Problem 83
Question
Write the Lewis structures of \(\mathrm{H}_{2} \mathrm{CNH}\) and \(\mathrm{H}_{3} \mathrm{CNH}_{2}\). Predict which molecule has the greater \(\mathrm{C}-\mathrm{N}\) bond energy.
Step-by-Step Solution
Verified Answer
\( \mathrm{H}_2 \mathrm{CNH} \) has the greater \( \mathrm{C}-\mathrm{N} \) bond energy due to its triple bond.
1Step 1: Determine Valence Electrons
To write the Lewis structures, we first need to calculate the total number of valence electrons in each molecule. For \( \mathrm{H}_2 \mathrm{CNH} \): 2 hydrogen atoms each with 1 electron, 1 carbon with 4, 1 nitrogen with 5, and a total of 2 electrons for hydrogen (\(2 \times 1 + 4 + 5 + 1 = 12\)). For \( \mathrm{H}_3 \mathrm{CNH}_2 \): 5 hydrogen atoms (\(5 \times 1 = 5\)), 1 carbon with 4, 1 nitrogen with 5, totaling 14 electrons (\(5 + 4 + 5 = 14\)).
2Step 2: Assign Connectivity
Establish the connectivity of atoms. \( \mathrm{H}_2 \mathrm{CNH} \) has a linear arrangement such that \( H - C \equiv N - H \). \( \mathrm{H}_3 \mathrm{CNH}_2 \) typically has \( H \) around \( C \), forming \( H_3C - NH_2 \), a standard arrangement in amines.
3Step 3: Draw Initial Structures
For \( \mathrm{H}_2 \mathrm{CNH} \): Start with single bonds \( H - C - N - H \) and add multiple bonds to satisfy valence electrons.For \( \mathrm{H}_3 \mathrm{CNH}_2 \): Begin with \( H_3C - NH_2 \) showing single bonds.
4Step 4: Complete Octets
Ensure each atom satisfies its octet (or duet for H):- \( \mathrm{H}_2 \mathrm{CNH} \): Carbon and nitrogen complete their octets by forming a triple bond, \( H - C \equiv N - H \). Each hydrogen forms a stable single bond. Count electrons to confirm total is 12.- \( \mathrm{H}_3 \mathrm{CNH}_2 \): The octets for \( \mathrm{C} \) and \( \mathrm{N} \) are completed by single bonds, affirm total number is 14 electrons.
5Step 5: Compare Bond Energies
In molecules, triple bonds (\( \mathrm{C} \equiv \mathrm{N} \)) have higher bond energies compared to single bonds (\( \mathrm{C}-\mathrm{N} \)). Hence, \( \mathrm{H}_2 \mathrm{CNH} \) with a triple bond should have a greater \( \mathrm{C}-\mathrm{N} \) bond energy compared to the single bond in \( \mathrm{H}_3 \mathrm{CNH}_2 \).
Key Concepts
Valence ElectronsC-N Bond EnergyOctet Rule
Valence Electrons
Valence electrons are the outermost electrons of an atom and they play a crucial role in chemical bonding. In the context of Lewis structures for molecules like \( \text{H}_2 \text{CNH} \) and \( \text{H}_3 \text{CNH}_2 \), we aim to account for the total number of valence electrons present in each molecule.
Understanding the count of valence electrons helps determine how atoms bond and organize themselves within a molecule, affecting its structure and properties.
Let's break down the counting process:
For \( \text{H}_3 \text{CNH}_2 \), the 5 hydrogens contribute 5 electrons, carbon contributes 4, and nitrogen adds 5 more, leading to a total of 14 valence electrons
By accounting for these valence electrons, we can accurately create the Lewis structure that visually represents the bonding and lone electron pairs.
Understanding the count of valence electrons helps determine how atoms bond and organize themselves within a molecule, affecting its structure and properties.
Let's break down the counting process:
- Hydrogen (H) has 1 valence electron.
- Carbon (C) has 4 valence electrons.
- Nitrogen (N) has 5 valence electrons.
For \( \text{H}_3 \text{CNH}_2 \), the 5 hydrogens contribute 5 electrons, carbon contributes 4, and nitrogen adds 5 more, leading to a total of 14 valence electrons
By accounting for these valence electrons, we can accurately create the Lewis structure that visually represents the bonding and lone electron pairs.
C-N Bond Energy
Bond energy refers to the energy required to break a bond between two atoms. A high C-N bond energy suggests a stronger and more stable bond.
In the Lewis structures for \( \text{H}_2 \text{CNH} \), there is a triple bond between carbon and nitrogen, whereas in \( \text{H}_3 \text{CNH}_2 \), there is a single bond.
Triple bonds, like \( \text{C} \equiv \text{N} \), involve more electron sharing than single bonds and are thus stronger, possessing higher bond energies. This means breaking a triple bond requires more energy compared to a single bond.
This concept explains why \( \text{H}_2 \text{CNH} \) has a greater \( \text{C}-\text{N} \) bond energy than \( \text{H}_3 \text{CNH}_2 \).
Bonds are crucial in determining the chemical stability and reactivity of a compound, with triple bonds rendering \( \text{H}_2 \text{CNH} \) more stable in this aspect compared to its counterpart with a single bond.
In the Lewis structures for \( \text{H}_2 \text{CNH} \), there is a triple bond between carbon and nitrogen, whereas in \( \text{H}_3 \text{CNH}_2 \), there is a single bond.
Triple bonds, like \( \text{C} \equiv \text{N} \), involve more electron sharing than single bonds and are thus stronger, possessing higher bond energies. This means breaking a triple bond requires more energy compared to a single bond.
This concept explains why \( \text{H}_2 \text{CNH} \) has a greater \( \text{C}-\text{N} \) bond energy than \( \text{H}_3 \text{CNH}_2 \).
Bonds are crucial in determining the chemical stability and reactivity of a compound, with triple bonds rendering \( \text{H}_2 \text{CNH} \) more stable in this aspect compared to its counterpart with a single bond.
Octet Rule
The octet rule is a chemistry principle that states that atoms tend to bond in such a way that each atom has eight electrons in its valence shell, resembling the electron configuration of a noble gas.
In the practice of constructing Lewis structures, it is important to apply this rule to ensure stability of the molecule.
For \( \text{H}_2 \text{CNH} \), carbon and nitrogen achieve their octets through a series of bonds, specifically the triple bond between them, while hydrogen achieves the 'duet' rule due to its capacity to hold only 2 electrons.
Meanwhile, in \( \text{H}_3 \text{CNH}_2 \), carbon and nitrogen complete their octets through single bonds, and the multiple hydrogen atoms each satisfy their duet rule.
This focus on fulfilling the octet or duet rule is a fundamental step when drafting Lewis structures. It not only solidifies the electron distribution but also enhances our understanding of molecular behavior and reactivity.
In the practice of constructing Lewis structures, it is important to apply this rule to ensure stability of the molecule.
For \( \text{H}_2 \text{CNH} \), carbon and nitrogen achieve their octets through a series of bonds, specifically the triple bond between them, while hydrogen achieves the 'duet' rule due to its capacity to hold only 2 electrons.
Meanwhile, in \( \text{H}_3 \text{CNH}_2 \), carbon and nitrogen complete their octets through single bonds, and the multiple hydrogen atoms each satisfy their duet rule.
This focus on fulfilling the octet or duet rule is a fundamental step when drafting Lewis structures. It not only solidifies the electron distribution but also enhances our understanding of molecular behavior and reactivity.
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