lectrochemistry: Voltaic Cells and Voltage

Finally, let’s examine our single silver electrode. We find that silver atoms have a lesser tendency to ionize than do hydrogen atoms; therefore the potential is higher across the platinum electrode. When the external circuit is connected, electrons travel down this potential energy gradient to the silver electrode to reduce aqueous silver ions. A voltmeter would report a voltage of + 0.80 V for this voltaic cell :

e^–_(electrode)  +  Ag⁺_(solution)  Ag _(electrode)

Does the silver electrode serve as the anode or cathode when the circuit is closed?

anode	cathode

E°_net for this cell is +0.80 V. If we were to look on a table of standard reduction potentials, we would see +0.80 V reported for the silver reduction half-reaction.

Note: Most voltmeters are polarized— the red(+) lead should be connected to the cathode (reduction electrode) and the black(–) lead should be connected to the anode (oxidation electrode). When the leads are connected 'in reverse', a negative voltage is measured; this allows us to quickly determine the cathode and anode in the laboratory! Remember, however, that E°net is always positive for a voltaic cell, regardless of what a voltmeter suggests.

Now that the potentials across our zinc and silver single electrodes have been defined, we can connect them directly to find the voltage of a zinc/silver { Zn|Zn²⁺, Ag⁺|Ag } voltaic cell.

E°_net =E°_ox + E°_red
E°_net=E°_Zn + E°_Ag
E°_net =(+0.76 V) + ( +0.80 )=1.56 V

Before leaving this module, you should understand how a potential is established on a single electrode and how the voltage of a voltaic cell, E°_net, is defined and measured.

Electrochemical Cells and Voltage