Question

Why is hydrogen fluoride more stable and has highest bond dissociation enthalpy then HCL HBR and HI.

Answer 1

the greater difference in electronegativity mean that the F would pull the electron density more strongly and make it more likely for the proton to "leave". Thus,Electronegativity plays a role, but there is an even bigger effect taking place here. As you go down a group, the shell number (n) increases, meaning that the outer valence electrons (the ones to make that bond with hydrogen) are further and further away from the nucleus. The bigger the element, the less "tightly" it can hold onto the hydrogen. For going across the periodic table, the trend does go that the more electronegative the atom, the stronger the acid, but that is because across a row, the size of the elements is hardly changing (they have the same n).

And hence HF has highest bond dissociation enthalpy among the four.