Question

Explain the following observations: (a) ${\mathrm{HNO}}_3$ is a stronger oxidizing agent than ${\mathrm{H}}_3{\mathrm{PO}}_4.$ (b) Silicon can form an ion with six fluorine atoms, ${\mathrm{SiF}}_6^{2-}$, whereas carbon is able to bond to a maximum of four, ${\mathrm{CF}}_4$ (c) There are three compounds formed by carbon and hydrogen that contain two carbon atoms each $({\mathrm{C}}_2{\mathrm{H}}_2,{\mathrm{C}}_2{\mathrm{H}}_4$, and ${\mathrm{C}}_2{\mathrm{H}}_6)$, whereas sili- con forms only one analogous compound $\left({\mathrm{Si}}_2{\mathrm{H}}_6\right)$.

Short answer

In summary, (a) HNO3 is a stronger oxidizing agent than H3PO4 because nitrogen has a higher electronegativity, which allows it to attract and withdraw electron density more effectively. (b) Silicon can form SiF62- because of its larger atomic radius and the availability of empty d orbitals, whereas carbon's smaller size and lack of empty d orbitals limit it to forming CF4. (c) Carbon can form more compounds with hydrogen than silicon due to its catenation property, which allows it to form extended chains with other carbon atoms, while silicon prefers to form multiple Si-H bonds instead of forming a longer Si-Si chain, resulting in only one stable compound, Si2H6.

Step-by-step solution

(a) Explaining the oxidizing power of HNO3 and H3PO4

To determine which species is a stronger oxidizing agent, we need to compare their electron-accepting abilities. HNO3 has a more electronegative central atom (N, electronegativity 3.04) compared to H3PO4 (P, electronegativity 2.19). This difference in electronegativity implies that nitrogen in HNO3 has a stronger ability to attract and withdraw electron density from the surrounding atoms, making it a more potent oxidizing agent than H3PO4.

(b) Comparing Silicon's and Carbon's ability to bond with fluorine

Carbon and silicon, both belong to Group 14 of the periodic table, meaning they have similar bonding behaviors. However, silicon has a larger atomic radius compared to carbon. This difference allows silicon to accommodate more fluorine atoms due to its larger size and the availability of empty d orbitals in its outer shell. As carbon has neither the size nor the empty d orbitals, it can form a maximum of four bonds with fluorine atoms, resulting in the compound CF4. In contrast, Si can form an ion with six fluorine atoms, SiF62-.

(c) Comparing the number of compounds formed by Carbon and Silicon with hydrogen

The difference in the number of compounds formed between carbon and silicon with hydrogen is due to the variation in their bonding characteristics. Carbon exhibits a property called catenation, which allows it to form extended chains with other carbon atoms easily, leading to the formation of numerous compounds (C2H2 - acetylene, C2H4 - ethylene, C2H6 - ethane). On the other hand, silicon can also form compounds with hydrogen, but the Si-Si bond is weaker than the C-C bond, and Si-H bond is stronger than the Si-Si bond. Thus, silicon prefers to form multiple Si-H bonds instead of forming a longer Si-Si chain. The only stable silicon hydride compound with two silicon atoms is Si2H6 (disilane).

Key concepts

Oxidizing Agents

In the realm of chemistry, oxidizing agents are substances that have the ability to accept electrons from other species. This process can lead to oxidation, where the oxidizing agent is reduced. One fundamental characteristic of an effective oxidizing agent is its electronegativity.

• Highly electronegative atoms, such as nitrogen in ${\mathrm{HNO}}_3$, improve its electron-attracting characteristics.
• In comparison, phosphorus in ${\mathrm{H}}_3{\mathrm{PO}}_4$ is less electronegative, thereby reducing its oxidizing capability.

This difference explains why ${\mathrm{HNO}}_3$ acts as a stronger oxidizing agent than ${\mathrm{H}}_3{\mathrm{PO}}_4$. The ability to draw electrons more effectively allows ${\mathrm{HNO}}_3$ to facilitate reactions where it is reduced, showcasing its greater oxidizing potential.

Bonding Capabilities of Silicon and Carbon

Silicon and carbon, though in the same group of the periodic table, exhibit different bonding capacities. This divergence arises from their varied atomic structures.
One major factor is the atomic size:

• Silicon, with a larger atomic radius, can form bonds with more atoms due to the additional space.
• This is exemplified by its ability to bond with six fluorine atoms in ${\mathrm{SiF}}_6^{2-}$.

In contrast, carbon, with a smaller atomic size, is limited to four bonds seen in ${\mathrm{CF}}_4$. Furthermore, silicon’s outer shell has access to vacant d orbitals, which facilitates the formation of additional bonds. Carbon, lacking these orbitals, is restricted in its bonding capacity.

Catenation in Carbon

Catenation refers to the unique ability of an element to form bonds with itself, creating long chains or structures. Carbon is renowned for this property due to the stability of the $\mathrm{C}-\mathrm{C}$ bond.

• This enables carbon to form robust and varied compounds like ${\mathrm{C}}_2{\mathrm{H}}_2$ (acetylene), ${\mathrm{C}}_2{\mathrm{H}}_4$ (ethylene), and ${\mathrm{C}}_2{\mathrm{H}}_6$ (ethane).

Silicon, on the other hand, has weaker $\mathrm{Si}-\mathrm{Si}$ bonds compared to $\mathrm{C}-\mathrm{C}$ bonds, limiting its ability to engage in catenation to the same extent as carbon. While silicon can still form chains, its bonds are not as stable, resulting in fewer silicone-based compounds with extended structures.

Silicon Hydrides

Silicon hydrides are compounds formed between silicon and hydrogen. These structures are similar to hydrocarbons but are generally less stable. One primary reason is the interference in bonding due to silicon's large atomic size and the varying strengths of $\mathrm{Si}-\mathrm{H}$ and $\mathrm{Si}-\mathrm{Si}$ bonds.

• The $\mathrm{Si}-\mathrm{Si}$ bond is relatively weak, making long chains less favorable.
• In contrast, the $\mathrm{Si}-\mathrm{H}$ bond is stronger, promoting the formation of structures with more silicon-hydrogen interactions.

Hence, disilane (${\mathrm{Si}}_2{\mathrm{H}}_6$) emerges as the primary stable silicon hydride with two silicon atoms, showcasing the preference for $\mathrm{Si}-\mathrm{H}$ over $\mathrm{Si}-\mathrm{Si}$ bonds.

Electronegativity Comparison

Electronegativity is a fundamental concept in chemistry, indicating an atom's tendency to attract electrons within a bond. In the comparison of nitrogen and phosphorus, nitrogen holds a higher electronegativity.

• This difference results in nitrogen being more effective at pulling electron density towards itself, which is pivotal in reactions involving electron transfer.

In molecules capable of oxidation-reduction, like ${\mathrm{HNO}}_3$ versus ${\mathrm{H}}_3{\mathrm{PO}}_4$, the variance in electronegativity influences their role as oxidizing agents. Consequently, in ${\mathrm{HNO}}_3$, the electron-withdrawing capacity of nitrogen facilitates its function as a stronger oxidizing agent compared to ${\mathrm{H}}_3{\mathrm{PO}}_4$, where the less electronegative phosphorus is present.