Q9.85CP - Chemistry: Molecular Nature Of Matter And Change

Question

There are two main types of covalent bond breakage. In homolytic breakage, each atom in the bond gets one of the shared electrons. In some cases, the electronegativity of adjacent atoms affects the bond energy. In heterolytic breakage, one atom gets both electrons and the other gets none; thus, a cation and an anion form.

a) Why is C-C bond in ${\mathbf{H}}_3\mathbf{C}\mathbf{-}{\mathbf{CF}}_3$  (423kJ/mol) stronger than that in  ${\mathbf{H}}_3\mathbf{C}\mathbf{-}{\mathbf{CH}}_3$ 

    (376kJ/mol)?

b) use bond energy and any other data to calculate the heat of reaction for the heterolytic cleavage of ${\mathbf{O}}_2$ . 

Step-by-Step Solution

Answer

a) The bond between C-C in ${\mathbf{H}}_3\mathbf{C}\mathbf{-}{\mathbf{CF}}_3$  is ionic bond and in ${\mathbf{H}}_3\mathbf{C}\mathbf{-}{\mathbf{CH}}_3$  is covalent. Ionic   

     bond is much stronger than covalent bond so, the strength of C-C bond is more in  

     ${\mathbf{H}}_3\mathbf{C}\mathbf{-}{\mathbf{CF}}_3$ .

b) heat of reaction for heterolytic cleavage of ${\mathbf{O}}_2$  is 498kJ/mol.

Definition

The ability of an atom or functional group to attract electrons to itself is called electronegativity in the chemical property.

A polar bond is a type of covalent bond in which electrons are shared unequally.

comparison of electronegativity and polarity

The most electronegative atom is the fluorine atom. hence, it is clear that it is more electronegative than carbon and hydrogen which means fluorine has a greater tendency to attract electrons than the hydrogen atom.

 

Thus, in heterolytic cleavage of the C-F bond fluorine attracts electrons from carbon atoms because of its high electronegativity. But there is a small difference between carbon and hydrogen electronegativity so, homolytic cleavage occurs in the C-H bond. 

 

As the electronegativity difference is more in the C-F bond of ${\mathbf{H}}_3\mathbf{C}\mathbf{-}{\mathbf{CF}}_3$  than the C-H bond in ${\mathbf{H}}_3\mathbf{C}\mathbf{-}{\mathbf{CH}}_3$, the polarity of the C-F bond is much more than the C-H bond which affects strength of the molecular structure. Therefore, the C-C bond in ${\mathbf{H}}_3\mathbf{C}\mathbf{-}{\mathbf{CF}}_3$  becomes more polar than the C-C bond in  ${\mathbf{H}}_3\mathbf{C}\mathbf{-}{\mathbf{CH}}_3$ due to polarity and electronegativity.

Thus, in heterolytic cleavage ionic species are formed and bond between C-C in  ${\mathbf{H}}_3\mathbf{C}\mathbf{-}{\mathbf{CF}}_3$ is ionic bond. And the bond form between C-C in ${\mathbf{H}}_3\mathbf{C}\mathbf{-}{\mathbf{CH}}_3$   is covalent. Because an ionic bond involves a complete transfer of electrons, it is more stronger than a covalent bond.

Therefore, more energy is required to cleavage the C-C bond in  ${\mathbf{H}}_3\mathbf{C}\mathbf{-}{\mathbf{CF}}_3$ due to the more strength of the C-C bond.

heterolytic cleavage of O 2

The oxygen atom has six valence electrons in its valence shell so it forms two bonds to complete its octet. In  ${\mathbf{O}}_2$  molecule, two oxygen atoms are bonded with double bonds.

In heterolytic cleavage of  ${\mathbf{O}}_2$  , one oxygen atom attracts all sharing electrons towards its side and becomes an anion species and the second oxygen atom becomes a cation species. 

 

     $\mathbf{O}\mathbf{=}\mathbf{O}\to {\mathbf{O}}^{2+}\mathbf{+}{\mathbf{O}}^{2-}$           

 

The heat of the reaction of heterolytic cleavage of ${\mathit{O}}_{\mathbf{2}}$   is equal bond energy of O=O molecule. So, the heat of reaction  $\mathbf{\Delta H}\mathbf{=}\mathbf{498}\mathbf{kJ}\mathbf{/}\mathbf{mol}$.