State which of the following compounds contain intermolecular hydrogen bonds: (i) hydrogen chloride, HCl (ii) water, H2O, (iii) ammonia, NH3 - Leaving Cert Chemistry - Question Question 1 - 2010

Hydrogen bonds occur in molecules where hydrogen is bonded to a small, highly electronegative element such as oxygen (O), nitrogen (N), or fluorine (F).

In water (H2O), each hydrogen atom is bonded to oxygen, which is a highly electronegative atom, allowing for strong hydrogen bonding. Similarly, in ammonia (NH3), hydrogen is bonded to nitrogen, another electronegative element, leading to the presence of hydrogen bonding. Thus, both H2O and NH3 exhibit intermolecular hydrogen bonds.

The boiling point of ammonia (–33 °C) is significantly lower than that of water (100 °C) due to the following reasons:

1. Hydrogen bonding in water is stronger than in ammonia because oxygen is more electronegative than nitrogen. This results in stronger dipole interactions in water.

2. Water has a higher ratio of lone pairs to hydrogen atoms (2:2) compared to ammonia’s (1:3), which enhances its ability to form hydrogen bonds and leads to a higher boiling point.

3. The effective hydrogen bonding in water is greater; thus, the additional energy required to overcome these bonds when transitioning to a gas phase is more substantial for water than for ammonia.