Electrochemistry-Electrode Potential

In a redox reaction, the reactant goes through oxidation in the anode and the reactant undergoing reduction is the cathode. Now, by convention, the potential of an electrode is a measure of attraction/repulsion of the electrons to electrodes. And since the cathode tends to shrink, i.e. it tends to attract electrons to itself, the electrode potential is usually a measure of its tendency to absorb (i.e. reduce) electrons.

When you hear the term electrode potential, it usually means reduction potential (potential or tendency to undergo reduction). That’s why if the E° value is positive, it means that the tendency to decrease is high. A negative value means that the reduction tendency is low, that is, the oxidation tendency is high. Therefore, the negative value of the reduction potential is called the oxidation potential.

What is Electrode Potential?

Suppose you have Zn as an electrode in a solution containing zinc ions. There will be a balance between them. But, zinc atoms tend to go into solution. This happens when atoms are converted into ions and electrons remain on the electrodes. Therefore, the region of the solution near the electrode is positive, and the electrode has a low accumulated negative charge.

Cu is then used as an electrode in the other half-cell, immersed in a solution containing Cu2+ ions. But, because the positive ions in the solution are lost near the rod, the opposite happens. Negative ions stick to the metal, and electrons are drawn out of the rod. The electric double layer is also applied here.

When the charges (positive and negative) are separated, we can measure the voltage. Thus, the voltage can be measured between the electrode and the surrounding solution. In simple terms, we can say that there is a changing electron pressure on the electrode.

The potential of an electrode can never be measured directly, only by comparing the electrode to a standard or reference electrode. Essentially, the electrode potential is the voltage at which the electrode is at, and it must be measured against a reference electrode. So, which electrochemical cell do we use? The answer is Voltaic (primary) cell and Electrolyzer.

Oxidation and Reduction

The word oxidation has two meanings. The general (narrow) meaning of oxidation is the chemical reaction (combination) between oxygen and any other element that results in the formation of oxides.

For example, calcium and oxygen combine to form calcium oxide:

Ca + 1/2O2 = CaO

Oxidation is a chemical process in which one element loses one or more electrons by donating it to another element. The element accepting electrons is not necessarily oxygen.

For example,  calcium and chlorine are considered to be oxidation:

Ca + Cl2 = CaCl2

In this reaction, a calcium atom oxidises to two chlorine atoms when it loses two electrons.

Both reactions can be expressed in ionic form as follows:

Ca = Ca2+ + 2e-

Reduction is the chemical process opposite to oxidation in which one element gains one or more electrons provided by another element.

For example, to reduce copper from copper oxide:

CuO + H2 = Cu +  H2O

In this reaction, the copper ion receives two electrons from two hydrogen atoms.

The reduction of copper to the ionic form can be expressed as follows:

Cu2+ + 2e- = Cu

Hydrogen that loses electrons is oxidised in this reaction.

Oxidation of an element is always accompanied by reduction of another, which is why the reaction is often referred to as redox or redox.

Applications:

1) Strength of oxidising and reducing agents, relatively.

2) pH

3) Free Energy (G)

4) Thermodynamic cell potential

5) Unknown concentration

6) Equilibrium constant

7) Solubility products

8) Dissociation constant

9) Titration curve

Dependence of Electrode Potential

• Concentration 
• Temperature
• Pressure (in gaseous directions only)

Example of Electrode Potential

A potential difference is accepted between the metal and its solution. Furthermore, half-cell reactions have their own electrode potentials because two different types of reactions take place:

For example, the Cu-AgNO3 reaction would proceed as follows:

Oxidation half-reaction: (electrode) Cu(s) (solution) Cu2++ (on electrode) 2e-

Reduction half-reaction: 2Ag+ + 2e- 2Ag

Global cellular response: Cu + 2Ag+ ⇌ Cu2+ + 2Ag

A cell can be represented as:

Cu(s) I Cu2+ (aq) II Ag+(aq) I Ag(s)

Difference Between Electrode Potential and Cell Potential

The potential difference between an electrode and its surrounding solution is called electrode potential. An electrical potential is created at the electrodes by the reactions that take place at the electrodes. Therefore, the potential generated by the reduction reaction is called the reduction potential, and the potential generated by the oxidation reaction is called the oxidation potential. If the conditions are standard, the potential is called the standard electrode potential. The cell potential is the algebraic sum of the reduction potential and the oxidation potential, and the standard cell potential is the algebraic sum of the standard reduction potential and the standard oxidation potential.

Conclusion

When a metal is put in a solution of its ions, the metal obtains a positive or negative charge relative to the solution. This creates a significant potential difference between the metal and the solution. This potential difference is called electrode potential. In order to compare the electrode potentials of different electrodes, it is necessary to specify the concentration of ions present in the solution in which the electrodes are immersed and the temperature of the half-cell. At 25 °C (298 K), the potential difference developed between the metal electrode and a unit molar concentration (1M) ionic solution is called the standard electrode potential. The electrode potential of the electrode measured under standard conditions is called the standard electrode potential or standard reduction potential.