Oxidation-Reduction Reactions

In this exercise, students observe simple oxidation-reduction reactions and rationalize the results in terms of the basic chemistry. The reagents chosen show color changes and have the additional advantage of modeling real biochemical redox reactions. The interconversions of ferrocyanide/ferricyanide and benzoquinone/hydroquinone model cytochrome and ubiquinone reactions. (The students are cautioned about the hazards of benzoquinone and hydroquinone; only a few drops of 0.1 M solutions are used to minimize risk.)

Several reagents are used in this exercise: 0.1 M solutions of potassium ferricyanide (keeps indefinitely) and potassium ferrocyanide (prepared fresh, keeps a month or so); 0.1 M solutions of benzoquinone and hydroquinone (prepared immediately before lab, decompose in hours); 5% sodium hypochlorite (commercial bleach; stable for over a year, but check before using); 3% hydrogen peroxide (stable for a long time but check before using); and 5% ascorbic acid or sodium ascorbate (make fresh; probably stable for a few days).

Students observe and record the color changes associated with reaction of 15 drops of either potassium ferrocyanide or potassium ferricyanide with four drops of either sodium hypochlorite, hydrogen peroxide, or ascorbic acid. They do the same for reaction of 15 drops of benzoquinone with one drop, and then five more drops, of ascorbic acid; and 15 drops of hydroquinone with one drop, and then five more drops of sodium hypochlorite (the solution begins to turn a dark brown a few seconds after addition of the hypochlorite; they are told to ignore this further color change). The two-step oxidation or reduction of the quinone species shows (I think) the intermediate free radical as a deeply-colored solution.

These reactions only take a few minutes to do. I give them several questions to help them interpret what they have seen. They first determine the oxidation state of the iron atom in the ferrocyanide and ferricyanide ions. From that distinction, they figure out whether hypochlorite, hydrogen peroxide, and ascorbic acid are oxidizing or reducing agents. They are asked to figure out the change in oxidation state of chlorine as hypochlorite changes to chloride. From the color changes in the quinone reactions, they are asked to identify the highly-colored intermediate. They are asked to determine the number of electrons gained or lost in these reactions. Finally, I try to connect what they've seen in lab with biochemical oxidations by asking them to determine how many cytochromes or quinones could be reduced by the electrons produced by oxidizing an alcohol to an aldehyde.