Problem 128 - A Complete Resource Book in Chemistry for JEE Main

Question

Phosphorus on reaction with conc. HNO \(_{3}\) gives an acid A which can also be formed by the action of dil. \(\mathrm{H}_{2} \mathrm{SO}_{4}\) on powdered phosphorite rock, the acid \(\mathrm{A}\) is (a) \(\mathrm{H}_{3} \mathrm{PO}_{3}\) (b) \(\mathrm{H}_{3} \mathrm{PO}_{4}\) (c) \(\mathrm{H}_{3} \mathrm{PO}_{2}\) (d) \(\mathrm{HPO}_{3}\)

Step-by-Step Solution

Answer

The acid A is H3PO4.

Step 1: Identify the Reaction with HNO3

Phosphorus reacts with concentrated nitric acid (HNO3) and is known to produce phosphorus pentoxide (P4O10), which then forms phosphoric acid (H3PO4).

Step 2: Action of H2SO4 on Phosphorite

Dilute sulfuric acid (H2SO4) acts on phosphorite rock (which contains apatite, Ca5(PO4)3F) to produce phosphoric acid (H3PO4) among other products.

Step 3: Compare Given Options with Reaction Products

The product formed in both reactions is phosphoric acid, which matches option (b) H3PO4. The other acids: (a) H3PO3, (c) H3PO2, and (d) HPO3 are not the same as the acid formed.

Key Concepts

Reaction with Nitric Acid

Phosphorus undergoes a chemical reaction when it interacts with concentrated nitric acid (HNO\(_3\)). This reaction is a two-step process where phosphorus initially reacts to form phosphorus pentoxide (P\(_4\)O\(_{10}\)).
The chemical equation for this primary reaction is:
\[\text{P} + \text{HNO}_{3} \rightarrow \text{P}_{4}\text{O}_{10} + \text{other products}\]
Upon further reaction, phosphorus pentoxide in the presence of water forms phosphoric acid (H\(_3\)PO\(_4\)). This occurs as follows:
\[\text{P}_{4}\text{O}_{10} + 6\text{H}_{2}\text{O} \rightarrow 4\text{H}_{3}\text{PO}_{4}\]
Thus, the main product of the reaction of phosphorus with concentrated nitric acid is phosphoric acid, also known as H\(_3\)PO\(_4\). By understanding these steps, one can better grasp how phosphorus leads to the formation of this common acid.

Sulfuric Acid on Phosphorite

Phosphorite rock, which is rich in the mineral apatite (Ca\(_5\)(PO\(_4\))\(_3\)F), is an essential source in the production of phosphoric acid (H\(_3\)PO\(_4\)). When dilute sulfuric acid (H\(_2\)SO\(_4\)) is added to phosphorite, a chemical reaction occurs that isolates phosphates and results in the formation of phosphoric acid.
One of the key reactions in this process involves:
\[\text{Ca}_{5}(\text{PO}_{4})_{3}\text{F} + 5\text{H}_{2}\text{SO}_{4} \rightarrow 3\text{H}_{3}\text{PO}_{4} + 5\text{CaSO}_{4} + \text{HF}\]
This demonstrates how effective dilute sulfuric acid is in extracting the needed phosphates to form phosphoric acid. Understanding this reaction helps clarify why phosphorite is a valuable rock in chemical manufacturing, particularly for fertilizers and other industries linked to phosphoric acid production.

Chemical Reaction Identification

Chemical reactions often produce specific products based on the reactants involved. When attempting to identify a reaction, chemists examine the known products and compare them to expected outcomes. In the case of phosphoric acid formation, both nitric and sulfuric acids play roles.
In the exercise, the key is to recognize which product is formed similarly by different methods. Phosphoric acid (H\(_3\)PO\(_4\)) is the common result from both the reaction of phosphorus with nitric acid and the treatment of phosphorite with sulfuric acid.
By comparing the possible candidates, it quickly becomes evident that neither H\(_3\)PO\(_3\), H\(_3\)PO\(_2\), nor HPO\(_3\) fits the results of these reactions. Phosphoric acid stands out as it is consistently produced by both pathways, confirming option (b) as the correct choice.