Which of the following statements are true based on the 4NH 3 (g)+5O 2 (g)→4NO(g)+6H 2 O(g)? (a) N gets oxidized. (b) O gets oxidized. (c) N gets reduced. (d) O gets reduced.

Correct — C. Track the oxidation numbers across 4NH₃ + 5O₂ → 4NO + 6H₂O.

Nitrogen in NH₃ is −3 (each H is +1). Nitrogen in NO is +2 (O is −2).
−3 → +2 is a rise, so nitrogen is oxidised — statement (a) is true.

Oxygen in O₂ is 0, because it is a free element. In NO and in H₂O it is −2.
0 → −2 is a fall, so oxygen is reduced — statement (d) is true.

Statements (a) and (d) together are option (c).

Why the others are wrong

• (a)
  (c) and (d) — This pairs (c) with (d). Statement (c) claims nitrogen is reduced, but nitrogen climbs from −3 to +2 — a rise in oxidation number is oxidation, not reduction.
• (b)
  (b) and (c) — This pairs (b) with (c), which is the reaction exactly reversed — it makes oxygen the oxidised element and nitrogen the reduced one, when the opposite is true of both.
• (d)
  (a) and (b) — Statement (a) here is right, but statement (b) says oxygen is oxidised. Oxygen starts at 0 in O₂ and ends at −2, so it gains electrons and is reduced.

An oxidation number is the charge an atom would carry if every bond were fully ionic. Two conventions settle most school reactions: a free element is 0, and in compounds hydrogen is +1 and oxygen −2.

Oxidation is a rise in that number, reduction a fall, and the two always happen together — the electrons one atom loses are the electrons another gains.

Here ammonia is burnt in oxygen. Nitrogen goes −3 to +2, oxygen 0 to −2, so ammonia is the reducing agent and O₂ the oxidising agent.

Key facts

• A free element such as O₂ has an oxidation number of 0.
• In compounds hydrogen is normally +1 and oxygen normally −2.
• Oxidation is an increase in oxidation number and reduction a decrease.
• This ammonia oxidation is the first step of the Ostwald process for manufacturing nitric acid, run over a platinum catalyst.

Common traps

• Assuming oxygen must be the element oxidised because oxidation is named after it.
• Forgetting that oxygen in the free molecule O₂ is 0, not −2.

No directly related past PYQ was found.