Transformation of matter from one kind
to another occurs through various types of reactions. One important
category of such reactions is the Oxidation and Reduction reactions.
These reactions find extensive use in the fields of
pharmaceutical, biological, industrial, metallurgical, space age
applications and agricultural sciences.
Oxidation is a chemical process in which there is a loss of electrons. The species, which gets oxidized becomes more positive or less negative, e.g.,
Na $\rightarrow$ Na+ + e- (Na is oxidized to Na+ )
Fe2+ $\rightarrow$ Fe3+ + e- (Fe2+ is oxidized to Fe3+)
Reduction is a chemical process in which there is a gain of electrons. The species, which gets reduced becomes more negative or less positive, e.g.,
Cl2 + 2e- $\rightarrow$ 2Cl- (Cl2 is reduced to Cl-)
S + 2e- $\rightarrow$ S2- (S is reduced to S2- )
The Oxidizing agent is the species which accepts electrons, e.g.,
Sn4+ + 2e- $\rightarrow$ Sn2+ (Sn4+ is an oxidizing agent)
F2 + 2e- $\rightarrow$ 2F- (F2 is an oxidizing agent)
The Reducing agent is the substance which loses electrons, e.g.,
Hg+ $\rightarrow$ Hg2+ + e- (Hg+ is a reducing agent)
Fe2+ $\rightarrow$ Fe3+ + e- (Fe2+ is a reducing agent)
Magnesium when burnt in air gives magnesium oxide in accordance with the reaction,
2Mg(s) + O2(g) $\rightarrow$ 2MgO(s)
Here, magnesium combines with oxygen to form magnesium oxide, i.e., oxygen is added to magnesium. According to the classical concept of oxidation and reduction, the addition of oxygen to magnesium is termed as the oxidation of magnesium.
The formation of magnesium oxide, on the basis of the electronic configurations of magnesium and oxygen, can be explained as follows
Magnesium oxide (MgO) consists of Mg2+ and O2-.
This means that during the formation of MgO, Mg is converted into Mg2+ and O into O2-. This is possible only when a magnesium atom (Mg) loses two electrons to form Mg2+ and an atom of oxygen (O) gains two electrons to form oxide ion (O2-). The following reactions should occur.
Mg $\rightarrow$ Mg2+ + 2e-
O + 2e- $\rightarrow$ O2-
O2 + 4e- $\rightarrow$ 2O2-
In the overall reaction between magnesium and oxygen, as electrons do not appear either on the reactant-side nor on the product-side, the number of electrons lost by magnesium must be equal to the number of electrons gained by oxygen. In this way, the number of electrons are balanced. To balance the electrons, the first reaction is multiplied by 2.
2Mg $\rightarrow$ 2Mg2+ + 4e-
Then, by adding the other two equations, one obtains:
Thus, two electrons are transferred from a magnesium atom to an oxygen atom in the reaction between magnesium and oxygen. This electron-transfer process is possible only when one of the species is capable of losing while the other is capable of gaining an electron, both being present in the reaction. The loss and the gain of electrons occurs simultaneously.
The process involving loss of electrons is termed as oxidation, while that involving gain of electrons is termed reduction. Therefore, oxidation and reduction reactions involve electron-transfer from one reactant to another.
Such reactions in which electrons are transferred from one reactant to another are called "redox reactions" involving both oxidation and reduction.
Oxidation is defined as the addition of oxygen/electronegative element to a substance or the removal of hydrogen/electropositive element from a substance.
Oxidation in terms of electron transfer : Loss of electron is called Oxidation .
Reduction is defined as the removal of oxygen/electronegative element from a substance or addition of hydrogen / electropositive element to a substance.
Reduction in terms of electron transfer : Gain of electron is called reduction.
Each redox reaction can be divided into two half reactions:
- Oxidation half reaction
- Reduction half reaction.
By taking the following examples, we can understand the half reactions better.
1. Cu2+(aq) + Mg(s)
→ Mg2+(aq) + Cu(s)
Oxidation half reaction is Mg(s) – 2e-
→ Mg2+ (aq)
(loss of electron)
Reduction half reaction is Cu2+(aq) + 2e-
(gain of electron)
2. Fe(s) + 2Ag+1(aq)
→ Fe2+(aq) + 2Ag(s)
Oxidation half reaction is Fe(s) – 2e-
→ Fe2+ (aq)
(loss of electron)
Reduction half reaction is 2Ag+1(aq) + 2e-
(gain of electron)
1. Combination reaction
All combustion reactions, which make use of elemental oxygen, as well as other reactions involving elements other than oxygen are redox reactions.
C(s) + O2(g) → CO2(g)
3Mg(s) + N2(g) → Mg3N2(s)
CH4(g) + 2O2(g) → CO2(g) + 2H2O(l)
2. Decomposition reaction
A decomposition leads to the breakdown of a compound into two or more components and at least one must be in the elemental state.
2KClO3(s) →2KCl(s) + 3O2(g)
2NaH (s) → 2Na(s) + H2(g)
3. Displacement Reaction:
In a displacement reaction, an ion (or an atom) in a compound is displaced by an ion (or an atom) of another element.
a) Metal displacement reaction
A metal in a compound is displaced by another metal atom in the free state.
Example :- CuSO4(aq) + Zn(s) → ZnSO4(aq) + Cu(s)
V2O5(s) + 5Ca(s) → 5CaO (s) + 2V(s)
b) Non metal displacement reaction
Non metal displacement redox reactions involve the displacement of Hydrogen by another metal atom.
Example : 2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
Zn(s) + 2HCl(aq) → ZnCl2(aq) + H2(g)
Non metal displacement by anther non metal:
Example :- Cl2(g) + 2KBr(aq) → 2KCl(aq) + Br2(l)
Br2(l) + 2KI(aq) → 2KBr(aq) + I2(s)
4. Disproportionation Reaction:
In a disproportionation reaction, an element simultaneously gets oxidized and reduced.
Example :- 2H2O2 → 2H2O + O2
Here the Oxygen in hydrogen peroxide is oxidized to elemental oxygen and is reduced to H2O.
Cl2 + 2OH- → ClO- + Cl- + H2O
Here Cl2 is oxidized to ClO- and reduced to Cl- .
Oxidation of Mg 2Mg(s) + O2(g)
Mg(s) + F2(g)
Mg(s) - 2e-
→ Mg 2+(g)
(aq) + H2(g)
(aq) + 2HCl(aq)
+ 2e → 2Cl- (g)
Below you could see examples
Identify the following reactions as Oxidation or Reduction
+ + 2e → Fe2+
2. 4P + 5O2
4. CuO + H2
→ Cu + H2
Oxidation --- 2, 3,5
Reduction ---- 1,4
Question 2: Solution: