# What intermolecular forces occur in water, and how are they manifested?

May 16, 2017

$\text{Hydrogen bonding}$, and $\text{dispersion forces............}$

#### Explanation:

Dispersion forces (transient polarization of electron clouds) exist between ALL molecules, but it is not the dominant intermolecular force in this instance.

The normal boiling point of $H F$ is $19.5$ ""^@C; that of $O {H}_{2}$ is $100$ ""^@C. These values are absurdly HIGH for such small molecules, and points to a special type of intermolecular interaction, that of hydrogen bonding, which occurs when hydrogen is bound to a strongly electronegative element, such as fluorine or oxygen. Hydrogen bonding thus decreases the volatility of each material.

The heteroatom strongly polarizes electron density towards itself to give a dipole that we could represent as ""^(-delta)OH_2^(delta+) or ""^(-delta)F-H^(delta+). In the condensed phase the dipoles line up appropriately, and this phenomenon is described as $\text{hydrogen bonding.}$ Ammonia, $N {H}_{3}$, also exhibits some degree of hydrogen bonding. It is the dominant intermolecular force.

Now compare the boiling points of $H F$ and ${H}_{2} O$ with the lower Group hydrides, ${H}_{2} S$, and $H C l$, or $P {H}_{3}$, where the dipole-dipole is not so strong due to the reduced electronegativity of the heteroatom. What do you find, and how does you account for it?