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Intermolecular Forces

Review - 6

Fluorine has a larger electronegativity than oxygen, so the H-F bond is more polar than the H-O bond. This means that individual hydrogen bonds between H-F molecules are stronger than individual hydrogen bonds between H2O molecules. However the effects of hydrogen bonding on the properties of water (unexpectedly high normal boiling point, the low density of solid water compared to liquid water) are much more pronounced than for HF. Why is this so?

Hint

The overall strength of hydrogen bonding in a substance is determined by the strength of the individual bonds and the total number of bonds which can form between the molecules.

Answer

Lewis structures for HF and water

A single hydrogen bond forms between a hydrogen atom bonded to F, O or N and a lone pair of electrons on another F, O or N atom. A hydrogen atom or lone pair involved in one hydrogen bond cannot take part in another hydrogen bond. In HF each molecule has one hydrogen atom which can form a hydrogen bond, and there are three lone pairs of electrons on the fluorine atom. The total number of hydrogen bonds is limited by the number of hydrogen atoms and on average each HF molecule will be involved in two hydrogen bonds. In H2O each molecule has two hydrogen atoms and two lone pairs of electrons on the oxygen atom. This means that there can be many more hydrogen bonds formed between water molecules than between HF molecules. Each water molecule can potentially be involved in up to four hydroen bonds. In fact water has the optimum number of H atoms and lone pairs to form an extensive network of hydrogen bonds.

 

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