Phosphorus react with concentrated $HNO_3$ to form

A. Phosphoric acid
B. Phosphorus acid
C. $NO_2$
D. NO

Choose the correct answer from the options given below:

A and C only

The correct answer is Option (1) → A and C only.

Let us delve into the details of the reaction between phosphorus and concentrated nitric acid, including the products formed, reaction mechanism, and the underlying chemistry.

When phosphorus (P) reacts with concentrated nitric acid (HNO₃), it undergoes oxidation. The primary products of this reaction are: Phosphoric Acid (H₃PO₄); Nitrogen Dioxide (NO₂)

The general reaction can be summarized as follows:

\(P + 5 HNO_3 \longrightarrow H_3PO_4 + 5 NO_2 + 2 H_2O\)

Mechanism of the Reaction

Step 1: Oxidation of Phosphorus

Phosphorus is oxidized by nitric acid. The concentrated nitric acid acts as a strong oxidizing agent. In this step, phosphorus is converted into its higher oxidation state, ultimately leading to the formation of phosphoric acid. 

Phosphorus in its elemental form has an oxidation state of 0. In phosphoric acid (H₃PO₄), the oxidation state of phosphorus is +5.

Step 2: Formation of Nitrogen Dioxide

During the reaction, nitric acid is reduced to nitrogen dioxide. The nitric acid donates oxygen to phosphorus and gets converted to NO₂ as a result of its reduction:

The nitrogen in nitric acid (with an oxidation state of +5) is reduced to +4 in nitrogen dioxide (NO₂).

Key Products

Phosphoric Acid (H₃PO₄)

Phosphoric acid consists of a phosphorus atom bonded to four oxygen atoms, three of which are involved in hydroxyl (-OH) groups, while the fourth is double-bonded as a phosphate group. Phosphoric acid is a triprotic acid, meaning it can donate three protons (H⁺) in aqueous solutions.

Nitrogen Dioxide (NO₂)

Nitrogen dioxide is a reddish-brown gas with a bent structure, consisting of one nitrogen atom bonded to two oxygen atoms. It is known for its toxic nature and is a significant air pollutant. NO₂ can dimerize to form dinitrogen tetroxide (N₂O₄) under certain conditions.

Examining the Options

Given the question asks about the products formed when phosphorus reacts with concentrated HNO₃, let’s evaluate the options provided:

A. Phosphoric acid: Correct. This is the main product of the reaction.

B. Phosphorus acid: Incorrect. Phosphorus acid (H₃PO₃) is not formed in this reaction. Phosphorus acid can be produced through different reactions, such as the hydrolysis of phosphorus trichloride, but it is not a product of the reaction with concentrated nitric acid.

C. NO₂: Correct. Nitrogen dioxide is indeed produced alongside phosphoric acid in this reaction.

D. NO: Incorrect. Nitric oxide (NO) is not produced in significant quantities in this reaction; instead, nitrogen dioxide is the primary nitrogen product.

Conclusion

Thus, when phosphorus reacts with concentrated nitric acid, the products formed are phosphoric acid (H₃PO₄) and nitrogen dioxide (NO₂). Therefore, the correct response to the question is A and C only.