Question

The bond energy for the$\mathit{C}\mathbf{-}\mathit{H}\mathbf{}$bond is about $\mathbf{413}\mathbf{}\mathit{k}\mathit{J}\mathbf{/}\mathit{m}\mathit{o}\mathit{l}$in CH₄ but $\mathbf{380}\mathbf{}\mathit{k}\mathit{J}\mathbf{/}\mathit{m}\mathit{o}\mathit{l}\mathbf{}$ in $\mathit{C}\mathit{H}\mathit{B}{\mathit{r}}_{\mathbf{3}}$. Although these values are relatively close in magnitude, they are different. Explain why they are different. Does the fact that the $\mathit{C}\mathbf{-}\mathit{H}\mathbf{}$bond energy in $\mathit{C}\mathit{H}\mathit{B}{\mathit{r}}_{\mathbf{3}}$is lower make any sense? Why?

Short answer

Answer

The$\mathrm{C}-\mathrm{H}$bond in bromoform is unstable due to the electronegativity of Br. Thesigma bond electron density is minimized. So, the$\mathrm{C}-\mathrm{H}$bond breaks.

Step-by-step solution

Describing theC-H bond energy in the methane molecule

The energy needed in a bond formation varies irregularly.The process of bond formation takes place in a clear way.

Process Energy required

$C{H}_{\mathit{4}\mathit{\left(}\mathit{g}\mathit{\right)}}\mathit{\to }C{H}_{\mathit{3}\mathit{\left(}\mathit{g}\mathit{\right)}}\mathit{+}{H}_{\left(g\right)}$ 435

$C{H}_{\mathit{3}\mathit{\left(}g\mathit{\right)}}\mathit{\to }C{H}_{\mathit{2}\mathit{\left(}g\mathit{\right)}}\mathit{+}{H}_{\left(g\right)}$ 453

$C{H}_{\mathit{2}\mathit{\left(}g\mathit{\right)}}\mathit{\to }C{H}_{\mathit{\left(}g\mathit{\right)}}\mathit{+}{H}_{\left(g\right)}$ 425

$C{H}_{\mathit{\left(}g\mathit{\right)}}\mathit{\to }C{H}_{\mathit{\left(}g\mathit{\right)}}\mathit{+}{H}_{\left(g\right)}$ 339

Total = 1652kJ/mol

Average = 1652 / 4 = 413

The average has been taken to calculate the$\mathit{}C\mathit{-}H\mathit{}$bond energy.

Describing the C-H  bond energy

The average of bond energy is used as a better method even though we assume the approximation of the energy correlation with the$\mathit{C}\mathbf{-}\mathit{H}$bond.

The energy needed to break the $C\mathit{-}H$ bond in methane =413 kJ/mol

The energy needed to break the $C\mathit{-}H$bond in methane =380kJ/mol

The degree of sensitivity of the bondcan be calculated by the experimental value of the energy required for the $C\mathit{-}H$bond.

The stronger bond requires a larger amount of energy. Thus, methane has a stronger bond while bromoform does not.