A chemical reaction is a change or a process that occurs when two or more molecules interact to form a new product(s). A redox reaction is a prominent reaction which is employed in numerous industries and field of work extensively. Let’s understand the concept of redox reactions in terms of electron transfer reaction.
Redox Reaction: Oxidation and Reduction
As the name suggests, a redox reaction is a combination of two chemical reactions- oxidation and reduction. If in a reaction oxygen/electronegative element is added to a substance or hydrogen/electropositive element is removed from a substance, we say that the substance has undergone oxidation.
Mg (s) + Cl2 (g) → MgCl2 (s)
2 H2S (g) + O2 (g) → 2 S (s) + 2 H2O (l)
When a substance undergoes reduction either oxygen/electronegative element is removed from the substance or hydrogen/electropositive element is added to the substance.
2 HgO (s) → 2 Hg (l) + O2 (g)
2 FeCl3 (aq) + H2 (g) → 2 FeCl2 (aq) + 2 HCl
Now, consider the following reaction:
3Fe3O4 (s) + 8 Al (s) → 9 Fe (s) + 4Al2O3 (s)
Here, oxygen is added to aluminium; hence aluminium is oxidised. While oxygen has been removed from ferrous ferric oxide (Fe3O4) and thus it is reduced. Thus, we can say that both oxidation and reduction occurred simultaneously and the reaction is an example of a redox reaction.
Redox Reaction in Terms of Electron Transfer
Redox reactions are not merely about addition or removal of oxygen/electronegative element hydrogen/electropositive element. It is also concerned with the oxidation state of the elements involved in the reaction i.e. electron transfer between the elements in the reaction. Let’s try to understand redox reaction in terms of electron transfer.
Zn + 2H+ → Zn2+ + H2
In the above reaction, the oxidation state of Zn changes from 0 to +2 and that of H changes from +1 to 0. Zinc is oxidized by losing electrons and H is reduced by gaining an electron. The modern definition of oxidation and reduction reaction states that loss of electrons is oxidation and gain of electrons is a reduction. Whenever reduction and oxidation occur simultaneously, such reactions are known as redox reactions.
Redox reactions consist of two half-reactions- the oxidation half-reaction and the reduction half-reaction. The oxidation half-reaction shows the oxidation reaction while the reduction half-reaction shows the reduction reaction. The substance that gets oxidized and helps another substance get reduced is known as the reducing agent and the one that gets reduced and helps to oxidize the other substance is known as the oxidizing agent.
In addition, the oxidation results in an increase in oxidation number and reduction result in a decrease in oxidation number. Since the electrons donated by reducing agent are accepted by an oxidizing agent, the loss of electrons in the overall reaction is zero.
Let’s summarise redox reaction in terms of electron transfer as follows:
- Loss of electrons is called oxidation (oxidation number increases).
- The gain of electrons is called reduction (oxidation number decreases).
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