Identify from the following reactions the reactants that undergo oxidation and reduction.

a. Fe + S → FeS

b. 2Ag₂O → 4 Ag + O₂

c. 2Mg + O₂→ 2MgO

d. NiO + H₂→ Ni + H₂O

a. Fe + S → FeS

In the given reaction, Fe is giving 2 electrons to Sulphur. Sulphur is accepting those electrons. This means:

⇒ Fe undergoes oxidation (to lose electrons)

⇒ Sulphur undergoes reduction (to accept electrons)

b. 2Ag₂O → 4 Ag + O₂

⇒ Ag in Ag₂O has oxidation state= +1

Ag in Ag(s) has oxidation state = 0

This means Ag undergoes reduction

⇒ O in Ag₂O has oxidation state = -2

O in O₂ has oxidation state = 0

This means oxygen undergoes oxidation

c. 2Mg + O₂→ 2MgO

First write the half reactions:

Mg⁰ → Mg²⁺ + 2e^-

O₂ + 4e^-→ O^2-

⇒ Oxygen goes from 0 to -2. Thus, it undergoes reduction.

⇒ Magnesium goes from 0 to +2. Thus, it undergoes oxidation

d. NiO + H₂→ Ni + H₂O

First write the half reactions:

2H⁰ - 2e^-→ 2H¹

Ni²⁺ + 2e^-→ Ni⁰

⇒ Hydrogen goes from 0 to 1. Thus, it undergoes oxidation.

⇒ Nickel goes from 2 to 0. Thus, it undergoes reduction.