Question

What happens when concentrated $HN{O_3}$ is added to iron metal?
A)Nitrogen is given out
B)Hydrogen is given out
C)The metal becomes passive
D)$Fe{(N{O_3})_2}$ is formed

Answer 1

Nitric Acid or $HN{O_3}$ is a highly corrosive mineral acid. It exhibits different characteristics while reacting with different substances. The type of products formed in the reactions involving $HN{O_3}$ varies greatly depending on the concentration of $HN{O_3}$ . When the concentration of nitric acid is dilute, it behaves like any other regular acid

Complete Step-by-Step Answer:
Before we move forward with the solution of the given question, let us understand some important basic concepts.
When a metal is reacted with dilute nitric acid, it results in the formation of the corresponding metal nitrate. This reaction also involves the release of hydrogen gas. A general representation of the chemical reaction between a metal and dilute nitric acid can be given as:
$M + 2HN{O_3} \to M{(N{O_3})_2} + {H_2}$ , where M is the metal.
Now, when a concentrated solution of the acid is used, it usually results in the oxidizing of the products and also releases nitrogen dioxide gas as a by – product. When this highly concentrated nitric acid is reacted with metals like aluminium, chromium, or iron, they generally tend to dissolve in the acid solution. Nitric acid forms a layer of metal oxide on the surfaces of these metals, which tends to protect the bigger bulk of the metal. The process of formation of this protective layer because of concentrated nitric acid is known as Passivation. Hence, the metal becomes chemically passive because of reacting with concentrated nitric acid.
Hence, Option C is the correct option

Note: Metals such as gold, Platinum do not react with nitric acid. They do not react with any strong acids for that matter. This is because the reactivity of these metals is extremely low. However, these metals react with a compound named Aqua Regia. Aqua Regia is made form nitric acid and hydrochloric acid.