Question

The bond energy for a \(\mathrm{C}-\mathrm{H}\) 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 C-H bond energy between CH4 and CHBr3 can be explained by the electronegativity of the surrounding atoms. Bromine atoms in CHBr3 have a higher electronegativity than hydrogen atoms in CH4, which weakens the C-H bond and results in a lower bond energy in CHBr3. This is consistent with the molecular structures and the effect of surrounding atoms on bond strength.

Step-by-step solution

Understanding bond energy

Bond energy is the amount of energy required to break a chemical bond between two atoms in a molecule. In this case, we are looking at the C-H bond in CH4 and CHBr3.

Comparing the molecular structures of CH4 and CHBr3

Methane (CH4) is a simple hydrocarbon molecule with a carbon atom that is bonded to four hydrogen atoms. On the other hand, CHBr3 (bromoform) is a halogenated hydrocarbon, where the carbon atom is bonded to one hydrogen atom and three bromine atoms.

Understanding the role of the surrounding atoms in bond energy

The chemical environment has a significant impact on the bond strength. In CHBr3, the carbon atom is bonded to three highly electronegative bromine atoms, which pull electron density away from the carbon atom. This reduces the effective positive charge on the carbon atom, weakening the C-H bond. In CH4, the carbon atom is only bonded to hydrogen atoms, which have lower electronegativity than the bromine atoms, resulting in a stronger C-H bond.

Explaining the difference in bond energy

The difference in C-H bond energy between CH4 and CHBr3 can be explained by the electronegativity of the surrounding atoms. Bromine atoms in CHBr3 have a higher electronegativity than hydrogen atoms in CH4, which causes a decrease in the effective positive charge on the carbon atom in CHBr3. This results in a weaker C-H bond and hence lower bond energy in CHBr3 compared to CH4.

Understanding whether the lower bond energy in CHBr3 makes sense

The fact that the bond energy is lower in CHBr3 does make sense. It is consistent with the molecular structures and the effect of surrounding atoms on bond strength. In CHBr3, the high electronegativity of bromine atoms weakens the C-H bond compared to the bond in CH4, resulting in a lower bond energy.