Problem 60 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following is not correct? (a) Nitric oxide in solid state exhibits diamagnetic property. (b) \(\mathrm{NH}_{3}\) is a weak reducing agent compared to \(\mathrm{PH}_{3}\) (c) Hydrolysis of \(\mathrm{NCl}_{3}\) gives \(\mathrm{NH}_{3}\) and \(\mathrm{HOCl}\) (d) \(\mathrm{NH}_{3}\) is less stable than \(\mathrm{PH}_{3}\)

Step-by-Step Solution

Answer

Option (d) is not correct as NH₃ is more stable than PH₃.

Step 1: Diamagnetism of Nitric Oxide

Nitric oxide (NO) in its gaseous state is paramagnetic due to the unpaired electron. However, in the solid state, nitric oxide forms dimers (5 N=O) that pair up all the electrons, making it diamagnetic. Therefore, option (a) is correct.

Step 2: Reducing Agent Strength Comparison

Ammonia (5 NH_3) is a known reducing agent, but its oxidizing power is less than that of phosphine (5 PH_3). Therefore, PH_3 is a stronger reducing agent compared to NH_3. Thus, option (b) is correct.

Step 3: Hydrolysis of Nitrogen Trichloride

When nitrogen trichloride (5 NCl_3) undergoes hydrolysis, it is converted into ammonia (5 NH_3) and hypochlorous acid (5 HOCl). This confirms that option (c) is correct.

Step 4: Stability of Ammonia vs Phosphine

Ammonia (5 NH_3) is more stable than phosphine (5 PH_3) due to stronger hydrogen bonding and higher bond energy in NH_3. Therefore, option (d) is not correct.

Key Concepts

Diamagnetism

Diamagnetism is a fascinating property of substances that arises when all the electrons in the atoms or molecules are paired. When exposed to an external magnetic field, diamagnetic substances create an induced magnetic field in the opposite direction, which slightly repels the applied field. It's a subtle effect but noticeable with experiments.
In the case of nitric oxide ( NO ), it behaves differently depending on its state. In the gaseous form, it has an unpaired electron, making it paramagnetic. However, when solidified, NO molecules form dimers ( N_2O_2 ), which pair up all the electrons, unlocking its diamagnetic nature.

Reducing Agents

Reducing agents play a vital role in redox reactions by donating electrons and getting oxidized themselves. Inorganic chemistry offers several examples of reducing agents with varying strengths. Among these, ammonia ( NH_3 ) and phosphine ( PH_3 ) are often compared.
Phosphine ( PH_3 ) is a stronger reducing agent than NH_3 . This is because phosphorus is less electronegative than nitrogen, facilitating easier donation of electrons. Therefore, PH_3 readily participates in reactions to reduce other substances.

Hydrolysis

Hydrolysis is a chemical reaction where water breaks down compounds. It's fundamental in many biological and inorganic processes. A typical example is the hydrolysis of nitrogen trichloride ( NCl_3 ).
When NCl_3 undergoes hydrolysis, it splits into ammonia ( NH_3 ) and hypochlorous acid ( HOCl ). This reaction showcases how water can transform complex molecules into simpler ones, providing substrates for further reactions.

Stability of Compounds

The stability of a compound is often influenced by the type of bonds it contains and intermolecular forces. Ammonia ( NH_3 ) and phosphine ( PH_3 ) provide a clear example of this concept.
Ammonia is more stable than phosphine because of stronger hydrogen bonds present in NH_3 . These hydrogen bonds result from nitrogen's higher electronegativity compared to phosphorus, allowing NH_3 to maintain a more robust structure. The bond energy of N-H in ammonia is also higher than that of P-H in phosphine, further enhancing its stability.