19.1 â Standard Electrode Potentials
19.1.1 – Describe the standard hydrogen electrode
The standard hydrogen electrode is made up of an acidic solution, containing H+ ions in a concentration of 1.00 mol dm-3. There is also a platinum electrode sitting inside a glass tube which bubbles H2 gas through the solution at a pressure of 1.01 x 102 kPa and a temperature of 298 K.
19.1.2 – Define the term standard electrode potential (Ef)
The voltage or potential or potential of a particular half-cell when it is connected to the standard hydrogen half-cell by an external circuit and salt bridge.
19.1.3 – Calculate cell potentials using standard electrode potentials
The oxidising agent has the more positive Eᶱ value when looking at the reduction potentials. This is because electrons always flow towards the half-cell with the highest Eᶱ value. It is crucial that you do not invert them before calculating the cell potential. Remember that oxidation occurs at the anode and reduction occurs at the cathode.
As a point of comparison, the standard hydrogen electrode is given the Eᶱ value of 0.00 V. The H+ ion is an oxidising agent and the H2 gas is a reducing agent. If the hydrogen half-cell is connected to another cell with a negative Eᶱ value, then the H+ ions will oxidise it. On the other hand, if it is connected to a half-cell with a positive Eᶱ value, then the H2 gas will reduce it.
In the cell made up of a Ni and a Cu half-cell, Ni(s) is the reducing agent, as it has the more negative reduction potential.
This means that Ni would be at the anode and Cu would be at the cathode.
They will react according to the equation:
Using the values given in the data booklet, the cell potential can be calculated:
0.34- -0.23=0.57V
You do not have to multiply the Eᶱ values to suit the molar ratios of the equation â just leave them as they are on the table.
19.1.4 – Predict whether a reaction will be spontaneous using standard electrode potential values
If the calculated cell potential is positive, then the reaction will be spontaneous. This must be found according to how the reaction is written. If the Eᶱ value for the cell is negative, then the spontaneous reaction would be the reverse reaction.
For example, if you were asked to determine whether this reaction is spontaneous:
You would calculate the cell potential to be:
Since the value is negative, this means the reaction is not spontaneous. However, if the reaction is reversed, then it would be a spontaneous reaction.
