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 more reactive than red phosphorus.

Short answer

PCl5 forms due to phosphorus utilizing its 3d orbitals, while nitrogen lacks accessible d orbitals to expand its octet, allowing only NCl3. H3PO2 is monoprotic as only 1 hydrogen is bonded to the electronegative oxygen. Phosphonium salts form under anhydrous conditions due to the solvation of ions in aqueous solutions. White phosphorus has a reactive P4 tetrahedral arrangement, while red phosphorus has a stable polymeric structure with favorable bond angles.

Step-by-step solution

Observation (a): PCl5 and NCl5 formation

Phosphorus forms a pentachloride (PCl5) while nitrogen does not. The reason for this observation lies in the atomic structure of phosphorus and nitrogen. Phosphorus has 5 valence electrons and can expand its octet structure by utilizing its empty 3d orbitals, thus it can form 5 covalent bonds with chlorine atoms. Nitrogen, on the other hand, has only 5 valence electrons and does not have any accessible d orbitals in the same energy range as its 2p orbitals to expand its octet. Therefore, nitrogen can only form 3 covalent bonds with chlorine atoms (forming NCl3), and cannot form pentachloride as phosphorus does.

Observation (b): Monoprotic nature of H3PO2

${\mathrm{H}}_3{\mathrm{PO}}_2$ is a monoprotic acid, which means it can donate only one hydrogen ion (${\mathrm{H}}^{+}$) to a base in an aqueous solution. The structure of H3PO2 can be represented as H2P(OH)(O)H, and we can see that only one of the three hydrogen atoms is bonded to the oxygen atom. Due to the higher electronegativity of oxygen, the $\mathrm{O}-\mathrm{H}$ bond is more polar, which makes the hydrogen atom easier to donate as a proton in an acid-base reaction. The other two hydrogen atoms are bonded to the less electronegative phosphorus atom, making the release of these hydrogen atoms as protons much less likely.

Observation (c): Formation of phosphonium salts

Phosphonium salts, such as PH4Cl, can be formed under anhydrous (non-aqueous) conditions but cannot be made in aqueous solutions. When phosphine (PH3) gas reacts with a hydrohalic acid (HX), for example HCl, a transfer of a proton from the acid to the lone pair of electrons on phosphorus occurs. This leads to the formation of phosphonium ion (PH4+). In anhydrous conditions, the resulting PH4+ ions can combine with chloride ions (Cl-) to form phosphonium chloride (PH4Cl) crystals. However, in an aqueous solution, these ions are solvated by water molecules and stay in the solution as separate hydrated ions, preventing the formation of a solid phosphonium salt.

Observation (d): Reactivity of white and red phosphorus

White phosphorus is more reactive than red phosphorus due to their differences in molecular structure and stability. White phosphorus consists of P4 tetrahedra, where each phosphorus atom is bonded to three other phosphorus atoms. This tetrahedral arrangement results in high angle strain, making white phosphorus highly reactive and prone to chemical reactions. Red phosphorus, on the other hand, has a more stable polymeric structure, where each phosphorus atom is connected to other phosphorus atoms forming chains or layers with more favorable bond angles. The more stable and lower energy structure of red phosphorus makes it less reactive than white phosphorus.

Key concepts

Chemical Bonding

Understanding chemical bonding is fundamental in explaining why certain elements can form different types of compounds. In the case of phosphorus forming a pentachloride (PCl₅), the ability of phosphorus to utilize its empty 3d orbitals allows it to form five covalent bonds with chlorine atoms. This is an example of expanding the octet beyond the typical eight electron limit, a concept tied to the availability of d orbitals in an element's valence shell.

Nitrogen, by contrast, lacks accessible d orbitals at an appropriate energy level to expand its octet, thus it's restricted to forming three covalent bonds, as seen in nitrogen trichloride (NCl₃). Phosphorus's ability to create more bonding interactions can be attributed to its position in the periodic table and the presence of extra orbitals available for bonding compared to nitrogen.

These observations outline the importance of atomic structure and electron configuration in determining an element's bonding capabilities and how those features affect the formation of different compounds.

Acid-Base Reactions

When studying acid-base reactions, the specific molecular structure of a molecule determines its behavior. H₃PO₂'s monoprotic nature is a great example. Being monoprotic indicates that H₃PO₂ can donate a single proton (H⁺) in a reaction with a base.

The molecular structure of H₃PO₂ has a crucial O-H bond with substantial polarity due to oxygen's electronegativity. This bond's polarity renders the hydrogen atom significantly more acquirable for donation as compared to hydrogen atoms attached to the less electronegative phosphorus atom.

This reflects a broader principle: the strength and number of acidic hydrogens in a molecule correspond to the polarity and number of hydrogen-oxygen bonds, among other factors. Hence, understanding chemical structure is pivotal in predicting acid strength and reactivity.

Molecular Structure

The crucial role of molecular structure in chemistry is clearly illustrated by comparing white and red phosphorus. White phosphorus consists of P₄ tetrahedra, creating a molecular entity with significant angle strain. This strain makes the molecules highly reactive.

On the other hand, red phosphorus exhibits a polymeric structure, lending it a network of stronger, more thermodynamically stable linkages. Angle strain is minimised and red phosphorus is less reactive.

Impact of Molecular Geometry

These differences are key in understanding reactivity. The tetrahedral shape of white phosphorus brings its atoms closer, increasing their energy and hence reactivity. Red phosphorus’s structure, conversely, allows for better distribution of electrons across the molecule, lowering the energy and reactivity. Therefore, the 3-dimensional arrangement of atoms within a molecule plays a crucial part in determining its chemical properties.

Reactivity of Elements

Investigating the reactivity of elements like phosphorus provides insight into how different allotropes, or forms of an element, can behave distinctly in chemical reactions. White phosphorus's high reactivity is a liability; it's highly dangerous and reacts vigorously with air. In stark contrast, red phosphorus is much more laid back, reacting slowly and predictably.

The essence of this difference is fundamental chemistry—bonding, energy levels, and molecular stability are the understated heroes that dictate how an element or compound will respond to a given environment or reactant.

Environmental Impact on Reactivity

Factors such as temperature, pressure, and the presence of other substances can influence reactivity as well. A safe and stable environment can mitigate the inherent reactivity of phosphorus allotropes, with practical applications in safety measures, storage, and transport. Recognising these nuances allows chemists to manipulate chemical reactions for desired outcomes, turning the volatile nature of some elements into controlled and useful reactions.