Problem 11 - 43 Years JEE ADVANCED (1978-2020) + JEE MAIN Chapterwise & Topicwise Solved Papers Chemistry

Question

The pair that contains two P-H bonds in each of the oxoacids is: [Main Jan. 10, 2019 (II)] (a) \(\mathrm{H}_{4} \mathrm{P}_{2} \mathrm{O}_{5}\) and \(\mathrm{H}_{4} \mathrm{P}_{2} \mathrm{O}_{6}\) (b) \(\mathrm{H}_{3} \mathrm{PO}_{2}\) and \(\mathrm{H}_{4} \mathrm{P}_{2} \mathrm{O}_{5}\) (c) \(\mathrm{H}_{3} \mathrm{PO}_{3}\) and \(\mathrm{H}_{3} \mathrm{PO}_{2}\) (d) \(\mathrm{H}_{4} \mathrm{P}_{2} \mathrm{O}_{5}\) and \(\mathrm{H}_{3} \mathrm{PO}_{3}\)

Step-by-Step Solution

Answer

Option (c) \(\mathrm{H}_{3}\mathrm{PO}_{3}\) and \(\mathrm{H}_{3}\mathrm{PO}_{2}\) contain two P-H bonds.

Step 1: Understanding Oxoacids

Oxoacids are acids that contain hydrogen, oxygen, and another element (in this case, phosphorus). In oxoacids, hydrogen is typically bonded to oxygen, but in some cases, it can also be bonded directly to phosphorus.

Step 2: Identify P-H Bonds

To identify if a P-H bond exists, we need to write the structure of each given compound: - **H₄P₂O₅**: This may represent pyrophosphoric acid which has no P-H bonds. - **H₄P₂O₆**: This is also unlikely to have P-H bonds based on typical structures. - **H₃PO₂**: Known as hypophosphorous acid, with one P-H bond per phosphorus atom. - **H₃PO₃**: Known as phosphorous acid, typically has one P-H bond.

Step 3: Analyze Each Option for P-H Bonds

- **Option (a) \(\mathrm{H}_{4}\mathrm{P}_{2}\mathrm{O}_{5}\)** and \(\mathrm{H}_{4}\mathrm{P}_{2}\mathrm{O}_{6}\): Neither has P-H bonds as per their conventional structures.- **Option (b) \(\mathrm{H}_{3}\mathrm{PO}_{2}\)** and \(\mathrm{H}_{4}\mathrm{P}_{2}\mathrm{O}_{5}\): Only \(\mathrm{H}_{3}\mathrm{PO}_{2}\) has P-H bonds.- **Option (c) \(\mathrm{H}_{3}\mathrm{PO}_{3}\)** and \(\mathrm{H}_{3}\mathrm{PO}_{2}\): Both have P-H bonds (1 in \(\mathrm{H}_{3}\mathrm{PO}_{3}\) and 1 in \(\mathrm{H}_{3}\mathrm{PO}_{2}\)).- **Option (d) \(\mathrm{H}_{4}\mathrm{P}_{2}\mathrm{O}_{5}\)** and \(\mathrm{H}_{3}\mathrm{PO}_{3}\): Only \(\mathrm{H}_{3}\mathrm{PO}_{3}\) has P-H bonds.

Step 4: Conclusion

After analyzing each pair of compounds, the correct answer is the pair that contains two P-H bonds, one in each compound. This is option (c) \(\mathrm{H}_{3}\mathrm{PO}_{3}\) and \(\mathrm{H}_{3}\mathrm{PO}_{2}\).

Key Concepts

P-H bonds

In the world of chemistry, P-H bonds stand for the bonds between a phosphorus (P) atom and hydrogen (H) atom within a molecule. These bonds are fairly unusual because, in many compounds, hydrogen prefers to bond with oxygen instead, especially in oxoacids.

Oxoacids are characterized by their structure containing hydrogen, oxygen, and another element, such as phosphorus. In certain cases, however, the hydrogen atom can directly bond with the phosphorus atom instead, creating a P-H bond.

This bond is less common and is mainly seen in specific phosphorus-containing acids such as hypophosphorous acid and phosphorous acid, where the direct P-H bond plays a key role in their chemical properties. These bonds are important due to their role in the reducing behavior of these compounds.

Hypophosphorous acid

Hypophosphorous acid, chemically denoted as \(\mathrm{H}_3\mathrm{PO}_2\), is a distinctive oxoacid of phosphorus. It is also known as phosphinic acid and possesses a unique structural characteristic, containing a single P-H bond for each phosphorus atom.

This composition gives it a strong reducing property, which is why it is considered a powerful reducing agent in various chemical reactions. The presence of one P-H bond per phosphate unit allows hypophosphorous acid to participate effectively in transformations and reductions of various organic and inorganic compounds.

Understanding its molecular structure reveals that of the three hydrogen atoms, two are bonded to the phosphorus (one P-H bond), and one is an acidic hydrogen attached to oxygen. It is vital to recognize that this acid, although simplistic, differs from its phosphoric \(\mathrm{H}_3\mathrm{PO}_4\) relatives due to those distinct P-H bonds influencing its reactivity.

Phosphorous acid

Phosphorous acid, or \(\mathrm{H}_3\mathrm{PO}_3\), is another notable phosphorus oxoacid and often confused with its other acidic forms due to how similar their names are. This compound, however, stands out because it contains one P-H bond for each phosphorus atom, much like hypophosphorous acid.

Its molecular arrangement consists of three oxygen atoms attached to a central phosphorus atom, with two of these bonded through P-O-H groups and one through a P-H bond. This configuration not only endows phosphorous acid with its unique chemical behavior but also forms the basis of its strong deoxidizing characteristics.

The presence of the P-H bond in phosphorous acid makes it an effective reducing agent, similar to, but not as potent as hypophosphorous acid. It is useful in various industrial applications, including the preparation of phosphite salts and as a water treatment agent to prevent corrosion.