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 \(\left(\mathrm{C}_{2} \mathrm{H}_{2}, \mathrm{C}_{2} \mathrm{H}_{4},\right.\) and \(\left.\mathrm{C}_{2} \mathrm{H}_{6}\right),\) whereas silicon forms only one analogous compound \(\left(\mathrm{Si}_{2} \mathrm{H}_{6}\right)\).

Short answer

The difference in the chemical properties and bonding in the given observations can be explained as follows: (a) HNO3 is a stronger oxidizing agent than H3PO4 due to the higher oxidation state and electronegativity of nitrogen compared to phosphorus, making it more likely to accept electrons. (b) Silicon can form SiF6^2- with six fluorine atoms because of its access to 3d orbitals, allowing for an expanded octet, while carbon is limited to a maximum of four bonded atoms in CF4 due to its available 2s and 2p orbitals. (c) Carbon can form three various compounds with hydrogen (C2H2, C2H4, C2H6) due to its ability to undergo multiple types of hybridization and form single, double, and triple bonds, whereas silicon forms only one analogous compound with hydrogen (Si2H6) as it does not readily form multiple bonds.

Step-by-step solution

Observation (a): HNO3 is a stronger oxidizing agent than H3PO4.

The difference in oxidizing strength between HNO3 and H3PO4 can be attributed to the difference in their structures and the electronegativities of nitrogen and phosphorus. In HNO3, nitrogen has a higher oxidation state (+5) while in H3PO4, phosphorus has a lower oxidation state (+3). In addition, nitrogen is more electronegative than phosphorus. The combined effect of a higher oxidation state and higher electronegativity makes nitrogen more likely to accept electrons, making HNO3 a stronger oxidizing agent compared to H3PO4.

Observation (b): SiF6^2- versus CF4

Silicon can bond to six fluorine atoms to form SiF6^2- while carbon can bond to a maximum of four fluorine atoms to form CF4. This difference is due to the fact that silicon, in the third period, has access to its 3d orbitals, which allows for the formation of an expanded octet. This expanded octet enables silicon to accommodate more than four bonded atoms. Carbon, on the other hand, is in the second period and only has 2s and 2p orbitals available for bonding, which limits it to a maximum of four bonded atoms.

Observation (c): Compounds of carbon and silicon with hydrogen

Carbon forms three different compounds with hydrogen containing two carbon atoms each (C2H2, C2H4, C2H6), while silicon forms only one analogous compound (Si2H6). The reason behind this difference lies in the multiple types of hybridization that carbon can undergo due to its small size and specific combination of valence electrons, which allows it to form single, double, and triple bonds with other carbon atoms. Silicon, on the other hand, is larger and less electronegative and tends to not form multiple bonds readily. As a result, it can only form one analogous compound with hydrogen, in which silicon atoms are single-bonded to each other.

Key concepts

Oxidizing Agents

An oxidizing agent, or oxidant, is a substance that has the ability to accept electrons from another species, which causes the oxidation of that species. In simpler terms, oxidizing agents gain electrons and get reduced in chemical reactions.

Understanding why \(\mathrm{HNO}_{3}\) is a stronger oxidizing agent than \(\mathrm{H}_{3} \mathrm{PO}_{4}\) relies on the concept of electronegativity and oxidation states. Nitrogen in \(\mathrm{HNO}_{3}\) has a high oxidation state of +5, which makes it more eager to accept electrons. Furthermore, nitrogen is more electronegative than phosphorus, thus it pulls electrons more strongly. These characteristics combined make \(\mathrm{HNO}_{3}\) a potent oxidizing agent. Phosphorus in \(\mathrm{H}_{3} \mathrm{PO}_{4}\) has a lower oxidation state of +3 and is less electronegative, diminishing its ability to oxidize other substances effectively.

Electronegativity

Electronegativity refers to the ability of an atom to attract and hold onto electrons in a chemical bond. This is an important factor in determining the behavior and strength of chemical bonds.

In the context of \(\mathrm{HNO}_{3}\) and \(\mathrm{H}_{3} \mathrm{PO}_{4}\), nitrogen's higher electronegativity compared to phosphorus plays a crucial role. Electronegativity influences how atoms share or transfer electrons, and in turn, affects the oxidizing abilities of molecules. Because nitrogen is more electronegative, it can attract electrons more powerfully than phosphorus, leading to its stronger oxidizing power in \(\mathrm{HNO}_{3}\).

Electronegativity differences are also fundamental when considering bond polarity and reaction tendencies of substances in various chemical processes.

Hybridization

Hybridization is a concept that describes the mixing of atomic orbitals to form new hybrid orbitals. These hybrid orbitals are involved in the bonding atoms within a molecule, influencing the geometry and the type of bonds that can form.

Carbon is well known for its versatile bonding capability through different types of hybridization. For example:

• sp hybridization allows for triple bonds as seen in \(\mathrm{C}_{2} \mathrm{H}_{2}\).
• sp\(^2\) hybridization supports double bonds as in \(\mathrm{C}_{2} \mathrm{H}_{4}\).
• sp\(^3\) hybridization leads to single bonds seen in \(\mathrm{C}_{2} \mathrm{H}_{6}\).

Silicon, however, is larger and less electronegative, limiting its ability to hybridize and form multiple bonds. Consequently, silicon tends to form single bonds, as observed in \(\mathrm{Si}_{2} \mathrm{H}_{6}\). The nature and extent of hybridization directly affect the diversity of compounds an element can form.

Expanded Octets

The concept of expanded octets revolves around certain elements being able to have more than eight electrons in their valence shell. This is possible mainly for elements that have d-orbitals available for bonding, often seen in elements from the third period and beyond, like silicon.

Silicon's ability to form \(\mathrm{SiF}_{6}^{2-}\) demonstrates expanded octet use, as it utilizes empty 3d orbitals to accommodate more than four bonding pairs of electrons. Carbon, a second-period element, cannot use d-orbitals, which makes its bonding capacity strictly limited to an octet, hence forming only \(\mathrm{CF}_{4}\).

Expanded octets allow for the formation of complex molecules that otherwise wouldn't be possible, influencing the chemistry of elements substantially. This capacity enables silicon and similar elements to form bonds and compounds with a larger set of atoms than is possible for elements without accessible d-orbitals.