Synthesis and Decomposition of Zinc Iodide: Model Reactions for Investigating Chemical Change in the Introductory Laboratory
The purpose of this article is to discuss two colorful reactions not widely used by chemical educators in high schools or college chemistry laboratories: The synthesis of zinc iodide from its elements, zinc and iodine, and the subsequent decomposition of zinc iodide back into its elements. These reactions are important for chemistry teachers to know about because they can be performed by introductory students to understand different aspects of chemical change such as the concepts of reaction, compound, bonding, excess and limiting reactants, an empirical formula, balanced chemical equation, the conservation of matter and energy, the Law of the Conservation of Mass, and the Law of Constant Composition. These concepts, in turn, are important because they are fundamental to chemistry, are widely taught by chemistry teachers, and are deceptively difficult for introductory chemistry students to understand. The synthesis of zinc iodide has many scientific advantages over current syntheses of binary compounds from elements such as the syntheses of copper sulfide and magnesium oxide. For example, zinc iodide can be synthesized to 1% of theoretical mass in less than a half an hour and can be readily analyzed qualitatively as well as quantitatively by two different titrations. As a set of reactions, the synthesis and decomposition of zinc iodide is safe to perform, reliable, inexpensive, and does not pose a threat to the environment.The author has developed a small collection of teacher activities describing the synthesis and decomposition of zinc iodide. The activities are innovative because they contain improvements not found in the existing literature. Appropriate for high school and first year college chemistry teachers, all of the activities contain detailed procedures and discussions as well as safety and disposal requirements.