Electrochemical Cells

A device that uses a chemical reaction to produce or use 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 that 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 that ionize to a high degree are referred to as strong electrolytes, while those that 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 of zinc sulfate into the left tube and a copper (II) sulfate solution into the one on the right. Then a strip of zinc metal is dipped into the ZnSO 4 solution; a piece of copper is inserted into the CuSO 4 solution; and the two ends of the metal strips are connected by wires to a 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.

The electric current is caused by a pair of redox reactions. At the zinc electrode, the metallic zinc is slowly being ionized by an oxidation reaction:

  equation

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 Cu 2+ cations to be deposited onto the electrode as copper metal:

  equation

Because negatively charged electrons are flowing from the anode to the cathode, the anode becomes the positive electrode. The cathode is, therefore, the negative electrode.

Figure 1. A voltaic cell.

figure

Adding the reactions at the two electrodes gives 

equation

which is the overall redox reaction in the zinc‐copper cell.