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Redox Reactions

What are redox reactions?

Redox reactions are oxidation-reduction reactions where reacting substances are changed their oxidation number or state. In another word, redox reactions are chemical reactions where one or more electrons are transferred between two reacting substances like atoms or ions participating in it. Redox reaction based on oxidation number and electronic concept in chemistry is given below the picture,

Redox reactions example according to oxidation number and electronic concept

Redox reactions have wide industrial applications like electricity generation, purification or extraction of metals like zinc, aluminum, chromium, and copper by electrolysis, and combustion of fuel in the fuel cell.

Oxidation and reduction

Oxidation and reduction are always found to go hand in hand during a redox reaction.

  1. Oxidation may be defined as a process where the oxidation number of reacting substances increases.
  2. Similarly, reduction may be defined as a process where the oxidation number of reacting substances decreases.

In a redox reaction, an oxidant is reduced and a reductant is simultaneously oxidized.

Indirect redox reactions take place in the electrochemical cell for the conversion of chemical energy into electrical energy by transfer of electrons. The electrolytic cell is another example where electrical energy is converted to chemical energy by the transfer of electrons.

Types of redox reactions

Redox reactions are generally the following types,

  • Decomposition reaction
  • Combination reaction
  • Displacement reaction
  • Disproportionation reaction

Examples of decomposition reaction

The chemical reaction where one reactant can be breakdown into two or more products is called decomposition reaction. Decomposition can be represented by,
AB → A + B

Breakdown of hydrogen peroxide or water into hydrogen and oxygen are examples of redox reactions where decomposition can occur.

Examples of combination reaction

In a combination reaction, a metal can be reacted with non-metal to form an ionic crystalline solid. For example, when magnesium metal burns in oxygen to produce magnesium oxide. Here two electrons can be transferred between the two reacting substances like magnesium and oxygen.

Examples of displacement reaction

A displacement reaction is a type of chemical reaction where one or more atoms can be displaced by another atom. For example, the electron charge is transferred from the zinc atom to the copper atom in this electrochemical cell.

Examples of disproportionation reaction

A disproportionation reaction is a type of redox reaction where one element of a reactant in a particular oxidation state can be simultaneously oxidized and reduced to form two products.

For example, the chemical reaction between phosphorus with sodium hydroxide. Here phosphorus can simultaneously be oxidized and reduced.

Examples of redox reactions

Oxidation of iron by potassium permanganate

Oxidation of ferrous ion to ferric ion by acidic potassium permanganate is an example of a redox reaction where oxidation and reduction occur simultaneously. The half-reaction is given below the picture,

Example of redox reaction for oxidation of iron by potassium permanganate in acid medium

Reaction of potassium dichromate with potassium iodide

Oxidation of potassium iodide by potassium dichromate in dilute sulfuric acid medium is another example of redox reaction. Here chromium (+6) is reduced to chromium (+3) and iodine is oxidized to form an elementary iodine molecule.

Redox reaction
Cr2O7−2 + 14 H+ + 6 I → 2 Cr+3 + 3 I2 + 7 H2O
Oxidation reaction
2I → I2 + 2e
Reduction reaction
Cr2O7−2 + 14 H+ + 6e → 2 Cr+3 + 7 H2O

Redox electrode examples

  • When oxidation (loss of electrons) reactions take place on the redox electrode, the chemical potential of the electrode is called oxidation potential.
  • When reduction (gain of electrons) takes place on a redox electrode is called reduction potential

If the oxidation potential of an electrode = x volt, then its reduction potential = − x volt.

Two half cells chemical reactions of ferric ion and ceric ion in the presence of dilute sulfuric acid are examples of redox electrodes. Two electrode potentials in balancing chemical equations can be given below the table,

Redox electrode examples
Ce+4 + Fe+2 ⇄ Ce+3 + Fe+3
Half cell reaction Electrode potential
Ce+4 + e ⇄ Ce+3 1.44 volt
Fe+2 ⇄ Fe+3 + e −077 volt

Applications of redox reaction

Redox reactions have wide industrial application, it is used for constructing electrochemical and electrolytic cells in our daily life.

Application of redox reactions in electrochemical cells

Redox reaction in electrochemistry

Redox reactions take place in the electrochemical cell convert chemical energy into electrical energy by electron transfer reaction. Therefore, the electrochemical or simply chemical cell is the device in which energy is produced due to the indirect redox reaction and converted electricity for mankind.

We used Galvanic and Voltaic cells according to the name of the scientist, Luigi Galvani, and Alessandro Volta.

Application of redox reaction in metallurgy

An electrolytic cell is a device where a non-spontaneous redox reaction takes place by an external source of electricity. The electrolysis process occurs in the electrolytic cells.

The very useful application of electrolysis is found in commercial electroplating and purification of metals. For copper, an electroplating reaction is carried out by dipping copper into the suspension of an aqueous copper sulfate solution.

The electrolysis or redox reaction at the two electrodes can be represented as follows,

Copper electroplating reaction
At cathode Cu+2 + 2 e → Cu
At anode Cu → Cu+2 + 2 e

Application of fuel cell

A fuel cell is an electrochemical cell where chemical energy available from the fuel like methane, hydrogen, solid oxide, and alkaline is converted into electrical energy by redox reactions.

At the cathodes of the cell, oxygen is dissolved to form OH ions. The cell has low voltage but is capable of supplying current for long periods.

Such cells provide power at comparatively low cost and low maintenance. The efficiency depends particularly on the surface of the electrodes. It serves as the chemical catalyst for the attainment of chemical equilibrium.

The electric polarization is due to the adoption and poisoning of the redox electrode. It reduces cell performance considerably. The density of the electrolyte and the temperature have a great influence on the potential of redox reactions.