A, Q, Y and D are 17th group elements of Modern periodic table. The correct trend of melting point of their hydrides is HA > HQ > HY > HD. The element ‘Q’ in the sequence is
Down the group as the size of the atom increases affinity of hydrogen towards central atom decreases. Therefore
a) X-H bond enthalpy decreases….HF>HCl>HBr>HI ;
b) Thermal stability decreases…..HF>HCl>HBr>HI ;
c) Reducing power increases…..HF<HCl<HBr<HI ;
d) Acidic strength (Ka) increases…..HF<HCl<HBr<HI ;
e) pKa decreases…..HF>HCl>HBr>HI ;
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.
B.P …...HCl<HBr<HI<HF ;
Volatility…..HCl>HBr>HI>HF ;
M.P….HCl<HBr<HF<HI