We can predict the voltage of a cell by using the table on the left of standard half cell potentials. These half cell potentials were measured against the half cell. The hydrogen half cell was arbitrarily assigned a value of 0V.

When the half cell is connected to the half cell a potential difference of 0.34V exists. The copper half cell is 0.34V greater in potential than the hydrogen half cell. Similarly when the half cell is connected to the hydrogen half cell a difference of 0.41V in favour of the hydrogen half cell exists.

In developing this half cell potential table we can now start to predict the potential difference of a given cell.

Example 1)

Using the table above, predict the cell potential difference when the two half cells below are connected.

and .

Step 1 Potential difference = highest - lowest

Step 2 Using the table above write down the values

potential difference = 1.49 - 0.34 =1.15V

Example 2)

Using the table above, predict the cell potential difference when the two half cells below are connected.

and

Step 1 Potential difference = highest - lowest

Step 2 Using the table above write down the values

potential difference = 0.34 - -2.38 = 0.34 + 2.38 = 2.72V