Oxidation-reduction or redox reactions

These are the reactions in which oxidation and reduction processes occur. Let us first learn what these processes are.

a) Oxidation: It is a process which involves loss of electrons. Earlier it was defined as a process involving addition of oxygen or loss of hydrogen.

b) Reduction: Reduction is a process which involves gain of electrons. Earlier it was defined as a process involving removal of oxygen or addition of hydrogen.

c) Redox reactions: From the above definitions, you must have noticed above that oxidation and reduction processes are just opposite to each other. None of these processes can occur alone. During a reaction if one substance gets oxidized the other gets reduced. Thus, both the processes occur simultaneously. That is why the reactions in which oxidation and reduction processes occur are called redox reactions or oxidation-reduction reactions. Now let us understand these processes with the help of some examples.

(i) Consider burning of coke (carbon) in presence of oxygen:

C(s)	+	O2(g)	—>	CO2(g)
carbon		oxygen		carbon dioxide

In this reaction carbon is getting oxidized as oxygen is added to it and oxygen is reduced.

(ii) When hydrogen sulphide reacts with sulphur dioxide the products are sulphur and water.

2H2S(g)	+	SO2(g)	—>	3S(s)	+	2H2O(l)
hydrogen sulphide		sulphur dioxide		sulphur		water

Here, hydrogen sulphide is oxidized to sulphur due to loss of hydrogen while sulphur dioxide is reduced to sulphur due to loss of oxygen.

(iii) When copper (II) oxide is treated with hydrogen, copper and water are produced.

CuO(s)	+	H2(g)	—>	Cu(s)	+	H2O(l)
cupric oxide		hydrogen		copper		water

Here cupric oxide is reduced to copper due to loss of oxygen while hydrogen is oxidized to water due to addition of oxygen.

(iv) When sodium metal reacts with chlorine it forms sodium chloride.

2Na(s)	+	Cl2(g)	—>	2NaCl(s)
sodium		chlorine		sodium chloride

Sodium chloride is an ionic compound. Sodium is present in it as sodium ion (Na⁺) and chlorine as chloride ion (Cl^-;). This reaction can be considered to occur in the following steps:

• Each sodium atom loses one electron and forms sodium ion. Since two sodium atoms are involved in the reaction, the process is:

2Na	—>	2Na+	+	2e-
sodium		sodium ion		carbon dioxide

Thus, sodium is oxidized due to loss of electron

• Each chlorine atom gains one electron and forms chloride ion. Since one chlorine molecule has two atoms of chlorine the process is:

Cl2	+	2e-	—>	2Cl-
chlorine				chloride ion

Thus, chlorine is reduced due to gain of electrons.

(v) When zinc is added to an aqueous solution of copper sulphate, it displaces copper.

Zn(s)	+	CuSO4(aq)	—>	ZnSO4(aq)	+	Cu(s)
zinc		copper sulphate		zinc sulphate		copper

Here zinc is oxidized to zinc ions and copper ions are reduced to copper. This reaction is displacement reaction as well as a redox reaction.

(d) Oxidizing and reducing agents : Consider the reaction between zinc and copper sulphate:

Zn(s)	+	CuSO4(aq)	—>	ZnSO4(aq)	+	Cu(s)
zinc		copper sulphate		zinc sulphate		copper

In this reaction zinc reduces cupric ions to copper. Such a substance which reduces another substance is called a reducing agent. Here, zinc is the reducing agent.
Also, in this reaction cupric ions oxidize zinc to zinc ions. Such a substance which oxidizes another substance is called an oxidizing agent. Here, cupric ions are the oxidizing agent.