A device that uses a chemical reaction to produce electricity is an
electrochemical cell, also known as a
voltaic cell. Because the liquid state allows reactions to occur more readily than in either solids or gases, most electrochemical cells are built using a liquid called an
electrolyte, a solution which contains ions and conducts electricity. This word has previously been mentioned with regard to ionic dissociation. Pure, distilled water is a very poor conductor of electricity, but a high concentration of dissolved ions leads to high conductivity. That is why acids, bases, and salts which ionize to a high degree are referred to as strong electrolytes, while those which ionize only slightly are referred to as weak electrolytes.
A simple electrochemical cell can be made from two test tubes connected with a third tube (the crossbar of the “H”), as shown in Figure
1 . The hollow apparatus is filled by simultaneously pouring different solutions into the two test tubes, an aqueous solution
(aq) of zinc sulfate into the left tube and a copper sulfate solution into the one on the right. Then a strip of zinc metal is dipped into the ZnSO4 solution, a piece of copper is inserted into the CuSO4 solution, and the two ends of the metal strips are connected by wires to an voltmeter. The lateral connecting tube allows ionic migration necessary for a closed electrical circuit. The voltmeter will show the electrical potential of 1.10 volts, which leads to the movement of electrons in the wire from the zinc electrode toward the copper electrode.
Figure 1
A voltaic cell.
The electric current is caused by a pair of redox reactions. At the zinc electrode, the metallic zinc is slowly being dissolved by an oxidation reaction:
An electrode at which oxidation occurs is called an
anode; it strongly attracts negative ions in the solution, and such ions are consequently called anions.
Simultaneously, a reduction reaction at the copper
cathode causes Cu2+ cations to be deposited onto the electrode as copper metal:
Because negatively charged electrons are flowing from the anode, it is the negative electrode. The cathode is the positive electrode.
Adding the reactions at the two electrodes gives
which is the overall redox reaction in the zinc-copper cell.
Elements
Electrochemistry
