P Block Elements – JEE Main ChemistryPractice Questions & PYQs

The outcome of the reaction between ammonia and chlorine depends strictly on the relative quantities of the reactants. When chlorine is present in excess, it acts as a strong halogenating agent that fully substitutes the hydrogen atoms in the ammonia molecule, yielding nitrogen trichloride and hydrogen chloride as the final products. This chemical transformation is expressed by the following equation:

$\text{NH}_3 + 3\text{Cl}_2 \rightarrow \text{NCl}_3 + 3\text{HCl}$

Materials that exhibit low melting points and fail to conduct electricity in both solid and liquid states are characterized as molecular solids, which consist of neutral molecules held together by weak intermolecular attractions. Because these substances do not contain free electrons or mobile ions, they function as insulators in any phase. Carbon tetrachloride ($CCl_4$) is a classic example of a molecular solid, making it the only substance among the choices that inherently lacks the charge carriers necessary for conductivity.

Thermal decomposition of lead(II) nitrate generates nitrogen dioxide gas, recognized by its distinct brown colour, acting as species (A). When cooled, this gas dimerizes to form the colourless dinitrogen tetroxide, which serves as species (B). Further interaction between dinitrogen tetroxide and nitric oxide during heating yields dinitrogen trioxide, the blue solid designated as species (C).

Determining the oxidation number of nitrogen in $\mathrm{N}_2\mathrm{O}_3$ involves balancing the total charge of the molecule, where the oxidation state of oxygen is assumed to be -2. Setting up the algebraic equation gives $2x + 3(-2) = 0$. Solving this results in $2x - 6 = 0$, which leads to $x = +3$.

Sulphite salts decompose when treated with strong mineral acids to liberate sulphur dioxide, identifying compound $Y$ as $\mathrm{SO}_2$ when $\mathrm{Na}_2\mathrm{SO}_3$ reacts with dilute $\mathrm{H}_2\mathrm{SO}_4$. This acidic gas reacts with the base $\mathrm{NaOH}$ to regenerate the original sulphite $\mathrm{Na}_2\mathrm{SO}_3$, which serves as compound $X$. Finally, the reaction of this sulphite $X$ with the gas $Y$ and water converts the mixture into sodium bisulphite, $\mathrm{NaHSO}_3$, establishing $Z$ as this acidic salt.

Pyrophosphoric acid, with the chemical formula $\text{H}_4\text{P}_2\text{O}_7$, is formed through the condensation of two orthophosphoric acid molecules, resulting in the two phosphorus centers being linked together by a shared oxygen atom. This structure creates exactly one $\text{P-O-P}$ bridging moiety at the core of the molecule. Each of the two phosphorus atoms is also double-bonded to an oxygen atom, giving a total of two $\text{P=O}$ bonds, while the remaining valencies of the phosphorus atoms are satisfied by four $\text{P-OH}$ groups.

The peroxodisulphate ion, $S_2O_8^{2-}$, is characterized by two $SO_4$ tetrahedra linked by a peroxide bridge, where each of the two sulphur atoms establishes four covalent bonds with oxygen atoms to yield a total of eight sulphur-oxygen bonds. In the case of rhombic sulphur, the atoms arrange themselves into a stable crown-shaped $S_8$ ring, where the continuous cyclic connectivity ensures that there are exactly eight sulphur-sulphur bonds.

To determine which compound does not fit the sequence, we must first identify the gases involved in the described reactions. Since the final product, gas (C), is basic and generated by the reaction of gas (B) with hydrogen, it is undoubtedly ammonia, $\mathrm{NH}_3$. This implies that gas (B) is nitrogen, $\mathrm{N}_2$, which is a major component of the atmosphere and reacts with hydrogen in the presence of a catalyst to produce ammonia. Therefore, the compound (A) must be a substance that yields nitrogen gas upon thermal decomposition.

Analyzing the decomposition of the given options reveals that $(\mathrm{NH}_4)_2\mathrm{Cr}_2\mathrm{O}_7$ undergoes reaction to produce $\mathrm{N}_2$, $\mathrm{Cr}_2\mathrm{O}_3$, and water, while $\mathrm{NaN}_3$ decomposes to release $\mathrm{N}_2$, and $\mathrm{NH}_4\mathrm{NO}_2$ evolves $\mathrm{N}_2$ alongside water. Conversely, the thermal decomposition of lead nitrate follows the equation $\mathrm{Pb}(\mathrm{NO}_3)_2 \xrightarrow{\Delta} \mathrm{PbO} + 2\mathrm{NO}_2 + \frac{1}{2}\mathrm{O}_2$. Because this process releases nitrogen dioxide rather than nitrogen gas, it fails to meet the criteria established by the reaction sequence.

The triple bond ($N \equiv N$) in the dinitrogen molecule possesses a very high bond dissociation energy, rendering it chemically inert at room temperature and therefore suitable to act as an inert diluent for reactive chemicals. Liquid dinitrogen is extensively utilized in cryosurgery to freeze and destroy diseased tissue, $N_2$ is diamagnetic due to its fully filled molecular orbitals, and the reaction with dioxygen requires temperatures on the order of 2000 K rather than $25^{\circ}\mathrm{C}$.

The reactivity of chlorine gas with alkaline hydroxides is governed by reaction conditions, which determine the oxidation state of the resulting chlorine species. When chlorine reacts with hot and concentrated $\mathrm{NaOH}$, it undergoes disproportionation to form sodium chlorate, $\mathrm{NaClO}_3$, as the primary oxy-salt. Conversely, the reaction of dry calcium hydroxide, $\mathrm{Ca(OH)}_2$, with chlorine gas yields calcium hypochlorite, $\mathrm{Ca(OCl)}_2$, which serves as the essential component of bleaching powder.

When nitric oxide ($\mathrm{NO}$) reacts with dinitrogen tetroxide ($\mathrm{N}_2\mathrm{O}_4$) at a temperature of 250 K, the specific chemical conditions favor the formation of dinitrogen trioxide ($\mathrm{N}_2\mathrm{O}_3$). This process is characterized by the production of a distinctive blue solid, as described by the following balanced equation:
$2\mathrm{NO} + \mathrm{N}_2\mathrm{O}_4 \rightarrow 2\mathrm{N}_2\mathrm{O}_3$

Borazine ($B_3N_3H_6$), known as inorganic benzene, is formed by the reduction of B-trichloroborazine ($H_3N_3B_3Cl_3$) using the hydride source lithium borohydride ($LiBH_4$), which replaces the chlorine substituents with hydrogen atoms. Conversely, the treatment of B-trichloroborazine with methylmagnesium bromide ($MeMgBr$) results in the nucleophilic substitution of chlorine atoms by methyl groups, generating $H_3N_3B_3(Me)_3$.

White phosphorus reacts with a concentrated aqueous sodium hydroxide solution in an inert atmosphere of $\mathrm{CO_2}$ to produce phosphine gas and sodium hypophosphite, which is represented as compound (X). When this sodium hypophosphite reacts with hydrochloric acid, it undergoes a chemical exchange to yield hypophosphorous acid, identified as compound (Y) with the molecular formula $\mathrm{H_3PO_2}$.

The basicity of an oxoacid is determined by the number of ionizable hydrogen atoms bonded directly to oxygen atoms, typically existing as -OH groups. In the chemical structure of $\mathrm{H_3PO_2}$, the central phosphorus atom is bonded to two hydrogen atoms directly, one oxygen via a double bond, and a single hydroxyl group. Because only the hydrogen atom within the hydroxyl group is acidic and capable of dissociation in aqueous solution, the basicity of the compound is 1.

Aqua regia, a highly reactive mixture of concentrated hydrochloric acid and nitric acid, is capable of dissolving noble metals because the nitric acid acts as a powerful oxidizing agent. During the dissolution of gold, the nitric acid undergoes reduction, specifically producing nitric oxide as a gaseous byproduct according to the balanced equation $\mathrm{Au} + \mathrm{HNO}_3 + 4\mathrm{HCl} \rightarrow \mathrm{HAuCl}_4 + \mathrm{NO} + 2\mathrm{H}_2\mathrm{O}$.

Magnesium reacts with nitrogen gas upon heating to form magnesium nitride ($Mg_3N_2$), which undergoes hydrolysis to yield ammonia gas according to the reaction $Mg_3N_2 + 6H_2O \to 3Mg(OH)_2 + 2NH_3$. This liberated ammonia acts as a ligand and coordinates with the copper(II) ions in the aqueous copper(II) sulfate solution to form the deep blue-violet complex $[Cu(NH_3)_4]^{2+}$, identifying the metal A as magnesium and the compound B as magnesium nitride.

Zone refining is a highly specialized purification technique grounded in the principle that impurities are generally more soluble in the molten state of a material than in its solidified form. By slowly passing a mobile heater along a rod of the substance, impurities are gradually concentrated into the molten zone and pushed toward one end, allowing the cooling material left behind to crystallize with exceptional levels of purity. Because of this efficiency in removing trace contaminants, this method is uniquely suited for producing the extremely high-purity boron and silicon required for modern semiconductor technologies.

Silicones are synthetic organosilicon polymers characterized by the repeating unit $(R_2SiO)_n$, a structure that imparts a naturally hydrophobic character and remarkable biocompatibility suitable for diverse medical and personal care applications. These materials are highly valued for their exceptional thermal stability and resistance to oxidative degradation, which allows them to function effectively as durable greases and lubricants under various harsh conditions. Consistent with their broad industrial utility, their specific dielectric properties and high heat resistance are fundamental characteristics that define their performance and reliability.

Hydrolysis of covalent chlorides generally requires the central atom to possess vacant orbitals, typically d-orbitals, which allow water molecules to coordinate with the central atom and initiate the reaction. In the case of carbon, the valence shell is limited to $n=2$, which contains only s and p orbitals and lacks the empty d-orbitals necessary to accommodate the incoming electron pair from water. Consequently, while silicon, tin, and lead can easily undergo hydrolysis, $CCl_4$ remains inert toward the process due to the absolute absence of any vacant orbital on the carbon atom.

The molecular structure of $B_2H_6$ consists of four terminal hydrogen atoms bonded to two boron atoms and two bridging hydrogen atoms that connect the two boron centers. Each of the four terminal $B-H$ bonds involves two electrons shared between two nuclei, which constitutes 2-center-2-electron bonds. The two $B-H-B$ bridging interactions involve two electrons delocalized over three atomic nuclei, identifying them as 3-center-2-electron bonds. This configuration results in a total of 4 terminal bonds and 2 bridging bonds.

This transformation represents a disproportionation reaction, where the chlorine element simultaneously undergoes both oxidation and reduction upon contact with hot, concentrated sodium hydroxide. In this specific chemical environment, the chlorine molecules redistribute their oxidation states to produce chloride ions ($Cl^{-}$) alongside chlorate ions ($ClO_3^{-}$). The balanced chemical equation governing this process is $3Cl_2 + 6OH^{-} \rightarrow 5Cl^{-} + ClO_3^{-} + 3H_2O$.

Silicate minerals such as feldspar, zeolites, mica, and asbestos are constructed from a repetitive arrangement of silicon-oxygen tetrahedra. In this geometry, a silicon atom sits centrally within four oxygen atoms positioned at the vertices of a tetrahedron, forming the fundamental anionic building block $(\mathrm{SiO}_4)^{4-}$. These tetrahedra link together by sharing one or more corner oxygen atoms to create the complex crystalline structures characteristic of these mineral classes.