NDA (II) 2025 General Ability Test · Question 104 of 147

Correct — B, N2H4 (hydrazine). A reducing agent is the species that is itself oxidised — it loses electrons and pushes them onto something else. In hydrazine the nitrogen is at oxidation state -2, and in the product N2 it rises to 0, so hydrazine is oxidised and therefore acts as the reducing agent. Hydrogen peroxide, whose oxygen falls from -1 to -2, is reduced and so is the oxidising agent.

• (a)H2O2 — Hydrogen peroxide is the oxidising agent, not the reducing agent. Its oxygen is reduced from the -1 state to -2 (in water), meaning it gains electrons rather than donating them.
• (c)N2 — Nitrogen gas is a product, not a reactant, so it cannot be the agent driving the reaction. It is simply the oxidised form of the nitrogen that started out in hydrazine.
• (d)H2O — Water is only a product of the reaction. It is neither oxidised nor reduced during the change, so it plays no role as an oxidising or reducing agent here.

In a redox reaction one species is oxidised (loses electrons, its oxidation number rises) while another is reduced (gains electrons, its oxidation number falls). The species that is oxidised is the reducing agent, because it supplies electrons to the other; the species that is reduced is the oxidising agent. Tracking oxidation numbers is the reliable way to tell which is which.

Assign oxidation numbers and watch which way each key element moves. Nitrogen climbs from -2 in N2H4 to 0 in N2, an oxidation, so hydrazine is the reducing agent; oxygen drops from -1 in H2O2 to -2 in water, a reduction, so peroxide is the oxidising agent. The trap is to pick the more reactive-sounding peroxide, which is in fact the oxidiser.

• The reducing agent is the species that is oxidised (loses electrons); the oxidising agent is the species that is reduced (gains electrons).
• In N2H4 nitrogen is at -2 and rises to 0 in N2, so hydrazine is oxidised.
• In H2O2 oxygen is at -1 and falls to -2 in water, so peroxide is reduced.
• Hydrogen peroxide can act as either an oxidising or a reducing agent depending on the partner, but here it is the oxidiser.

• The reducing agent is the one that gets oxidised — the naming feels back-to-front, so track the electrons, not intuition.
• A product such as N2 or H2O can never be the agent; only the reactants can be oxidising or reducing agents.

Redox is asked as 'identify the oxidising or reducing agent' — assign oxidation numbers to the reactants and see which element goes up (reducing agent) and which goes down (oxidising agent).

• practice — not a real PYQ

  In the reaction Zn + CuSO4 → ZnSO4 + Cu, the reducing agent is

  • (a)Cu
  • (b)Zn
  • (c)CuSO4
  • (d)ZnSO4

  Answer(b) Zn — zinc is oxidised from 0 to +2, so it is the reducing agent.

• practice — not a real PYQ

  A reducing agent in a redox reaction is the species that

  • (a)gains electrons
  • (b)is reduced
  • (c)loses electrons
  • (d)acts as a catalyst

  Answer(c) loses electrons — it is itself oxidised while reducing the other species.