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By "Half Reaction Method" (or) " Ion-Electron Method"


According to modern views, the process of electron(s) being lost by a species is known as oxidation, whereas gaining of electron(s) is known as reduction. The species that loses electron(s) is said to be oxidized, whereas the species that gains electron(s) is said to be reduced. An oxidizing agent is species that can cause oxidation of some other species. During this oxidation process, the oxidizing agent absorbs the emitted electronics from the other species. In other words, by causing oxidation in other species, the oxidizing agent gets reduced. Similarly, a reducing agent is a species that can cause reduction of some other species. During this reduction process, the reducing agent supplies electrons that are absorbed by other species. In other words, by causing reduction in other species, the reducing agent gets oxidized.
The transfer of electron(s) from a reducing agent (known as reductant) to an oxidizing agent (known as oxidant) implies that both reduction and oxidation process occur simultaneously and such a reaction is known as a redox (red from reduction and ox from oxidation) reaction.
The few examples may be reviewed in terms of the modern views on oxidation and reduction.
  1. In the reaction Mg + Magnesium atom has undergone the following change
    Mg Mg2+ + 2e- whereas oxygen molecule has undergone the change
    Thus, we say that
    1. Magnesium has undergone oxidation (removal of electrons) and oxygen had undergone reduction (addition of electron),
    2. Magnesium is the reducing agent and oxygen is the oxidizing agent, and
    3. Magnesium had reduced oxygen to oxide ion, whereas oxygen has oxidized magnesium to magnesium ion.
  2. In the reactionhydrogen has undergone the change  H2 2H+ + 2e- and oxygen and undergone the change where as 2H+ and O2- combine to give H2O.
    Thus, we say that
    1. hydrogen has undergone oxidation (removal of electron) and oxygen has undergone reduction (addition of electron),
    2. hydrogen is the reducing agent and oxygen is the oxidizing agent, and
    3. hydrogen has reduced oxygen to oxide ion, whereas oxygen has oxidized hydrogen to hydrogen ion.
  3. For the reaction involving KMnO4 and FeSO4 in acidic medium, we write the partial reaction involving those species, which undergo oxidation and reduction. We have
    MnO4- + 8H+ + 5e-Mn2+ + 4H2O + [Fe2+ Fe3+ + e-] 5
    MnO4- + 5Fe2+ + 8H+ Mn2+ + 5Fe3+ + 4H2O
    Thus we say that
    1. Fe2+ has undergone oxidation and MnO4- has undergone reduction,
    2. MnO4- is an oxidizing agent and Fe2+ is the reducing agent, and
    3. MnO4- has oxidized Fe2+ to Fe3+, whereas Fe2+ has reduced MnO4- to Mn2+.
The above way of writing half-reactions is more realistic since there does occur transfer of electrons from one species to another. Moreover, the reaction involves only those species, which are actually required (or have undergone oxidation and or reduction).

A Few More Examples of Redox Reactions are Given Below:
 

Undergone Half Reactions

Agent

Reaction

Oxidation

Reduction

Oxidizing

Reducing Cu + 2Ag+ Cu2+ + 2Ag

Cu Cu2+ + 2e-

2Ag+ + e- Ag

Ag+

Cu

Zn + Cu2+ Zn2 + Cu

Zn Zn2 + 2e-

Cu2+ + 2e- Cu

Cu2+

Zn

Cr2o72- + 6Fe2+ + 14H+ 2Cr3+ + 6Fe3+ + 7H2O

6Fe2+ 6Fe3+ + 6e-

Cr2o72- + 14H+ + 6e- 2Cr3+ + 7H2O

Cr2

Fe2+

2S2o32- + I2 S4O + 2I-

2S2O32- S4o62- + 2e-

I2 + 2e- 2I-

I2

 

S2 O3 2-

2Fe3+ + 2I- I2 2Fe2+

2I- I2 + 2e-

2Fe3+ + 2e- 2Fe2+

Fe3+

I-

 




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