Electrochemistry Tutorial: Galvanic Cells and the Nernst Equation >> Step 3: Measuring cell potentials

Electrochemistry: Galvanic Cells and the Nernst Equation

Step 3: Measuring cell potentials

In the previous step, we harnessed the energy of the chemical reaction by creating a galvanic cell to power a stopwatch. In this section we will look at how to measure the energy produced in a galvanic cell. Using this information we can understand why some of the reactions powered the stopwatch and others did not.

To measure the energy produced in a galvanic cell, we will use a voltmeter, placing it in the pathway of the electrons (on the wire). In the following movie, we demonstrate how to measure the voltage for a Zinc/Copper galvanic cell as one would in a lab.

There are two aspects of the flow of electrons in an electrochemical cell that relate to the energy:

Current: The number of electrons flowing per second.
Potential: The energy associated with each electron.

We can understand the distinction between these aspects by drawing analogies with water. Consider a very high waterfall that has a trickle of water flowing over it. This corresponds to a low current (not much water is flowing) but high potential (each drop of water has a lot of energy when it hits the ground). The greater the difference in the water level at the top and bottom of the waterfall, the greater the energy behind the flow of water. A large river flowing down a gradual slope has high current with low potential. Niagara falls has both high potential and high current.

When we hook up a voltmeter to an electrochemical cell, we are measuring only the potential, or the energy of the electrons.

The unit of the cell voltage, volt (V), is the energy per unit charge; 1 Volt = 1 Joule/Coulomb.

The potential difference of 1.63V read by the voltmeter in the above movie signifies an energy of 1.63 J for every coulomb of charge passing through the electric circuit.

The cell voltage is also called a cell potential and represented by the symbol E_cell.

Remember, as we discussed above, that when the direction of the spontaneous flow of electrons is from the black (-) terminal of the voltmeter to the red (+) terminal, then the voltmeter will show a positive voltage. A negative voltage indicates that the spontaneous direction of electron flow is from the red (+) terminal to the black (-) terminal.

In this next activity, you will use electrochemical cell simulation to measure the voltages for the reactions in the table below.