Question

Answer the following questions:
Arrange the following in the order of property indicated against each set:
a. ${\text{HF, HCl,}}\,{\text{HBr, HI}}$ - increasing bond dissociation enthalpy.
b. ${{\text{H}}_2}{\text{O, }}{{\text{H}}_2}{\text{S,}}\,{{\text{H}}_2}{\text{Se, }}{{\text{H}}_2}{\text{Te}}$ - increasing acidic character.

Answer 1

Hint: As the stability of the bond increases the bond dissociation enthalpy increases. More number of bonds more will be its dissociation enthalpy. The stability of the bond decreases, the acidic character increases.

Complete answer:
(a) The bond dissociation enthalpy depends upon the stability of the bond. On increasing the stability of the bond, bond dissociation enthalpy increases.
Stability of the bond depends upon the electronegativity of the atoms as follows:
The decreasing order of electronegative of the halogens is as follows:
${\text{F}}\,{\text{ > }}\,{\text{Cl}}\,{\text{ > }}\,{\text{Br}}\,{\text{ > }}\,{\text{I}}$
Fluorine has the highest electronegativity so, the electrons of the hydrogen-fluorine bond are more attracted to the fluorine atom so, fluorine gets a partial negative charge and hydrogen gets a partial positive charge. So, the hydrogen-fluorine bond is more polar than other hydrogen-halogen bonds.
Due to the small size and high electronegativity of the fluorine, the hydrogen-fluorine bond is most stable, so it requires high enthalpy for the dissociation of the bond.
So, the increasing order of bond dissociation enthalpy is as follows:
${\text{HI}}\, < \,{\text{H}}\,{\text{Br}}\,{\text{ < }}\,{\text{HCl < }}\,{\text{HF}}$
(b) The acidity of binary acid depends upon the stability of the bonds.
Stability of the bond depends upon the size of the atoms as follows:
The increasing order of the size of the oxygen family is as follows:
${\text{O}}\,{\text{ < }}\,{\text{S}}\,{\text{ < }}\,{\text{Se}}\,{\text{ < }}\,{\text{Te}}$
The acidic character depends upon the size of the atom. On the increasing size of the atom, bond length increases so, bond becomes weak so, easily breaks. So, acidity increases.
So, the order of increasing acidic character in oxides of oxygen family is as follows:${{\text{H}}_2}{\text{O}}\,{\text{ < }}\,{{\text{H}}_2}{\text{S}}\,{\text{ < }}\,{{\text{H}}_2}{\text{Se}}\,{\text{ < }}\,{{\text{H}}_2}{\text{Te}}$

Note: Down in a group, the electronegativity of the atom decreases. As the electronegativity of the atom decreases the size of the atom increases and the stability of the bond decreases. Fluorine is the most electronegative element in the periodic table.