The C-Cl bond distance in CH3Cl and CF3Cl are 1.78Å and 1.75Å respectively. Comment on this difference with the help of Bent's rule?

The C-F bonds are formed from sp3+x hybrids with slightly more p-character and less s-character than an sp3 hybrid and the hydrogens are bonded by sp3-x hybrids with slightly less p-character and slightly more s-character and in accordance with Bent’s rule.

Since F is more electronegative than Cl therefore it will occupy the orbitals with more p-character.The C-F bond thus becomes highly polar where C gets slightly positive charge and thereby the electronegativity difference between C and Cl increases leading to higher forces of attraction and hence bond-length decreases.