The nitrogen atoms in N2 participate in multiple bonding, whereas those in hydrazine, N2H4, do not.
(a) Draw Lewis structures for both molecules.
(b)What is the hybridization of the nitrogen atoms in each molecule?
(c)Which molecule has the stronger N-N bond?
The correct answer and explanation is :
Answer:
(a) Lewis Structures
- Nitrogen (N₂):
The Lewis structure of (N_2) consists of a triple bond between two nitrogen atoms, with each nitrogen having a lone pair.
Structure:
:N≡N: - Hydrazine (N₂H₄):
The Lewis structure of hydrazine features a single bond between the two nitrogen atoms, with each nitrogen bonded to two hydrogen atoms and possessing a lone pair.
Structure:
H-N-N-H
| |
H H
(b) Hybridization of Nitrogen Atoms
- N₂: Each nitrogen in (N_2) forms a triple bond consisting of one sigma (σ) bond and two pi (π) bonds. Since sigma bonding involves hybrid orbitals, the nitrogen atoms in (N_2) are sp-hybridized.
- N₂H₄: In hydrazine, each nitrogen forms single bonds with another nitrogen and two hydrogen atoms. It also has a lone pair, meaning its bonding arrangement is tetrahedral. This corresponds to sp³ hybridization.
(c) Bond Strength
- N₂ (Triple Bond): The triple bond in (N_2) consists of one sigma and two pi bonds, making it extremely strong and short. The bond dissociation energy is around 945 kJ/mol, making it one of the strongest known bonds.
- N₂H₄ (Single Bond): The nitrogen-nitrogen bond in hydrazine is a single sigma bond, which is much weaker than a triple bond. Its bond dissociation energy is only about 159 kJ/mol.
Explanation (300 words)
The difference in bonding between nitrogen molecules ((N_2)) and hydrazine ((N_2H_4)) is due to the number of shared electron pairs and the type of hybridization involved. In nitrogen gas ((N_2)), each nitrogen atom has five valence electrons. To complete their octets, the two nitrogen atoms form a triple bond, sharing six electrons. This results in a very short and strong bond with a bond length of about 109 pm. The presence of two pi bonds in (N_2) contributes significantly to the bond strength.
On the other hand, in hydrazine ((N_2H_4)), each nitrogen forms only a single bond with the other nitrogen, while also bonding to two hydrogen atoms. The nitrogen atoms are sp³ hybridized, leading to a bond angle close to 109.5°. The N-N single bond is much weaker than the triple bond in (N_2) because it consists of only one sigma bond with no pi bonding contribution.
The difference in hybridization also affects the bond strength. In (N_2), the sp hybridization results in stronger, more directional bonds due to greater s-character. In contrast, the sp³ hybridization in (N_2H_4) leads to a longer, weaker bond because the bond has more p-character.
Thus, (N_2) has the stronger N-N bond due to its triple bond, while (N_2H_4) has a much weaker single bond due to electron repulsion from the lone pairs and reduced bond order.