A Redox reaction is a type of chemical reaction in which both oxidation and reduction occur simultaneously, which involves a transfer of electrons between two species.

Initially, oxidation was used to describe the addition of oxygen to an element or a compound. The following reactions represent oxidation processes according to the limited definition of oxidation:

  1. 2Mg(s) + o2(g)  \rightarrow 2MgO(s)
  2. S(s) + O2(g)  \rightarrow SO2(g)

Develoepment of concept of Redox reaction

In the reactions above, the elements magnesium and sulphur are oxidised on account of addition of oxygen to them. Similarly, methane is oxidised on account of addition of oxygen to them. Similarly, methane is oxidised owing to addition of oxygen to it.

3. A careful examination of reaction 3 in which hydrogen has been replaced by oxygen prompted chemists to reinterpret oxidation in terms of removal of hydrogen from it and therefore, scope of the term of oxidation was broadened to include the removal of hydrogen from a substance.

As knowledge of chemists grew, they had to extend the term of oxidation to reactions involving electronegative elements as well.

4. Mg(s) + F2(g)  \rightarrow MgF2(s)
5. Mg(s) + Cl2(g)  \rightarrow MgCl2(s)
6. Mg(s) + S(s) \rightarrow MgS(s)

Soon, the removal electro-positive elements from a compound was also included into the definition of oxidation by chemists. The chemists went through the same procedure in discovering the definition of Reduction. They also discovered that reduction and oxidation always occur simultaneously. As shown in this example:

7. 2HgCl2(aq) + SnCl2(aq) \rightarrow Hg2Cl2(s) + SnCl4(aq)

In reaction #7, there is a reduction of mercury to mercuric chloride and simultaneous oxidation of stannous chloride to stannic chloride.

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