chemistry 2.7 - hydrogen bonding

What are hydrogen bonds?

They are the strongest intermolecular force

When do hydrogen bonds occur (which elements) and why?

When hydrogen is covalently bonded with oxygen, nitrogen or fluorine as they are very electronegative

What happens in hydrogen bonds?

The bond is very polar and the hydrogen has a very high charge density that the hydrogen forms weak bonds with the lone electron pair with the oxygen, nitrogen or fluorine in other molecules

Why does hydrogen have a high charge density when covalently bonded to oxygen, nitrogen or fluorine?

Its small size means when the electrons are pulled away exposing its nucleus it has a very strong partial charge that is concentrated in a small volume giving it a high charge density

What are example of molecules that form hydrogen bonds?

Alcohols (-OH), amines (-NH) and water (H2O)

What can make molecules bonded why hydrogen bonds have a higher melting point

The higher the number of lone pair of the O, N or F the higher the melting points as more hydrogen bonds can be formed (e.g. in H2O the O has 2 lone pairs = to hydrogen bonds can be formed)

Why does HCl have a lower boiling point than HBr despite there being a bigger difference in electronegativity so stronger dipole - dipole interactions?

Because in Br there are more electrons than in Cl so the London forces are stronger

What is the structure of ice?

Simple molecular

Why does ice float in water?

It is less dense as when water freezes the molecules are arranged so the maximum number of hydrogen binds are firmed which wastes lots of space whereas when water melts some of the hydrogen bonds are broken are the lattice breaks down allowing molecules to fill the spaces

How does hydrogen bonding in alcohols affects their properties?

They have low volatility (high boiling bonding) compared to non-polar compounds (e.g. alkanes of similar size and number of electrons)