lectrochemistry: Voltaic Cells and Voltage
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Now that we have a standard, we can find the potential difference of the zinc half-cell by comparison. However, before we connect the external circuit, remember the following equilibrium reactions are occurring on each electrode:
H2 (Pt
electrode) 
2H+(solution) + 2
e–(electrode)
Zn(electrode)
Zn2+(solution) + 2
e–(electrode)
Because zinc has a greater tendency to ionize than does hydrogen gas, a higher potential will be established across the zinc electrode. So, when we close the circuit and connect the wires, the electrons will flow down the potential gradient from the zinc strip to the platinum electrode. This difference in potential is what causes the electrons to flow. As part of this larger cell, the zinc electrode is acting as an anode:
Zn(electrode) 
Zn2+(solution)
+ 2 e–(electrode)
Simultaneously, the reduction of hydrogen ion is occurring on the platinum strip; this electrode is acting as a cathode:
2e–(electrode) +
2H+(solution) H2 (Pt electrode)
The potential difference that is driving the electron flow from zinc atoms to hydrogen ions is called the electromotive force and can be measured with a voltmeter. This force is simply the addition of the potentials generated by the oxidation and reduction half-cells:
E°net =E°ox + E°red
When the flow of electrons is spontaneous, E°net is positive. In this reaction, because our standard is defined as contributing exactly 0 V (at standard conditions), E°net will be the potential contribution from the oxidation reaction occurring at the zinc electrode:
E°net
=E°ox + E°red
E°net=E°Zn + E°SHE
E°zn=E°net – E°SHE
E°zn=E°net – ( 0 )=E°net
Activate Figure 4 to find the cell potential, measured in volts, that is driving electrons between the zinc and standard hydrogen electrodes.
Click on figure to find cell potential.
Good!
The
reduction of Zn will have the same magnitude potential as the oxidation.
Remember
the standard potential for the SHE is 0 V.
Before moving to the next page, you should understand how a potential gradient is generated across a single electrode and between two half-cells. You should also understand the operation and function of a standard hydrogen electrode (SHE).
Electrochemical Cells and Voltage 
