Which one of the following hydride of 17th group is highest p^K_a value

Down the group as the size of the atom increases affinity of hydrogen towards central atom decreases. Therefore

a) X-H bond enthalpy decreases

b) Thermal stability decreases

c) Reducing power increases

d) acidic strength (K_a) increases

e) p^K_a decreases….p^K_a value is positive for HF but negative for other hydrides.

Down the group as vanderwaal's forces increases

Melting and boiling points increases

Expected BP and MP….HF<HCl<HBr<HI

Due to H-bonding in HF, it has higher BP and MP than expected.

BP trend…...HCl<HBr<HI<HF

MP trend….HCl<HBr<HF<HI