Hydrogen bonding  

​​In a nutshell

Hydrogen bonding is the strongest type of intermolecular force. Hydrogen bonding forms when hydrogen is covalently bonded to either fluorine, nitrogen or oxygen. 

Hydrogen bonding 

​​Definition 

Hydrogen bonding is formed from hydrogen atoms being covalently bonded to either fluorine, nitrogen or oxygen. It is known as the strongest type of intermolecular force. 

How does hydrogen bonding work?

Electrons are drawn away from a hydrogen atom by the strongly electronegative atoms fluorine, nitrogen or oxygen. The bond becomes strongly polarised, leaving weak bonds between the hydrogen atom with a high charge density and the lone pair of electrons on the electronegative atom. 

Example 

Water molecules will have hydrogen bonding between them.    

The lone pairs of electrons on the oxygen atom are attracted to the hydrogen atoms in other surrounding water molecules, forming hydrogen bonds. 

Note: Organic substances, such as amines, alcohols and carboxylic acids will form hydrogen bonds between molecules. 

Boiling points 

Lots of energy is needed to overcome the strong hydrogen bonds that are formed, this makes boiling points high. 

Group 7 hydrides 

Hydrogen bonds form between $$HF$$ molecules, so more energy is required to break these strong forces leaving a higher boiling point. The rest of the Group $$7$$​ hydrides will not experience hydrogen bonding.

An increase in the number of electrons between $$HCl$$ and $$HI$$ results in more London forces, requiring more energy to be broken and an increase in boiling point. This effect will override the decreasing permanent dipole-permanent dipole interactions between these Group $$7$$​ hydrides. 

These trends can be seen on the graph below.  

Group 6 hydrides  

Water ($$H_2O$$) will have the highest boiling point as it contains hydrogen bonding.  

​An increase in London forces between $$H_2S$$​ and $$H_2Te$$​ results in an increase in boiling point. This effect will override the decreasing permanent dipole-permanent dipole interactions between these Group ​$$6$$​ hydrides. ​

These trends can be seen on the graph below. 

Properties of water

The $$H_2O$$ molecules in ice are arranged to experience the maximum number of hydrogen bonds, forming a lattice structure. As the ice melts to form water, this lattice starts to break down as the hydrogen bonds are broken. This explains why ice is less dense than water and is able to float. 

Alcohols 

Alcohols contain $$OH$$ groups that can form hydrogen bonds.   

Alcohols have higher boiling points compared to other non-polar organic structures, such as alkanes. Therefore, alcohols have a lower volatility compared to non-polar organic structures.