Which of the following nitrogen containing substances can act both as an oxidising agent and as a reducing agent, depending on the reaction conditions?

Introduction / Context:
This question explores redox chemistry and the concept of a compound that can behave both as an oxidising agent and as a reducing agent. These so called intermediate oxidation state compounds can either gain or lose electrons depending on the reacting partner. We focus here on nitrogen oxy compounds and related substances.

Given Data / Assumptions:

• The candidates are nitric acid (HNO3), ammonia (NH3), nitrous acid (HNO2), nitrogen dioxide (NO2) and dinitrogen gas (N2).
• We need to identify which substance can show both oxidising and reducing behaviour.
• We assume aqueous or typical laboratory conditions where these substances commonly react.
• Oxidising agents accept electrons, reducing agents donate electrons.

Concept / Approach:
Compounds of nitrogen exhibit many oxidation states, and those in intermediate states often can act both as oxidising and reducing agents. In nitrous acid, nitrogen has an oxidation number of +3, which lies between higher states like +5 in nitric acid and lower states like +1 in nitrous oxide or -3 in ammonia. Because of this intermediate position, HNO2 can be oxidised to nitrate or reduced to products like NO. Therefore, it can either gain electrons (behaving as an oxidising agent) or lose electrons (behaving as a reducing agent) depending on the reaction partner.

Step-by-Step Solution:
1) Determine approximate oxidation states: in nitric acid (HNO3), nitrogen is in +5; in nitrous acid (HNO2), nitrogen is in +3; in ammonia (NH3), nitrogen is in -3; in nitrogen dioxide (NO2), nitrogen is around +4; in dinitrogen gas (N2), nitrogen is in 0.2) A species at a very high oxidation state like +5 in HNO3 usually acts as an oxidising agent and tends not to behave as a reducing agent in ordinary reactions.3) A species at a very low oxidation state like -3 in NH3 typically acts as a reducing agent and rarely as an oxidising agent.4) Nitrous acid with nitrogen at +3 can be oxidised to nitrate (nitrogen at +5) or reduced to NO or other lower oxidation state species, so it can both accept and donate electrons.5) Therefore, nitrous acid behaves as both an oxidising and a reducing agent in different chemical reactions.6) Nitrogen dioxide (NO2) can undergo disproportionation, but in typical introductory chemistry and exam questions, nitrous acid is more commonly highlighted for dual behaviour.

Verification / Alternative check:
Example reactions demonstrate this dual behaviour. As an oxidising agent, HNO2 can oxidise iodide ions to iodine while itself being reduced. As a reducing agent, it can be oxidised to nitrate in the presence of strong oxidisers such as KMnO4. Textbooks often explicitly state that nitrous acid is capable of acting either as an oxidising agent or as a reducing agent, confirming that it has amphoteric redox behaviour typical of an intermediate oxidation state compound.

Why Other Options Are Wrong:
Nitric acid (HNO3): Nitrogen is in the highest common oxidation state of +5 and mainly acts as a powerful oxidising agent, not as a reducing agent.
Ammonia (NH3): Nitrogen is in a very low oxidation state of -3, so ammonia usually acts as a reducing agent and is not used as an oxidising agent.
Nitrogen dioxide (NO2): While it can participate in complex redox chemistry, standard exam answers emphasise nitrous acid as the classical example of a species acting as both oxidising and reducing agent.
Dinitrogen gas (N2): Molecular nitrogen is very stable and relatively inert under normal conditions and is not commonly described as both oxidising and reducing in basic chemistry questions.

Common Pitfalls:
Students sometimes select nitric acid simply because they know it is a strong oxidising agent, forgetting that the question asks for a substance that also shows reducing behaviour. Others may be unsure and pick ammonia, knowing it is reducing, but they overlook that it does not act as an oxidiser. A good strategy is to check the oxidation state of the central atom: intermediate values often indicate potential for both oxidation and reduction. Remember that nitrogen in nitrous acid (oxidation state +3) can move either up to +5 or down to lower states, making HNO2 a classic example of a compound that acts both as oxidising and reducing agent.

Final Answer:
The nitrogen compound that can act as both an oxidising and a reducing agent is nitrous acid (HNO2).