chemistry

1.
think about repulsion between non-bonding electrons.
H has no non-bonding electrons and C has less than 3 non-bonding electrons (in the outermost shell).
in contrast, each chlorine has 6 non-bonding electrons, and they tends to repel other electrons on another atom. this weakens the bonds.

on the other hand, thin about distance between bonding electrons and nuclei of chlorine atoms. the distance is large compared to C-H's, so attractive force is weaker for this bond.

or think in his way: Cl makes a bond with 3p-electrons, which are high in energy; C with 2p or sp(x)-hybridized orbital electrons, and H with 1s, which are lower in energy. the bonds formed by overlapping of low-energy orbitals are stronger and more stable, while bonds with high-energy orbitals are less stable and more prone to be broken.


2.
refer to above argument, especially attraction between two nuclei and the bonding electrons.