A Knowledge of Electrolysis Thwarts a Scam

A salesman knocks at your door, selling home water distillers.  

He claims your municipal tap water contains dissolved heavy metal ions, which can be removed when you use his fancy-schmancy distiller. When you question the salesman, he produces an electric-powered gizmo, pictured in Figure 1, that supposedly precipitates heavy metal cations in your tap water. He does a side-by-side test (see the two drinking glasses) using your municipal tap water and distilled water produced by the home distiller. After a short while, you notice a brown precipitate, whose amount increases with time, forming in the tap water. The distilled water (from his high-priced distiller), of course, does not react and remains precipitate-free. He then pressures you to buy his expensive home distiller, so that “you, too, can enjoy pure water”.

You smell a rat.

Instead of buying the home distiller, you manage to purchase the gizmo (Figure 1), which you believe the salesman used to create some kind of electrolytic cell.

Figure 1. The electrolysis apparatus used by the door-to-door salesman. Note that it can simultaneously electrolyze two solutions.⁴

You bring the electrolysis apparatus to the Chemistry lab and conduct some experiments.

Here are your findings:

• The electrolysis apparatus (Figure 1) converts AC to DC
• Of each pair of electrodes, one is iron, the other is aluminium
• The brown precipitate produced in the tap water is determined to be Fe(OH)₃⁵
• The quantity of Fe(OH)₃ increases in direct proportion to how long a current is passed through the tap water
• In the tap water, bubbles are produced at the Al electrode
• There is no visible reaction in the distilled water sample

Questions

1. Why was a reaction observed in the tap water sample and NOT with the distilled water?
2. Sketch a fully labelled diagram of the electrolytic cell occurring in the tap water. Include the relevant half equation for each electrode. Label each electrode as either Fe or Al; indicate which is the anode and cathode; label their respective polarities. Show the direction of electron and ion movement.
3. Provide the balanced chemical equation for the overall reaction; calculate E^o_cell. 

See supporting materials for student-ready handouts and answer key.