Which one of the following reactions will give NO (nitric oxide) gas as one of the products?

The reaction of metals with nitric acid (HNO3) is a complex redox process where the reduction product depends on the concentration of the acid and the reactivity of the metal. When copper (Cu) reacts with dilute nitric acid, it reduces the nitrogen from a +5 oxidation state to +2, resulting in the evolution of nitric oxide (NO) gas. The balanced equation is 3Cu + 8HNO3 (dilute) → 3Cu(NO3)2 + 2NO + 4H2O. In contrast, concentrated nitric acid reacts with copper to produce nitrogen dioxide (NO2), a brown gas, as seen in the reaction Cu + 4HNO3 (conc) → Cu(NO3)2 + 2NO2 + 2H2O. Zinc (Zn), being more reactive, typically reduces dilute nitric acid further to nitrous oxide (N2O) or ammonium nitrate (NH4NO3) rather than NO. Copper's specific reduction potential with dilute HNO3 uniquely favors NO production among the given options.

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 1: Chemical Reactions and Equations > CuO +H Cu+H O 2 2 → Heat (1.29) > p. 12