Science practice 29

There is some evidence that ancient civilizations knew placing various metals together could create an electrical current. In the year 1800, Alexander Volta published experiments outlining his discovery of the voltaic pile, a device commonly referred to as the first electric battery. Volta stacked two different metals on either side of a wet felt disk and found that certain combinations produced an electrical voltage. By the early 1800s, many scientists were expanding on the idea of the voltaic pile by making apparatuses now known as voltaic cells. These cells generally contain two separate jars connected by a salt bridge, or porous membrane. Each jar contains a certain metal and a solution of the positive ions of the same metal. When different jars containing different metals are connected, an electrical voltage can be produced. A theoretical example of a copper/zinc voltaic cell is shown in Figure 9.1.

Figure 9.1

The last two centuries have seen a marked spike in demand for smaller batteries that can produce higher voltages for longer periods of time. This led a chemistry student to become interested in how using different metals in the voltaic cell could increase the voltage output of that cell. To conduct the experiment she chose four different types of metals that were available in strips from her local hardware store. These metals were zinc (Zn), lead (Pb), copper (Cu), and silver (Ag). She used 1-molar concentrated solutions of each of the various metal ions. She then made a salt bridge out of filter paper soaked in a potassium chloride brine solution. Many different combinations of metals were attempted, and the voltage output was measured with a standard voltmeter. The results of the experiment are recorded in Table 9.1.

TABLE 9.1

After the experiment was completed, the chemistry student looked at the literature to make sense of her results. She found two definitions particularly helpful. The anode was defined as the metal strip where oxidation occurs. The metal atoms were losing electrons and dissolving into the solution as metal ions. Electrons from the anode were free to move through the wire toward the cathode. The cathode was defined as the metal strip where reduction occurs. The cathode had an abundance of electrons from the anode. Metal ions in the solution around the cathode accepted those electrons and joined the strip as additional solid metal atoms.

The student also found tables of standard reduction potentials (Table 9.2). These tables compared how much voltage should be produced when the metal is placed in an electrochemical cell with a standard electrode. One table showed each metal as a cathode, and the other showed each as an anode. The student learned that these tables were used to calculate the theoretical voltage output of any electrochemical cell. (Any electrochemical cell has to have both a cathode and an anode.)

TABLE 9.2

The theoretical output voltage of an electrochemical cell is the sum of the standard potentials of the anode and the cathode.