. Gold is currently about 23,000 times more expensive than aluminum. This is not surprising, especially given the fact that gold is very rare, whereas aluminum is the third most abundant element in the Earth's Crust (after oxygen and silicon). It might be hard to believe that there was a time when aluminum was much more expensive than gold. For decades since the first preparation of aluminum metal by the Danish physicist Hans Christian Ørsted in 1825, aluminum was much more expensive than gold. Although naturally occurring Al compounds were abundant, the challenge was to convert them to aluminum metal. In the mid-1850s, the French chemist Henri Étienne Sainte-Claire Deville solved the problem by developing the first industrial method to make aluminum. His method was based on electrolysis. In 1886, the advanced Hall-Héroult aluminum smelting process was developed, which was also based on electrolysis. This process is still used nowadays to produce aluminum on a tremendous scale, over 60 million metric tons a year worldwide.
Electrolysis has played a very important role in the development of chemistry, particularly in the discovery of many most highly reactive simple substances. For example, potassium and sodium metals were prepared by Humphry Davy using electrolysis in 1807. Likewise, electrolysis was used to make and isolate fluorine gas (F2
), the most aggressive and reactive simple substance. This discovery was made by Henri Moissan in 1886, who was awarded the 1906 Nobel Prize for his work. For exactly 100 years since Moissan's electrochemical synthesis of fluorine, scientists were convinced that F2
could be prepared only by electrolysis. In 1986, however, the discovery of the first electricity-free, purely chemical method to make fluorine was reported.
If electrical energy can be used to force a reaction that otherwise does not occur, can a reverse process be created to make electricity from the energy released by a spontaneous chemical reaction? Such a process could be compared to cycling downhill in contrast with electrolysis that is like cycling up the hill (Figure 2-97). The answer is "yes", and I bet you have many times seen and used such chemical reactors producing electricity. These reactors are batteries. There are a number of different chemical reactions that are used in batteries. However, not every chemical reaction can be used as a source of electricity. For instance, the reverse of the electrolytic decomposition of HCl (H2
= 2 HCl) and CuCl2
(Cu + Cl2
) cannot be used to make electricity for a number of reasons. Perhaps the oldest chemical system to generate an electric current is the galvanic cell
employing Zn and Cu metals and their sulfates. As the Cu2+
discharges at the copper cathode by accepting two electrons (Cu2+
+ 2 e-
= Cu), the Zn metal that the anode is made of dissolves while releasing two electrons (Zn - 2 e-
). The electric current in the Cu-Zn galvanic cell is a flow of electrons from the Zn metal to the Cu2+
cations. [Note that in a galvanic cell the anode is the negatively charged electrode and the cathode is the positively charged electrode.]