A new classification scheme for teaching reaction types in general chemistry

General chemistry is a freshman-level class required for most science majors and pre-professional undergraduates. A variety of concepts are introduced in this course, including stoichiometry of formulas and equations, chemical reactions, electronic structure and periodic properties of elements, chemical bonding and molecular geometry, gases, liquids and solids, solutions and colloids, kinetics, equilibrium, thermodynamics, electrochemistry, transition metals and coordination chemistry, and nuclear chemistry. Among these topics, chemical reactions are fundamental, but remain a central component of chemistry.

There are millions of known compounds and millions of possible chemical reactions. To better understand how substances interact with one other, chemists group chemical reactions into classes based on their reaction patterns. There are different ways to classify chemical reactions; the most commonly used classification system is introduced to students via general chemistry textbooks.

The author investigated 10 textbooks widely used for general chemistry teaching (Table 1). All of them introduce the three major classes of chemical reactions: precipitation reactions, acid-base reactions, and oxidation-reduction (redox) reactions. However, significant differences exist between the treatments of the topic. Some textbooks do not indicate that precipitation and acid-base reactions are subclasses of double-displacement/exchange/metathesis reactions; others do not point out the subtypes of redox reactions when elements are involved in the reaction, such as combination redox, decomposition redox, single-displacement/replacement, and combustion reactions. In addition, some textbooks do not clearly indicate that there are combination and decomposition reactions not belonging to the redox reaction subgroup; others do not provide examples of redox reactions not involving elements.

The inconsistency of reaction classifications in general chemistry textbooks may cause confusion: First, students may not know what double-displacement/metathesis reactions are if they are only taught the three major classes of chemical reactions (i.e. precipitation reactions, acid-base reactions, and oxidation-reduction reactions); second, although detailed examples are given from one or more subclasses of redox reactions when teaching the redox reaction category, students may not realize that certain combination and decomposition reactions, along with single-displacement and combustion reactions, are subtypes of redox reactions if the names of these subclasses are not made clear; third, if combination and decomposition reactions are only introduced under the redox category, students may not realize that there are also combination and decomposition reactions not belonging to the redox category; and fourth, if only the redox subclasses involving elements are introduced, students may not realize that there are also redox reactions not involving elements. The author has not found any publication that addresses the differences between reaction classification systems in chemistry textbooks, nor on clarifying the confusion that arises from current classification systems. In order to address these problems, the author suggests a modified classification scheme for teaching about reaction types in general chemistry.

The author groups the major chemical reactions into four categories: (1) double-displacement reactions, (2) oxidation-reduction (redox) reactions, (3) combination reactions that are not redox reactions, and (4) decomposition reactions that are not redox reactions. Precipitation and acid-base reactions are classified as separate subtypes of double-displacement reactions. The redox category is first divided into two subgroups: redox reactions not involving elements, and redox reactions involving elements. The combination redox, decomposition redox, single-displacement, and combustion reactions then become four subclasses of “redox reactions involving elements,” along with the fifth subclass called “other redox reactions involving elements as reactants and/or products.”

Assigning the two major classes of chemical reactions – precipitation and acid-base reactions – both to group (1) allows students to observe their similarity (i.e. both are double-displacement reactions). Dividing group (2) – redox reactions – into two subgroups helps students realize that redox reactions may or may not involve elements; as long as electron transfer occurs (i.e. a change in oxidation number for atoms in a reaction), it is a redox reaction. Including five subclasses in the “redox reactions involving elements” subgroup provides students a complete view of the possible ways to arrange elements in both reactants and products. Furthermore, presenting groups (3) and (4) allows students to understand that not all combination and decomposition reactions are redox reactions.

Reaction examples and notations are associated with each reaction category, as shown below. By using this classification method, undergraduate students should gain a more complete and nuanced view of chemical reactions while taking general chemistry, which will build a stronger foundation for learning advanced chemical reactions from the domains of organic, inorganic, and biochemistry.