Redox reactions: a conceptual mess?

Redox reactions are fundamental chemical processes that include an heterogeneous range of phenomena (from photosynthetic and respiratory processes to combustion, corrosion and a number of organic reactions). Indeed redox processes are a central topic of any introductive chemistry course, at the level of secondary school as well as at the University. Learning such an intrinsically complex subject implies dealing with conceptual obstacles that are often magnified, rather than overcome, by the teacher’s choices. Two most frequent mistakes in teaching redox are: i) the implicit reference to distinct models (e.g. oxidation numbers and electron transfer) without pointing them out or highlighting their different nature and realm of validity; ii) to mix up the formal level of explanation with the level of physical evidence. Four models are usually employed to interpret and classify redox phenomena. They refer to the transfer of oxygen, hydrogen and electrons, respectively, or to a change in oxidation numbers. The first three models are empirical, they overlap only partially and have limited validity. The fourth is a purely formal model and it has wider applicability. Reactions that are not recognized as redox by the first three models may be included in the redox family based on the change of oxidation numbers. The coexistence of distinct models is often overlooked in redox teaching: this may result in considerable learning problems. A critical discussion of these educational implications will be proposed.