Question

Although water and ammonia differ in molar mass by only one unit, the boiling point of water is over 100 " \({ }^{\circ} \mathrm{C}\) higher than that of ammonia. What forces in liquid water that do \(n o t\) exist in liquid ammonia could account for this observation?

Short answer

The difference in boiling points between water and ammonia can be attributed to the stronger intermolecular forces in water, primarily due to stronger hydrogen bonding and greater dipole-dipole interaction. The higher electronegativity of oxygen in water allows it to form stronger hydrogen bonds with hydrogen atoms in neighboring water molecules, creating a more interconnected network of molecules and increasing the boiling point.

Step-by-step solution

Understand boiling points

The boiling point of a substance is influenced by the strength of the forces present between individual molecules (intermolecular forces). The stronger these forces, the higher the boiling point because more energy is required to break the bonds and change the substance from a liquid into a gaseous state. Water boiling point: \(100^{\circ}\mathrm{C}\) Ammonia boiling point: \(-33^{\circ}\mathrm{C}\)

Identify molecular structure

Water (H₂O) has a bent molecular structure with an oxygen atom in the center bonded to two hydrogen atoms. Ammonia (NH₃) has a pyramidal molecular structure with a nitrogen atom at the apex and three hydrogen atoms bonded at the base. Therefore, the molecular structures of water and ammonia are different.

Intermolecular forces in water

There are several types of intermolecular forces that can influence the boiling point. In the case of water, these forces include: 1. Hydrogen bonding: Due to its unique molecular structure, water can form strong hydrogen bonds between its molecules. The oxygen atom has two lone electron pairs, allowing it to form two hydrogen bonds. This results in a strong network of interconnected water molecules. 2. Dipole-dipole interaction: Water is a polar molecule due to the significant electronegativity difference between oxygen and hydrogen, resulting in a dipole-dipole interaction between the molecules.

Intermolecular forces in ammonia

In the case of ammonia, these forces include: 1. Hydrogen bonding: Like water, ammonia can also form hydrogen bonds between its molecules, but it is weaker due to the lower electronegativity of nitrogen compared to oxygen. 2. Dipole-dipole interaction: Ammonia is also a polar molecule due to the electronegativity difference between nitrogen and hydrogen. However, the polarity is weaker than that of water.

Compare the intermolecular forces

Comparing the forces present in water and ammonia, we can see that the hydrogen bonding and dipole-dipole interaction are stronger in water. The stronger hydrogen bonding in water is primarily due to the higher electronegativity of oxygen, allowing it to form stronger hydrogen bonds with hydrogen atoms in neighboring water molecules. This creates a more interconnected network of molecules, increasing the boiling point.

Conclusion

The significant difference in boiling points between water and ammonia can be attributed to the stronger intermolecular forces present in water, primarily due to stronger hydrogen bonding and greater dipole-dipole interaction. These stronger forces result in a higher amount of energy required to break the bonds between water molecules, causing the boiling point of water to be much higher than that of ammonia.

Key concepts

Hydrogen Bonding

Hydrogen bonding is a special type of intermolecular force that occurs when a hydrogen atom, bound to a highly electronegative atom like oxygen or nitrogen, gets attracted to another electronegative atom with lone pairs.
This bonding is one of the strongest types of intermolecular interactions and plays a vital role in determining boiling points.
In water, each molecule can form up to four hydrogen bonds, thanks to the two lone pairs on oxygen and the two hydrogen atoms. This leads to a highly interconnected and stable structure, requiring more energy to break apart when water boils.

• Water molecules form strong networks due to hydrogen bonding.
• Oxygen has higher electronegativity compared to nitrogen, resulting in stronger bonds in water.

In contrast, ammonia can form hydrogen bonds, but they are weaker because nitrogen is less electronegative than oxygen, limiting the strength and stability of the network that ammonia can form. This results in weaker overall attractions in liquid ammonia, contributing to its lower boiling point.

Molecular Structure

The molecular structure of a substance significantly affects how it interacts with other molecules, especially regarding intermolecular forces.
Water has a bent shape due to the two lone pairs of electrons on the central oxygen atom, which repel the hydrogen atoms downwards. This polar shape allows water molecules to form stronger hydrogen bonds and dipole-dipole interactions.

• The bent molecular geometry of water enhances its polarity.
• Polarity leads to strong dipole-dipole interactions, which are important for intermolecular attractions.

On the other hand, ammonia has a pyramidal shape because of the lone pair on nitrogen pushing the hydrogen atoms into a triangular base form. Although ammonia is also polar and participates in similar interactions, its geometry and weaker polarity result in less extensive hydrogen bonding compared to water. These structural differences explain why water's intermolecular forces are stronger, leading to a higher boiling point.

Boiling Point

The boiling point of a substance is the temperature at which its molecules have enough energy to overcome intermolecular forces and transition from liquid to gas.
Substances with strong intermolecular forces will generally have higher boiling points because more heat energy is required to overcome these forces.
Water, with its extensive hydrogen bonding and strong dipole interactions, necessitates a significant amount of thermal energy to reach its boiling point of 100°C.

• Water's high boiling point is due to strong hydrogen bonding.
• Strong intermolecular forces mean greater energy needed to break liquid bonds to reach gas state.

Ammonia, despite being able to form hydrogen bonds, has a much lower boiling point of -33°C. This is because its intermolecular forces are weaker, primarily due to less electronegativity in the nitrogen and a less polar structure compared to water. This means less energy is required to disrupt these bonds, allowing ammonia to boil at a much lower temperature.