When a substance gains oxygen during a reaction, it is said to be oxidized and when a substance loses oxygen during a reaction, it is said to be reduced. During the reaction process, one reactant gets oxidized while the other gets reduced. Such reactions are called oxidation reduction reaction or Redox Reactions.
2Cu (s) + O2 (g) + heat → 2CuO (s)
In the above reaction, copper gains oxygen and is said to be oxidized.
CuO (s) + H2 (g) → Cu (s) + H2O (l)
In the above reaction, copper oxides loses oxygen and is said to be reduced. Hydrogen in this reaction is gaining oxygen and is thus being oxidized.
Thus, in this reaction, during the reaction process, one reactant gets oxidized while the other gets reduced. Such reactions are called oxidation reduction reaction or Redox Reactions.
The CuO provides oxygen and therefore is an oxidizing agent and hydrogen takes this oxygen and therefore is a reducing agent. In a redox reaction, an oxidizing agent is reduced and a reducing agent is oxidized.
Other Examples of Redox Reaction
ZnO (s) + C (s) + heat → Zn (s) + CO (g)
MnO2 (s) + 4HCl (aq) → MnCl2 (aq) + 2H2O (l) + Cl2 (g)
In all redox reactions, one species is oxidized and the other is reduced. There is no oxidation without reduction and there is no reduction without oxidation. This aspect of redox reactions can also be explained in terms of electron gain and electron loss.
A reaction in which a species loses electrons is called an oxidation reaction and a reaction in which a species gains electrons is called a reduction reaction. For example,
Cu (s) + I2 (s) → CuI2 (s)
Above reaction can be written in two steps:
Step (i): Cu → Cu2+ + 2e–
Step (ii): I2 + 2e– → 2I–
In step (i) one copper atom loses two electrons to become a cupric ion, Cu2+ and in step (ii) iodine gains two electrons and gets converted into two iodide ions. Copper is oxidized by losing electrons and iodine is reduced by gaining electrons.
The substance which oxidizes the other substance is known as an oxidizing agent. An oxidizing agent gets reduced during the reaction. Likewise, the substance which reduces the other substance is known as a reducing agent. A reducing agent gets oxidized during the reaction.
Gain of electron is reduction and loss of electron is oxidation.
Fe2O3 (s) + 2Al (s) ⎯⎯→ Al2O3 (s) + 2Fe (s)
2Na (s) + Cl2 (g) ⎯⎯→ 2NaCl (s)
2Mg (s) + O2 (g) ⎯⎯→ 2MgO (s)