Ted Weiland really nails the lesson on metals’ redox properties in his introductory chemistry lab at Kirkwood Community College in Iowa. He asks students to dip two iron nails into oxalic acid and wrap one of them with zinc. Both nails then go into a Petri dish of agar with extra salt to facilitate ion transfer and phenolphthalein to visualize pH changes. When students arrive in lab the following week, the metallic iron in the plain nail, Fe0, has oxidized to Fe2O3—in other words, rusted. Meanwhile, the zinc-covered nail is surrounded by a white solid and a halo of pink that indicates a high concentration of hydroxide ions in the agar. Weiland explains that zinc is more redox-active than iron, so it oxidizes first, reducing the surrounding water in the process to hydrogen gas and hydroxide ions, some of which pair with Zn2+ ions to form solid zinc hydroxide. By doing this experiment, the students learn why galvanized metal, which is encased in a protective layer of zinc, does not rust.
Credit: Ted Weiland
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