Finding Cell Potential of an Electrochemical Cell

Before we dive deep into the procedure of finding the cell potential, let us understand some important terms. 

The first term is cell potential. The cell potential is the potential difference between the half-cells of an electrochemical cell. It is the driving force that allows electrons to flow from one electrode to another. The unit of cell potential is volt. For example, if we take two electrodes, zinc and copper, the electrons flow from the zinc half-cell to the copper half-cell because the former has a higher electrical potential than the copper half-cell. The magnitude of the cell potential completely depends on the half-cells. 

Standard and Non-Standard conditions

Standard conditions are the conditions during which the standard temperature and pressure must be taken. Standard temperature is taken as 273.15 K, and standard pressure is taken to be 105 Pa. Standard conditions must not be confused with the standard state that is generally used in Gibbs Energy of a reaction.

Non-standard conditions refer to the conditions when the temperature can be any temperature, but 273.15 K and the pressure can be anything but 105Pa. 

Standard cell potential

The standard cell potential, E°cell, is calculated through a control. The standard half-cell potential is determined by connecting the half-cell of a substance to a standard hydrogen electrode. This standard hydrogen electrode acts as a reference cell. 

The reference hydrogen electrode consists of a platinum electrode in a solution containing 1M H+ ions where the hydrogen gas is decomposed. The potential of this half-cell has an assigned value of zero. When a particular half-cell is connected to the standard hydrogen electrode, the potential measured under this condition is the standard half-cell potential of the substance.

Oxidation and Reduction

In electrochemistry, you will often encounter the word “redox” many times. The word refers to reduction-oxidation. It refers to the electrochemical process involving the transfer of electrons to or from a molecule hence, altering its state of oxidation. This reaction occurs only when an external voltage is applied. 

Oxidation and reduction tell about the oxidation state of an atom, molecule, or ion involved in an electrochemical reaction. If an atom gives up its electron to any other atom or ion, then the oxidation state is said to decrease. If an atom or ion gains an electron, its oxidation state is said to increase. The increase and decrease of oxidation state happen simultaneously. 

Standard Electrode Potential

To predict the cell potential of a cell, the standard cell potential’s tabulations are available. These tabulations are known as standard hydrogen electrodes. The standard hydrogen electrode goes through the reaction 

2H+ + 2e– → H2

This is a reduction reaction; however, the standard hydrogen electrode can act both as a cathode and anode depending upon the oxidation/ reduction potential of the other electrode. It can be connected to any electrode with the help of a salt bridge hence forming a cell. When the second electrode connected to the standard hydrogen electrode is also in standard conditions, the cell potential that is measured is also known as standard potential for the electrode.

Determination of negative or positive reduction potential

As the value of half-cell potential is taken against a standard hydrogen electrode, substances with a positive reduction potential are easier to reduce compared to H+ ions. The substances with a negative reduction potential are more difficult to reduce. 

Anode half-cell reaction:

Zn(s) → Zn2+(aq) + 2e–; E°Zn→Zn2+ = 0.76V

Cathode half-cell reaction

Cu2+(aq) + 2e– → Cu(s); E°Cu2+ + 2e– → Cu(s) = 0.34V

The overall reaction will be Zn(s) + Cu2+ → Zn2+(aq) + Cu(s)

E°cell= E° Zn + E° Cu = 0.76 + 0.34 = 1.10V

Dependence of Cell Potential On Concentration

The relationship between the concentration of an electrolyte and the cell potential is given by the Nernst Equation.

Ecell= E°cell – (RT/nF) lnQ

Ecell is the cell potential under non-standard conditions, while E°cell is the cell potential under standard conditions. R is the universal gas constant, F is Faraday’s constant, and Q is the reaction quotient. The value of RT/F = 0.0257 V

Therefore, the Nernst Equation boils down to

Ecell = E°cell –(0.0257/n) lnQ

Conclusion

These are the steps to find out the cell potential of an electrochemical cell. Apart from the cell potential, we can also find out the cell potential under non-standard conditions by using the Nernst Equation.