Question

While sulphur dioxide and hydrogen peroxide can act as oxidising as well as reducing agents in their reactions, ozone and nitric acid act only as oxidants. Why?

Answer 1

Answer:

The oxidising or reducing behaviour of a substance depends on the oxidation state of the central atom and whether it can both increase or decrease its oxidation state.

---

Sulphur dioxide (SO₂) and hydrogen peroxide (H₂O₂):

In sulphur dioxide, sulphur is in an intermediate oxidation state of +4. Hence, sulphur can either be oxidised to a higher oxidation state (+6) or reduced to a lower oxidation state (0 or −2). Therefore, SO₂ can act both as an oxidising agent and a reducing agent.

In hydrogen peroxide, oxygen is in an oxidation state of −1, which is also an intermediate value. Thus, oxygen can be oxidised to 0 (in O₂) or reduced to −2 (in H₂O). Hence, H₂O₂ shows both oxidising and reducing properties.

---

Ozone (O₃) and nitric acid (HNO₃):

In ozone, oxygen effectively acts as a strong oxidising species and readily decomposes to give O₂, releasing nascent oxygen. It does not undergo further oxidation and therefore acts only as an oxidising agent.

In nitric acid, nitrogen is already present in its highest oxidation state of +5. Since nitrogen cannot increase its oxidation state further, HNO₃ cannot act as a reducing agent and thus behaves only as an oxidising agent.

---

Conclusion:

SO₂ and H₂O₂ act as both oxidising and reducing agents because their central atoms are in intermediate oxidation states, whereas O₃ and HNO₃ act only as oxidising agents because their atoms are either highly oxidising or already in the maximum oxidation state.