Problem 65 - Chemistry: The Central Science

Question

Account for the following observations: (a) Phosphorus forms a pentachloride, but nitrogen does not. (b) \(\mathrm{H}_{3} \mathrm{PO}_{2}\) is a monoprotic acid. (c) Phosphonium salts, such as \(\mathrm{PH}_{4} \mathrm{Cl}\), can be formed under anhydrous conditions, but they can't be made in aqueous solution. (d) White phosphorus is extremely reactive.

Step-by-Step Solution

Answer

(a) Phosphorus forms a pentachloride because it can expand its octet using its d orbitals, while nitrogen cannot do the same due to its position in the periodic table, forming only \(\mathrm{NCl}_{3}\). (b) \(\mathrm{H}_{3} \mathrm{PO}_{2}\) is monoprotic because it can only release one proton, specifically from the P-OH bond. (c) Phosphonium salts like \(\mathrm{PH}_{4} \mathrm{Cl}\) can be made in anhydrous conditions but not in aqueous solution as PH3 reacts with water instead, forming \(\mathrm{PH}_{3}\cdot\mathrm{H}_{2}\mathrm{O}\). (d) White phosphorus is extremely reactive due to the strain within its P4 tetrahedral structure, which releases a lot of energy when a P-P bond is broken.

Step 1: (a) Phosphorus forms a pentachloride, but nitrogen does not.

Phosphorus has five valence electrons and forms a pentachloride, \(\mathrm{PCl}_{5}\), by expanding its octet using its d orbitals. Nitrogen, on the other hand, has only three valence electrons and does not have access to d orbitals since it belongs to the second period in the periodic table. Nitrogen can only form \(\mathrm{NCl}_{3}\); thus, it does not form a pentachloride.

Step 2: (b) \(\mathrm{H}_{3} \mathrm{PO}_{2}\) is a monoprotic acid.

The structure of \(\mathrm{H}_{3} \mathrm{PO}_{2}\) has two P-H bonds and one P-OH bond. Since only the hydrogen atom in a P-OH bond can be donated in an acid-base reaction, \(\mathrm{H}_{3} \mathrm{PO}_{2}\) can only release one proton, making it a monoprotic acid.

Step 3: (c) Phosphonium salts, such as \(\mathrm{PH}_{4} \mathrm{Cl}\), can be formed under anhydrous conditions, but they can't be made in aqueous solution.

Phosphonium salts, like \(\mathrm{PH}_{4} \mathrm{Cl}\), are formed when phosphine (PH3) reacts with a halogen, such as chlorine (Cl2), under anhydrous (absence of water) conditions. However, in an aqueous solution, PH3 would preferentially act as a Lewis base reacting with the water (due to the lone pair of electrons on the phosphorus atom), forming \(\mathrm{PH}_{3}\cdot\mathrm{H}_{2}\mathrm{O}\) and preventing the formation of phosphonium salts.

Step 4: (d) White phosphorus is extremely reactive.

White phosphorus consists of discrete P4 tetrahedra, where each phosphorus atom is bonded to three other phosphorus atoms, leading to significant strain in the molecule. This strain makes white phosphorus highly reactive, as breaking a single P-P bond releases a significant amount of energy. In contrast, red phosphorus has a more stable, polymeric structure with less molecular strain, making it much less reactive than white phosphorus.

Key Concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial in determining an element's chemical properties, especially its reactivity and ability to bond with other elements. For instance, phosphorus, with five valence electrons, can form five covalent bonds as in phosphorus pentachloride (\(PCl_5\)), utilizing its d orbitals to expand its valence shell beyond the typical eight-electron octet.

In contrast, nitrogen, which also aims to achieve stability in bonding, has only three valence electrons and is limited to an octet as it lacks access to the d orbitals found in the third period elements like phosphorus. This limitation explains why nitrogen does not form a nitrogen pentachloride (\(NCl_5\)) and is restricted to forming trichloride (\(NCl_3\)).

d Orbitals

The d orbitals in an atom's electron configuration begin to fill with electrons from the fourth period onward on the periodic table. These orbitals can participate in bonding by offering additional space for electrons, thereby allowing elements like phosphorus to accommodate more than an octet in certain compounds. This ability to use d orbitals for bonding is crucial in the formation of compounds with high coordination numbers, such as phosphorus pentachloride, where phosphorus can expand its valence shell to accommodate ten electrons in total.

Monoprotic Acid

A monoprotic acid is an acid that can donate only one proton (hydrogen ion) per molecule in an acid-base reaction. The acid \(H_3PO_2\), known as hypophosphorous acid, exemplifies this behavior; it has two hydrogen atoms bonded to phosphorus, but only the hydrogen in the P-OH bond is acidic and can be donated. The other hydrogen atoms, linked directly to phosphorus as P-H bonds, are not released as protons in solution. This characteristic defines the acid's monoprotic nature and influences its reactivity and strength as an acid.

Phosphonium Salts

Phosphonium salts, such as \(PH_4Cl\), are ionic compounds that form when a phosphorus compound, typically phosphine (\(PH_3\)), reacts with a halogen like chlorine. These salts can only form under anhydrous conditions because phosphine acts as a Lewis base in the presence of water, preferring to bond with water molecules rather than form the salts. The lone pair of electrons on the phosphorus in phosphine is donated to a proton in water, thus preventing the formation of phosphonium salts in aqueous environments.

Molecular Reactivity

The molecular reactivity of an element or compound is influenced by factors such as electronic configuration, bond strength, molecular geometry, and the presence of any molecular strain. For example, white phosphorus is composed of \(P_4\) tetrahedra units, with significant strain due to the angles between bonds that deviate from the ideal tetrahedral angle. This strain puts stress on the bonds, making them weaker and more reactive. Atoms will often undergo reactions to relieve this strain and move to a more thermodynamically stable arrangement, as seen with phosphorus, which is more reactive in its white form than in the red or black forms.

Lewis Base

A Lewis base is a molecule or ion that can donate a pair of electrons to form a coordinate covalent bond with a Lewis acid. In the context of phosphorus chemistry, phosphine (\(PH_3\)) acts as a Lewis base due to its lone pair of electrons on the phosphorus atom. This electron pair can be donated to other compounds, such as water, to form complexes or react and produce other species. The Lewis base behavior of phosphine is critical in explaining why phosphonium salts cannot be formed in aqueous solutions, as the phosphine prefers to react with water rather than halogens in such conditions.

Molecular Strain

Molecular strain refers to the stress within a molecule resulting from bond angles or lengths that deviate from their optimal values. Strain can occur in cyclic structures, small rings, and molecules like white phosphorus (\(P_4\)) where the P-P-P bond angles are compressed. This strain is a form of potential energy that makes molecules less stable and more chemically reactive. As the molecules react and the strain is released, the potential energy decreases, driving the reaction energetically towards products with a more favorable, lower-energy configuration.