Important Points to Remember

There are two types of \(\mathrm{H}\)-boding (i) Intermolecular \(\mathrm{H}\)-bonding (association).

\(\mathrm{H}\)-bonding involving two or more molecules.

(ii) Intramolecular \(\mathrm{H}\)-bonding (chelation). \(\mathrm{H}\)-bonding taking place within single molecule.

Applications of Intermolecular H-Bonding

(i) Water: Water has the lowest molecular weight among the hydrides of group 16 elements yet it has the highest melting and boiling points.

(ii) Ice has less density than water

Amides associate and have higher melting and boiling points.

Applications of Intramolecular H-Bonding

(i) Salicylic acid is stronger acid than \(o\)-methoxy benzoic acid

(ii) Ethyl aceto acetate - It exists in two forms

Keto form

Enolic form is more volatile due to chelation

(iii) Maleic acid is stronger acid than fumaric acid

Maleate ion can be stabilised by Chelation because hydrogen and oxygen responsible for forming hydrogen bond are very near to each other. On the other hand fumerate ion cannot stablise by chelation because hydrogen and oxygen are on opposite sides to each other. Hence formation of fumerate ion does not take place.

(iv) Acid character of nitrophenols. It follows the following order \(p\) - nitrophenol \(>0\) - nitrophenol \(>m\) - nitrophenol Acid character of o - nitrophenol is suppressed by chelation

(v) o-nitrophenol is more volatile (b.pt \(214^{\circ} \mathrm{C}\) ) as compared to meta (b.pt \(290^{\circ} \mathrm{C}\) ) and para (b.pt \(279^{\circ} \mathrm{C}\) ). It is due to chelation

b) \(\Delta \mathrm{H}_{\mathrm{f}}=\Delta \mathrm{H}_{\mathrm{sub}}+\frac{1}{2} \mathrm{DH}_{\mathrm{d}}+\mathrm{I} \cdot \mathrm{E} .+\mathrm{E} \cdot \mathrm{A} \cdot+\mathrm{U}\)

where,

\(\left.\begin{array}{l}\Delta \mathrm{H}_{\text {sub }}=\text { Enthalpy of formation } \\ \Delta \mathrm{H}_{\text {sul }}=\text { Enthalpy of sublimation } \\ \Delta \mathrm{H}_{\mathrm{d}}=\text { Enthalpy of dissociation } \\ \text { I.E. = Ionization energy } \\ \text { E.A. = Electron affinity } \\ \mathrm{U}=\text { Lattice energy or Lattice enthalpy }\end{array}\right\}\) positive

In order to determine the number of \(p p-p p\) or \(p p-a p\) bonds, the following formula can be used. (i) Calculate the number of \(s\) bonds = number of surrounding atoms

(ii) \(\mathrm{p}\) bonds = number of oxygen atoms - number of negative charge

(iii) Lone pair \(=1 / 2\) (Valence electron - number of covalent bond)

(iv) Hybridization \(=s\) bonds + number of lone pairs

(v) Now, number of \(p p-p p\) bonds = number of unhybridized \(p\)-orbitals left

(vi) \(p p-d\) p bonds will be formed when \(p\) bonds are more than the number of unhybridized \(p\)-orbitals left.

The more the electronegativity of atom involved in \(\mathrm{H}\)-bonding, the more is the bond strength eg. \(\mathrm{HLF} \quad>\mathrm{HLO} \quad>\mathrm{HLN}\)

\(10 \mathrm{kcal} / \mathrm{mole} \quad>7 \mathrm{kcal} / \mathrm{mole} \quad>2.0 \mathrm{kcal} / \mathrm{mole}\)