Question

Arrange the following in order of property indicated for each set:
a. $F_2,C{l}_2,B{r}_2,I_2$ - increasing bond dissociation enthalpy.
b. $HF,HCl,HBr,HI$ - increasing acid strength.
c. $N{H}_3,P{H}_3,As{H}_3,Sb{H}_3,Bi{H}_3$ - increasing base strength.

Answer 1

(i) $I_2<F_2<B{r}_2<C{l}_2$ The electron affinity of Chlorine is maximum in the periodic table and so the bond dissociation enthalpy. Flourine has lower bond dissociation enthalpy than $B{r}_2$ and $C{l}_2$. Due to small size electronic repulsion is very high. $I_2$ has lowest bond dissociation enthalpy due to quite larger size it is easiest to break the bond.

(ii) $HF<HCl<HBr<HI\Rightarrow$Acidic strength of Halogen acids increases down the group.

(iii) $Bi{H}_3<Sb{H}_3<As{H}_3<P{H}_3<N{H}_3\Rightarrow$ Basic strength decreases down the group because as we go down the group the size of the atom and thus the electron density decreases. These compounds are lewis base having 1 lone pair on the central atoms. As the electron density decrease the tendency to donate electron pair decreases.