Why is the carbon-fluorine bond stronger than the other haloalkanes?
From wikipedia article of carbon-fluorine bond,


  The high electronegativity of fluorine (4.0 for F vs. 2.5 for carbon)
  gives the carbon–fluorine bond a significant polarity/dipole moment.
  The electron density is concentrated around the fluorine, leaving the
  carbon relatively electron poor. This introduces ionic character to
  the bond through partial charges ($\ce{C^{δ+}—F^{δ−}}$). The partial
  charges on the fluorine and carbon are attractive, contributing to the
  unusual bond strength of the carbon–fluorine bond. The bond is
  labeled as "the strongest in organic chemistry," because fluorine
  forms the strongest single bond to carbon. Carbon–fluorine bonds can
  have a bond dissociation energy (BDE) of up to 544 kJ/mol. (This is why) The BDE
  (strength of the bond) is higher than other carbon–halogen and
  carbon–hydrogen bonds. For example, the molecule represented by
  $\ce{CH3X}$ has a BDE of 115 kcal/mol for carbon–fluorine while values
  of 104.9, 83.7, 72.1, and 57.6 kcal/mol represent $\ce{C–X}$ bonds
  to hydrogen, chlorine, bromine, and iodine, respectively.