Question

Hydrogen bonds are formed in many compounds e.g.,

. The boiling point of such compounds depends to a large extent on the strength of hydrogen bond and the number of hydrogen bonds. The correct decreasing order of the boiling points of above compounds is

• A. $\mathrm { HE } > \mathrm { H } _ { 2 } \mathrm { O } > \mathrm { NH } _ { 3 }$
• B. $\mathrm { H } _ { 2 } \mathrm { O } > \mathrm { HF } > \mathrm { NH } _ { 3 }$
• C. $\mathrm { NH } _ { 3 } > \mathrm { HF } > \mathrm { H } _ { 2 } \mathrm { O }$
• D. $\mathrm { NH } _ { 3 } > \mathrm { H } _ { 2 } \mathrm { O } > \mathrm { HF }$

Answer 1

Answer: (B)

$\mathrm { H } _ { 2 } \mathrm { O } > \mathrm { HF } > \mathrm { NH } _ { 3 }$

Answer 2

: Strength of hydrogen bonding depends on the size and electroengativity of the atom. Smaller the size of the atom, greater is the electronegativity and hence stronger is the H-bonding. Thus the order of strength of H – bonding is H ….F>H…O > H…N. But each HF molecule is linked only to two other HF molecules while each $\mathrm { H } _ { 2 } \mathrm { O }$molecule is linked to four other molecules through H-bonding.

Hence, the decreasing order of boiling points is $\mathrm { H } _ { 2 } \mathrm { O } > \mathrm { HF } > \mathrm { NH } _ { 3 }$