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?
In and , the central elements are in intermediate oxidation states, allowing them to either increase or decrease their oxidation numbers. Therefore, they can act as both oxidising and reducing agents.
In nitric acid (), nitrogen is in its highest oxidation state of +5 (as seen in the ion). Since it cannot increase its oxidation number further, it can only decrease it, acting solely as an oxidant. Similarly, ozone () is an elemental form of oxygen where oxygen is in the zero oxidation state. It can only decrease its oxidation number to negative values, thus acting only as an oxidant.
Explanation
The ability of a substance to act as an oxidant or reductant depends on the oxidation state of its central element. If the element is in its highest oxidation state, it can only be reduced (act as an oxidant). If it is in an intermediate state, it can be both oxidised and reduced. The context highlights that nitrogen in is in the +5 state, which is its maximum, and that elemental oxygen (like in ) is in the 0 state, which typically decreases in reactions.