Question

The strength of HF is stronger than H2O. But still HF is gas at room temperature whiles H2O is liquid at room temperature. Explain the concept

Answer 1

The HF molecule forms only 2 hydrogen bonds per molecule due to high electronegativity of fluorine atom which does not share its lone pair of electrons which can potentially be used to form more hydrogen bonds.

The H₂O molecules forms 4 hydrogen bonds per molecule because both lone pair of electrons on oxygen molecule is available for bonding as electronegativity of oxygen is less than fluorine and oxygen readily shares its non-bonding electrons.

Due to this increased number of hydrogen bonds in H₂O, the hydrogen bonding is more extensive and the H₂O compound exists as liquid at room temperature as molecules experience higher attractive forces than HF which exists as gas at room temperature.

Also energy required to break 4 hydrogen bonds in H₂O is higher than energy required to break 2 hydrogen bonds in HF. Due to this reason HF gains energy quickly to break hydrogen bonds and convert from liquid to gas phase whereas H₂O is unable to do so due to high energy requirements and exist as liquid.