A,B,X and E are 16th group elements of Modern periodic table. The correct trend of bond enthalpy is A-H > B-H > X-H > E-H. The element ‘B’ 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….O-H>S-H>Se-H>Te-H;
b) Thermal stability decreases….H2O>H2S>H2Se>H2Te;
H2O and H2S are exothermic products, remaining three hydrides are endothermic products.
c) Reducing power increases…..H2O<H2S<H2Se<H2Te;
d) Acidic strength (Ka) increases…..H2O<H2S<H2Se<H2Te;
e) pKa decreases…..H2O>H2S>H2Se>H2Te;
Down the group as vanderwaal's forces increases
Melting and boiling points increases
Expected BP and MP….H2O<H2S<H2Se<H2Te
Due to H-bonding in H2O, it has higher BP and MP than expected.
BP trend…...H2S<H2Se<H2Te<H2O
MP trend….H2S<H2Se<H2Te<H2O
Central atom in these hydrides contain two lone pairs, according to Bent rules
As the Electronegativity of central atom decreases,
Bond angle decreases…..H2O>H2S>H2Se>H2Te;