Question

Iron is rendered passive in:

• A. aquaregia
• B. conc. H${}_2$SO${}_4$
• C. con. HNO${}_3$
• D. conc. HCl

Answer 1

Answer: (C)

con. HNO${}_3$

Reaction of iron with nitric acid:

Products of Iron (Fe) reacting with Nitric Acid (HNO${}_3$) depends on the temperature of reactants as well as the concentration of HNO${}_3$.

1. With dilute HNO${}_3$ Iron gives Ferrous Nitrate [Fe(NO${}_3$)${}_2$], water and Nitric oxide (NO).

3Fe + 8HNO${}_3$ $\to$ 3Fe(NO${}_3$)${}_2$ + 4H${}_2$O + 2NO

2. With concentrated HNO${}_3$ iron gives Ferric nitrate [Fe(NO${}_3$)${}_3$], water and Nitrogen dioxide (NO${}_2$).

Fe + 6HNO${}_3$ $\to$ Fe(NO${}_3$)${}_3$ +3H${}_2$O + 3NO${}_2$

Reaction of iron with concentrated Nitric acid Renders it passive to further reaction due to the formation of a thin oxide layer on the surface.