\ {{\text{acid}}_{\text{1}} + {\text{base}}_{\text{2}} \to {\text{base}}_{\text{1}} + {\text{acid}}_{\text{2}} } \hfill \\ \end{array} $$

  • Some substances, for example, water, can act both as an acid and as a base depending on the substance with which they react.

  • Salt and water are formed... there should be an acid and a base as well...perhaps you can identify NaCl as an acid, I am not too familiar with that.

  • It would have been better to learn Brønsted from the beginning. It gets messy changing models when you’ve already learned it one way.

    $${\text{HCl}} + {\text{NaOH}}$$

• 5. Students were asked to explain, on a particle level, the reaction between the substances HCl and NaOH.

  • Perhaps it strives to turn into water. If these two (Na⁺ and Cl⁻) are attracted more to each other and NaCl is a salt, right? Perhaps it has something to do with a metal and a non-metal attraction. They want to form a salt. And if OH and H have a stronger attraction to each other, I do not know.

Buffer Solutions

• 6. Students were asked to explain how buffer solutions work.

  • How can it work? You cannot have an acid and a base in the same solution. In that case the acid and the base should consume each other.

Excerpts from Textbooks

Since the teachers used different books, different excerpts were discussed. As the excerpts from the different books were quite similar, however, an example from each type of excerpt is presented below.

• 1. The books define acids according to their general properties, e.g., sour taste, colouring litmus, reaction with non-precious metals, etc (for example, textbook 1, page 103).

  The substances that for many years were called acids have many common properties. For instance, they have a sour taste and it is said that their reactions are sour.

• 2. The books define acids according to Brønsted (for example, textbook 2, page 79).

  A reaction in which protons are donated or accepted is called a proton transfer reactions. The substance or ion that donates protons is an acid, according to Brønsted’s definition of acids.

• 3. The books’ explain acidic solution/basic solution (for example, textbook 2).

  The oxonium ions give water solutions their sour taste and sour reaction. In colloquial speech, that sour solution is called an acid (page 78)...The rule is to pour the acid into water (page 80; with acid, the authors refer to a water solution of an acid.)

• 4. The books use redox reactions, e.g., reactions with non-precious metals (for example, textbook 3, page 161).

  Now that we know that acids are proton acceptors, we can write formulas for acidic solutions’ reactions with carbonate compounds and with non-precious metals....

  Reaction with carbonate

  $${\text{CaCO}}_{\text{3}} \left( {\text{s}} \right){\text{2H}}^{\text{ + }} \left( {{\text{aq}}} \right) \to {\text{Ca}}^{2 + } \left( {{\text{aq}}} \right) + CO_2 \left( {\text{g}} \right) + {\text{H}}_{\text{2}} {\text{O}}\left( {\text{l}} \right)$$

  Reaction with non-precious metal

  $${\text{Mg}}\left( {\text{s}} \right) + {\text{2H}}^{\text{ + }} \left( {{\text{aq}}} \right) \to {\text{Mg}}^{2 + } \left( {{\text{aq}}} \right) + {\text{H}}_{\text{2}} \left( {\text{g}} \right)$$

• 5. The books use formula equations and ionic equations according to, for instance, neutralisation (for example, textbook 1, page 112).

  $${\text{Na}}^{\text{ + }} + {\text{OH}}^{\text{ - }} + {\text{H}}_{\text{3}} {\text{O}}^{\text{ + }} + {\text{Cl}}^{\text{ - }} \to {\text{2H}}_{\text{2}} {\text{O}} + {\text{Na}}^{\text{ + }} + {\text{Cl}}^{\text{ - }} $$

  We can see that the ions Na ⁺ and Cl ⁻ do not participate in the reaction. They are called counter ions or “spectator ions.” Thus, only the following reaction takes place.

  $$\mathop {{\text{OH}}^{\text{ - }} }\limits_{{\text{base}}} + \mathop {{\text{H}}_{\text{3}} {\text{O}}^{\text{ + }} }\limits_{{\text{acid}}} \to {\text{H}}_{\text{2}} {\text{O}} + {\text{H}}_{\text{2}} {\text{O}}$$

  “The reaction between an acid and a base is called neutralization. A salt is formed.”

None of the textbooks explained the following about acids and bases:

• That models are used

• What model is in use at the moment

• Why different models are used in parallel

• The scope and limitations of each model.

Textbook 1: Andersson, S., Sonesson, A., Stålhandske, B., & Tullberg, A. (2000). Gymnasiekemi A. Stockholm: Liber.

Textbook 2: Borén, H., Larsson, M., Lif, T., Lilleborg, S., & Lindh, B. (2000). Kemiboken A 100p. Stockholm: Liber.

Textbook 3: Henriksson, A. (2000). Kemi kurs A. Malmö: Gleerups Förlag.