Hydrogen bonding

hydrogen bond Formation of hydrogen bonds between HF molecules. Electrostatic attraction exists between partial positive charge of H atom and the lone pair electrons of F atom of another HF.

hydrogen bond Formation of hydrogen bonds between H2O molecules. Electrostatic attraction exists between partial positive charge of H atom and the lone pair electrons of O atom of another H2O.

hydrogen bond Formation of hydrogen bonds between NH3 molecules. Electrostatic attraction exists between partial positive charge of H atom and the lone pair electrons of N atom of another NH3.

Structure and bonding of ice The oxygen atom of each water molecule forms hydrogen bonds with two hydrogen atoms of nearby water molecules. a water molecule hydrogen bond hydrogen bond hydrogen atom oxygen atom

The central oxygen atom of each water molecule has a tetrahedral arrangement of two lone pairs (forming hydrogen bonds) and two bond pairs. 1 4 2 A water molecule can form hydrogen bonds with four other water molecules. 3

open structure collapses In this open structure, water molecules are further apart than they are in liquid water. ice liquid water melts open structure collapses water molecules tend to pack more closely together Think about

Molecular size of hydride molecules increases down a group  the VDW forces between molecules increases down a group

V

Besides van der Waal’s forces, there are hydrogen bonds between H2O molecules. However, In H2S, H2Se, H2Te , there are weak van der Waal’s forces between molecules only. Hydrogen bond is stronger than van der Waal’s forces A lot more energy is needed to break hydrogen bonds between molecules  The melting and boiling pt of water are much higher than expected.

Effects of intermolecular hydrogen bonding Association Higher melting and boiling points. Solubility Effects of intermolecular hydrogen bonding

Association: the molecule connected by intermolecular hydrogen bond associated with one another to form bigger units each of which contain two more units. Example :

Fig. 28.4 The structure of ice. hydrogen bond empty space (ball) hydrogen bond empty space a water molecule

Higher boiling points Increase intermolecular force of attraction requires to separate molecules, leads to higher melting and boiling point Example: The hydrogen atom of an ethanol molecule can form a hydrogen bond with the oxygen atom of another ethanol molecule. High boiling point

Ethanol CH3CH2OH Solubility: intermolecular hydrogen bonding b/w solute and solvent leads to higher solubility. hydrogen bond The hydrogen atom of an ethanol molecule can form a hydrogen bond with the oxygen atom of another water molecule. The hydrogen atom of an ethanol molecule can form a hydrogen bond with the oxygen atom of another ethanol molecule. High Solubility in water