Question

The bond energy for a \(C-H\) bond is about 413 \(\mathrm{kJ} / \mathrm{mol}\) in \(\mathrm{CH}_{4}\) but 380 \(\mathrm{kJ} / \mathrm{mol}\) in \(\mathrm{CHBr}_{3}\) . Although these values are relatively close in magnitude, they are different. Explain why they are different. Does the fact that the bond energy is lower in \(\mathrm{CHBr}_{3}\) make any sense? Why?

Short answer

The difference in bond energy between the C-H bond in CH₄ and CHBr₃ can be attributed mainly to factors like electronegativity difference, size and shielding effect, and steric effect. In CHBr₃, the presence of larger, more electronegative bromine atoms nearby the C-H bond influences the bond strength by inducing electron cloud distortion, increasing the shielding effect, and causing steric repulsions. These factors contribute to a weaker C-H bond in CHBr₃ compared to the C-H bond in CH₄, hence the lower bond energy in CHBr₃ makes sense.

Step-by-step solution

Understanding bond energy

Bond energy is the amount of energy required to break a chemical bond between two atoms. It is affected by factors such as the type and size of atoms, the distance between them, and their neighboring atoms' influence.

Analyzing the molecular structure of CH₄

CH₄ (methane) is a simple, symmetrical molecule with four hydrogen atoms bonded to a central carbon atom. The four C-H bonds in CH₄ are all single bonds with equal bond lengths and bond strengths.

Analyzing the molecular structure of CHBr₃

CHBr₃ is called Bromoform, which contains one hydrogen atom and three bromine atoms bonded to a carbon atom. The molecule is more complex than CH₄, with different bond lengths and electronegativities between carbon and its bonded atoms.

Factors contributing to the difference in C-H bond energy

The difference in bond energy between the C-H bond in CH₄ and CHBr₃ can be attributed mainly to the following factors: 1. Electronegativity difference: In CH₄, the carbon and hydrogen atoms have a small difference in electronegativity, causing a bond polarity. In CHBr₃, the carbon-bromine bond is even more polar due to the larger electronegativity difference between the atoms, which induces electron cloud distortion in the C-H bond, decreasing its strength. 2. Size and shielding effect: The bromine atoms in CHBr₃ are much larger than the hydrogen atoms in CH₄. As a result, the electron cloud in the C-H bond can experience more shielding from the large electron cloud of the bromine atoms, reducing the bond strength. 3. Steric effect: Due to the bulky size of bromine atoms, the bond strength of the C-H bond may be affected by the repulsive forces between the electron clouds of the hydrogen and bromine atoms. So steric effect can also contribute to a lower bond energy in CHBr₃ than in CH₄.

Does the lower bond energy in CHBr₃ make sense?

Yes, the lower bond energy of the C-H bond in CHBr₃ does make sense because of the factors mentioned above. The presence of larger, more electronegative bromine atoms nearby the C-H bond influences the bond strength by inducing electron cloud distortion, increasing the shielding effect, and causing steric repulsions. All these factors contribute to a weaker C-H bond in CHBr₃ compared to the C-H bond in CH₄.

Key concepts

Electronegativity

Electronegativity is a crucial concept in chemistry, helping us understand why atoms share electrons unequally in a molecule. It refers to the ability of an atom to attract shared electrons towards itself. Each element has a characteristic electronegativity value that influences bond properties such as bond strength and bond polarity.
The bond energy differences we see in molecules such as methane \(CH sub{4}\) and bromoform \(CHBr sub{3}\) largely stem from differences in electronegativity between the bonded atoms. \In methane, both carbon and hydrogen have similar, relatively low electronegativity values, leading to a relatively nonpolar C-H bond.
Conversely, in bromoform, bromine has a much higher electronegativity than carbon. This results in a greater polarity and distortion of the electron cloud in the C-H bond. \This distortion makes the C-H bond weaker, lowering the bond energy.

• Methane: Carbon & hydrogen have similar, lower electronegativities
• Bromoform: Bromine has higher electronegativity, inducing polarity

Understanding how electronegativity affects bonds helps us predict reactivity and chemical behavior in various substances.

Molecular Structure

Molecular structure is fundamental to understanding the differences in bond energies in molecules. It involves the 3D arrangement of atoms in a molecule, which significantly affects physical and chemical properties.
In methane (CH₄), the molecule is tetrahedral, with four hydrogen atoms symmetrically arranged around a central carbon. This symmetry contributes to uniform bond energies because the C-H bonds are in similar environments with consistent distances.
In bromoform (CHBr₃), the molecule is more complex. Three bromine atoms and one hydrogen atom are bonded to a carbon atom. This results in different bond lengths and angles due to the varying sizes and electronegativities of the atoms involved.

• Methane has a symmetrical, tetrahedral structure
• Bromoform has an asymmetrical, complex structure

As seen, molecular structure plays a significant role, with asymmetry and differing atomic sizes creating variability in bond energies, making some bonds, like the C-H in CHBr₃, weaker.

Steric Effect

The steric effect is an important concept to understand when analyzing bond energies, particularly in crowded or complex molecules. It occurs due to the repulsion between electron clouds in atoms, often affecting bond strength and molecular stability.
In Bromoform (CHBr₃), the presence of large bromine atoms creates "steric hindrance." This term refers to the spatial arrangement preventing optimal overlap of electron clouds between carbon and hydrogen, resulting in weaker C-H bonds due to increased repulsive forces.
This steric hindrance reduces bond strength and increases bond lengths, resulting in lower energy required to break these bonds in CHBr₃ compared to methane.

• Bromoform experiences steric hindrance due to large bromine atoms
• The steric effect causes increased repulsion and weaker bonds

Understanding steric effects helps us appreciate how molecular shape and size influence chemical interactions and stability, key to explaining variations in bond energies.