Standard electrode potential is the potential that arises in an electrochemical cell under standard conditions (pressure = 1atm, temperature = 298K, concentration = 1M). The symbol ‘Eocell’ represents the standard electrode potential of a cell.
Need for Standard Electrode Potential
- All electrochemical cells are based on redox reactions, which are made up of two half-reactions.
- The oxidation half-reaction occurs at the anode and it involves a loss of electrons.
- Reduction takes place at the cathode, involving a gain of electrons. Thus, the electrons flow from the anode to the cathode.
- The electric potential that arises between the anode and the cathode is due to the difference in the individual potentials of each electrode (which are dipped in their respective electrolytes).
- The cell potential of an electrochemical cell can be measured with the help of a voltmeter. However, the individual potential of a half-cell cannot be accurately measured alone.
- It is also important to note that this potential can vary with a change in pressure, temperature, or concentration.
- In order to obtain the individual reduction potential of a half-cell, the need for standard electrode potential arises.
- It is measured with the help of a reference electrode known as the standard hydrogen electrode (abbreviated to SHE). The electrode potential of SHE is 0 Volts.
- The standard electrode potential of an electrode can be measured by pairing it with the SHE and measuring the cell potential of the resulting galvanic cell.
- The oxidation potential of an electrode is the negative of its reduction potential. Therefore, the standard electrode potential of an electrode is described by its standard reduction potential.
- Good oxidizing agents have high standard reduction potentials whereas good reducing agents have low standard reduction potentials.
- For example, the standard electrode potential of Ca+ is -3.8V and that of F2 is +2.87V. This implies that F2 is a good oxidizing agent whereas Ca is a reducing agent.
The calculation of the standard electrode potential of a zinc electrode with the help of the standard hydrogen electrode is illustrated below.
It can be noted that this potential is measured under standard conditions where the temperature is 298K, the pressure is 1 atm, and the concentration of the electrolytes is 1M.
Predicting Spontaneity of Redox Reactions
If a redox reaction is spontaneous, the ΔGo (Gibbs free energy) must have a negative value. It is described by the following equation:
ΔGocell = -nFE0cell
Where n refers to the total number of moles of electrons for every mole of product formed, F is Faraday’s constant (approximately 96485 C.mol-1).
The E0cell can be obtained with the help of the following equation:
E0cell = E0cathode – E0anode
Therefore, the E0cell can be obtained by subtracting the standard electrode potential of the anode from that of the cathode. For a redox reaction to be spontaneous, the E0cell must have a positive value ( because both n and F have positive positive values, and the ΔGo value must be negative)
This implies that in a spontaneous process, E0cell > 0; which in turn implies that E0cathode > E0anode
Thus, the standard electrode potential of the cathode and the anode help in predicting the spontaneity of the cell reaction.
It can be noted that the ΔGo of the cell is negative in galvanic cells and positive in electrolytic cells. To learn more about this concept and other concepts related to electrochemistry, register with BYJU’S and download the mobile application on your smartphone.