differences are purely a function of the electronegativities of
F, O, and Cl, that is, the strength with which each atom pulls electrons
After combustion, the electrons that were shared equally between
carbon and hydrogen in methane are drawn toward the Cl atoms when
Cl2 is the combustion
medium, drawn more strongly toward O atoms, and most strongly of
all toward F when F2 is
the combustion medium.
The tighter the electrons are held after combustion, the more stable
the product molecules are, and the more energy is given off as heat.