First slide

Chemical properties

Question

Which one of the following hydride of 17th group has the least melting point

Easy

A

HI
B

HF
C

HCl
D

HBr

Solution

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….HF has positive p^K_^a value,others have negative p^K_^avalues.
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

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