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With this paper, our main aim is to contribute to the realisation of the chemical reactivity concept, tracing the historical evolution of the concept of chemical affinity that eventually supported the concept of chemical equilibrium. We will concentrate on searching for the theoretical grounds of three key chemical equilibrium ideas: 'incomplete reaction', 'reversibility' and 'dynamics'. In addition, the paper aims to promote teachers' philosophical/ historical chemical knowledge. The starting point of this historical reconstruction will be the state of the art in the construction of the first affinity tables, based on the concept of elective affinities, during the 18th century. Berthollet reworked this idea, considering that the amount of the substances involved in a reaction was a key factor accounting for the chemical forces. Guldberg and Waage attempted to measure those forces, formulating the first affinity mathematical equations. Afterwards, the first ideas providing a molecular interpretation of the macroscopic properties of equilibrium reactions are presented. Eventually, theoretical chemists integrated previous findings into a new field: thermodynamics. This historical approach may serve as a base for an appropriate sequencing of the teaching and learning of chemical equilibrium. Hence, this paper tries to go beyond the simple development of teachers' conceptions of the nature of chemistry, for it gives suggestions about how teachers may translate such understandings into classroom practice.

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This study analyzes the misrepresentation of Gibbs energy by college chemistry textbooks. The article reports the way first-year university chemistry textbooks handle the concepts of spontaneity and equilibrium. Problems with terminology are found; confusion arises in the meaning given to ΔG, Δ r G, ΔG°, and Δ r G°, which results in many textbooks not differentiating between ΔG and Δ r G. Also, there is confusion over when standard conditions apply and when they do not. A problem with the proper use of units is also found. Finally, it is suggested that most of these difficult concepts could be removed from the first-year university chemistry syllabus because (i) an accurate presentation of Gibbs energy would be far beyond an introductory chemistry level and (ii) current attempts to introduce those difficult concepts in first-year university chemistry courses are usually full of misleading formulations.

THE PRACTICE OF CHEMISTRY EDUCATION (PAPER)Concept teaching and learning/ History and philosophy of science (HPS)