Question

When lead dioxide reacts wod conc. nitric acid gas evolved is oxygen why not nitrogen dioxide . Nitric acid is also a strong oxidizing agent.

Answer 1

We know metal oxides are bases. Therefore Lead(IV) Oxide (PbO₂) here acts as a base.

HNO₃ (nitric acid) is an acid.

ACID + BASE → SALT + WATER

Thus general equation

Metal oxide + HNO₃ ​ → Metal Nitrate + H₂O

Thus only metal nitrate and water are the confirmed product in the reaction of any metal oxide with nitric acid. The gas evolved is the byproduct and may change with the nature of metal oxide.

Thus the gas evolved may be nitrogen dioxide or Oxygen gas in case of different metal oxides.

here in this case the balanced reaction is as follows :

2 PbO₂ (s) + 4 HNO₃ (aq) → 2 Pb(NO₃)₂ (aq) + 2H₂O (l) + O₂ (g)