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Galvanic cells

In a galvanic cell, the oxidation half-reaction occurs at the anode. Since electrons are deposited on this electrode, it is the negative electrode. The reduction half-reaction occurs at the cathode, which loses electrons and is therefore the positive electrode. The two half-reactions are physically separated, which forces the electrons to move through an external circuit (see Figure 1). This is what generates electricity.

Two almost full beakers are placed beside each other. One beaker is full of copper sulfate and it contains a piece of copper, which is the anode. The other beaker is full of silver nitrate and it contains a piece of silver, which is labeled as the cathode. A wire is connecting the anode and the cathode, which allows the transfer of electrons from the anode to the cathode. This wire also has a voltmeter that is measuring the electric potential. The value is 0,460 volts. There is a tube that contains salt connecting both solutions from the two beakers. The salt bridge is filled with potassium nitrate. In the first beaker, the copper ions are moving toward the salt bridge from the anode and the nitrate ions are moving from the bridge into the beaker. In the other beaker, the silver ions are moving towards the cathode and the nitrate ions towards the salt bridge.

Figure 1: Galvanic cell with two half-cells, external circuit (wires) connecting the anode and cathode, a voltmeter for measuring the cell potential, and a salt bridge.

To allow the reaction to continue, the charges need to be balanced and ions need to be able to move between the cells. Without this movement of ions, one half of the cell would build up a net positive charge and the other half would build up a net negative charge and the reaction would stop. Both anions and cations contribute to the balancing of the charge and the completion of the electric circuit. In a galvanic cell, the ions can move through a salt bridge (see Figure 1) or a porous membrane between the two half-cells.