Passage V

A zinc-air battery (ZAB) produces an electrical current when O₂ in the air flowing into the ZAB reacts with zinc metal to produce solid zinc oxide:

2Zn + O₂ → 2ZnO

Two experiments were done using the apparatus shown in the diagram below.

For each trial, Steps 1-5 were followed:

1. Three 1.4-volt batteries were stacked in series (so that their voltages added together) in the airtight chamber. Each battery was either a ZAB or an SLB, a sealed lithium battery (the type of battery used in watches).

2. A small buret (a graduated tube that could be opened or closed using a stopcock) was inserted through a port into the chamber. A bead of H₂O was introduced into the buret to act as a free-moving barrier to the surrounding air.

3. A current regulator was connected to the batteries to ensure they would generate a constant current.

4. The chamber was sealed, and then the stopcock was opened (at time = 0 sec) to allow only the air from the buret to enter the chamber. As the Zn in a ZAB reacted with O₂, more air was drawn into the chamber.

5. As each 0.050 mL of air entered the chamber—indicated by the movement of the bead toward the chamber—the elapsed time was recorded. When 0.40 mL of air had entered the chamber, the stopcock was closed.

Experiment 1

Three trials were done at a current of 20 milliamps (mA). In Trial 1, 1 ZAB and 2 SLBs were tested. In Trial 2, 2 ZABs and 1 SLB were tested.In Trial 3, 3 ZABs were tested (see Figure 1).

Experiment 2

Trial 3 was repeated in 2 trials except that in Trial 4 the current was 10 mA, and in Trial 5 the current was 30 mA (see Figure 2).