why does HF have a boiling point of 19°c and H2O 100°c
The difference in boiling points between hydrogen fluoride (HF) and water (H2O) can be attributed to the differences in their molecular structures and intermolecular forces.
Hydrogen fluoride (HF) is a small, polar molecule composed of one hydrogen atom and one fluorine atom. The polar nature of the HF molecule arises from the electronegativity difference between hydrogen and fluorine, causing a partial positive charge on hydrogen and a partial negative charge on fluorine. These partial charges create strong dipole-dipole forces between HF molecules. However, HF does not form extensive hydrogen bonding, which typically results in higher boiling points.
On the other hand, water (H2O) is a larger, bent-shaped molecule consisting of two hydrogen atoms and one oxygen atom. Oxygen is highly electronegative, giving rise to significant polarity in the molecule. This polarity leads to strong hydrogen bonding between adjacent water molecules. Hydrogen bonding is an exceptionally strong intermolecular force that results from the attraction between the oxygen atom of one water molecule and the hydrogen atom of a neighboring water molecule. These hydrogen bonds require more energy to break, which is why water has a higher boiling point compared to HF.
The boiling point of a substance is the temperature at which its vapor pressure is equal to the atmospheric pressure. For example, at sea level, water boils at 100°C because the average atmospheric pressure is about 1 atmosphere. In contrast, HF boils at a much lower temperature, approximately 19°C, due to the weaker intermolecular forces between HF molecules.
In summary, the difference in boiling points between HF and H2O can be attributed to the strength and extent of intermolecular forces. HF has lower boiling point due to weaker interactions between molecules, while water has a higher boiling point due to the presence of strong hydrogen bonding.