(i) C is a reducing agent while O2 is an oxidising agent. If excess of carbon is burnt in a limited supply of O2, CO is formed in which oxidation state of C is +2 but when O2 is in excess CO formed gets oxidised to CO2 in which oxidation state of C is + 4.
(a)

2C(s)

(Excess)

+O2(g) →

+2

2CO(g)

;
(b) C(s)+

O2(g)

(Excess)

→

+4

CO2(g)

(ii) P4 is a reducing agent while Cl2 is an oxidising agent. When excess of P4 is used, PCl3 is formed in which the oxidation state of P is +3. When excess of Cl2 is used, the initially formed PCl3 reacts further to form PCl5 in which the oxidation state of P is +5.
(a)

P4(s)

(Excess)

+6Cl2(g) →

+3

4PCl3

(b) P4(s)+

10Cl2(g)

(Excess)

→

+5

44PCl5

(iii) Na is a reducing agent and O2 is an oxidising agent. When excess of Na is used, sodium oxide is formed in which the oxidation state of O is –2. If, excess of O2 is used, Na2O2 is formed in which the oxidation state of O is –1.
(a)

4Na(s)

(Excess)

+O2(g) → 2Na2

−2

O(s)

(b) 2Na(s)+

O2(g)

(Excess)

→ Na2

−1

O2(s)