Potential In Electrochemistry

Electrode Potential or cell potential( Ecell)

It can be defined as:

· The tendency of an electrode to lose or gain electron when it is in contact of its own ion is called electrode potential.

· It is also called electromotive force (emf) or cell voltage. It is the difference in potential between two electrodes.

· A potential difference that exists across a metal and solution interface.

Ecell = E_R - E_O

Ecell = E_O + E_R

Ecell = Ecathode - Eanode

Ecell = Ereduction -Eoxidation

Electrode or cell potential can be oxidation or reduction potential.

· Oxidation potential: Tendency of an anode to lose e^- when in contact with its ions is called oxidation potential or anode potential [E_anode ,E_oxidation]. It occurs at anode and is the oxidation potential of oxidation reaction.

M → Mⁿ⁺ + ne^-

· Reduction potential: Tendency of cathode to gain e^- when in contact with its own ions is called reduction or cathode potential [E_cathode, E_reduttion]. It occurs at cathode and is the reduction potential of reduction reaction.

Mⁿ⁺ + ne^{- →} M

Example:

Anode potential of Zn: Zn → Zn⁺² + 2e^- [-0.76]

Cathode potential of Cu: Cu⁺² + 2e^- → Cu [0.34]

Electrode potential: Ecell = Ecathode - Eanode

Ecell = 0.34 – (-0.76)

Ecell = 1.1 V

	Oxidation potential	Reduction potential
Cu	-0.34	0.34
Zn	-0.76	0.76

Note: electrode with greater reduction potential will be cathode and electrode with smaller reduction potential will be anode.

Oxidation or reduction potential can also be defined as half-cell or single cell potential.

Single cell potential:

Potential difference of electrical double layer that is formed at the interface of metal electrode and electrolyte solution is called single electrode potential. [electrical double layer is the layer of e^-s of electrode and electrolyte which may deposits or dissolve].

Standard Electrode Potential E°:

When concentration of an electrolyte/ions is 1M, electrode is pure (no impurity), temperature is 25°C and pressure is 1 atm (in case of gaseous specie if present) then it is called standard electrode potential (SEP).

E⁰cell = E⁰_R + E⁰_O

E⁰cell = E⁰cathode + E⁰anode

E⁰cell = E⁰reduction +E⁰oxidation

Self-Potential:

The Sum of potentials of two electrodes is called self-potential. Or Potential difference between anode and Cathode is called self-potential.

Unit of cell potential:

Cell Potential is measured in volts. Purpose of voltaic cell is to get energy (due to flow of e^-ss or charges).

unit of charge = 1 Coulomb

unit of energy = 1 Joule

when an energy of 1J is obtained by flow of 1 coulomb of charge in circuit between two electrodes then the potential difference between two electrodes is said to be 1 volt.

1V = J/C (energy per charge)

Determination of electrode potential

Standard electrode is used for the measurement of relative electrode and is also called reference electrode. Potential of a single electrode cannot be determined at alone. It is always determined as relative potential to a standard potential. The potential of a standard electrode is standard potential. It's value is 0.00 Volt. Different standard electrodes are used for the determination of electrode potential, the most common is standard hydrogen electrode (SHE).

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Applications of Electrode Potential

1. Determination of relative strength of oxidizing and reducing agent"

These electrode potentials can be arranged or written in ascending or descending order. The arrangement gives a series or table which called “table of emf series" or Electrochemical series". This series can be used to determine the relative strength of oxidation and reduction agent. The more positive the 'E⁰' value, the more readily reduction will occur.

If we compare two oxidizing agents,

Cu⁺² [ E⁰ -0.337 V] and H⁺ [E⁰ -0.00 V]

Reduction of Cu⁺² is easier than H⁺because Cu⁺² is a stronger oxidizing agent than H⁺. If we arrange in descending order than, down the group the strength of oxidizing agent decreases. Then

· At bottom: weaker oxidizing agent and stronger reducing agent

· At top: stronger oxidizing agent and weaker reducing agent

Also down the group strength of reducing agent increases. Thus oxidizing strength (increasing order) is Cu⁺² > H⁺ > Zn⁺². Similarly, reducing agent strength order is Zn > H₂ > Cu. Thus the advantage is that we can compare many redox reactions.

2. To write the spontaneous Electrochemical runs.

Emf series can be used to write spontaneous electrochemical reactions which then can be used to construct a voltaic cell. Stronger oxidizing agents reacts readily with stronger reducing agents. If both of the agents or one of them will be weak then the reaction will be non- spontaneous.

For example:

Cu + Zn⁺² → Zn + Cu⁺²	Zn + Cu⁺² → Cu + Zn⁺²
Weaker R.A Weaker O.A	Stronger R.A Stronger O.A
Zn⁺² E⁰_Zn= -0.76	Cu⁺² E⁰_Cu= 0.337
Non- spontaneous reaction	spontaneous reaction