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 not exist in liquid ammonia could account for this observation?

Short answer

The difference in boiling points between water and ammonia is due to the stronger hydrogen bonding present in water compared to ammonia. Water has a stronger hydrogen bond because oxygen is more electronegative than nitrogen, resulting in a stronger attractive force between water molecules. This stronger attractive force requires more energy to separate water molecules during evaporation, leading to a higher boiling point (\(100^{\circ}\mathrm{C}\)) compared to ammonia (\(-33^{\circ}\mathrm{C}\)).

Step-by-step solution

Identify the molecular structure and polarity of water and ammonia

First, let's look at the molecular structure and polarity of both water and ammonia. Water (H2O) has a bent molecular structure with a bond angle of approximately \(104.5^\circ\), while ammonia (NH3) has a trigonal pyramidal structure with a bond angle of approximately \(107^\circ\). Both molecules are polar due to the differences in electronegativity between hydrogen and oxygen/nitrogen atoms.

Determine intermolecular forces in water

In water, there are two main types of intermolecular forces at play: 1. Hydrogen bonding: The highly polar O-H bonds in water molecules form strong hydrogen bonds with other water molecules. This strong attraction between water molecules results in a high boiling point as it takes a significant amount of energy to break these hydrogen bonds. 2. Dipole-dipole interactions: As water is a polar molecule, it has regions with partial positive charges and regions with partial negative charges. These regions interact with the oppositely charged regions on adjacent water molecules, creating additional forces between the water molecules.

Determine intermolecular forces in ammonia

In ammonia, the main types of intermolecular forces present are: 1. Hydrogen bonding: Like water, ammonia can also form hydrogen bonds, but these bonds are weaker than those in water because nitrogen is less electronegative than oxygen. 2. Dipole-dipole interactions: Similar to water, ammonia is a polar molecule, and its partial charges interact with neighboring ammonia molecules, leading to dipole-dipole interactions.

Compare the forces in water and ammonia

Both water and ammonia have hydrogen bonding and dipole-dipole interactions, but the strength of these forces differs significantly between the two substances. The hydrogen bonding in water is much stronger than the hydrogen bonding in ammonia due to the greater electronegativity of oxygen compared to nitrogen. This results in a more considerable intermolecular force between water molecules compared to ammonia molecules.

Conclude by relating the forces to the boiling point difference

The stronger hydrogen bonding in water leads to a stronger attractive force between water molecules compared to ammonia molecules. Consequently, it takes more energy to separate water molecules and cause the liquid to evaporate. This energy requirement is reflected in the much higher boiling point of water (\(100^\circ \mathrm{C}\)) compared to ammonia (\(-33^\circ \mathrm{C}\)). Hence, the difference in the strength of hydrogen bonding between water and ammonia molecules accounts for the observation of the higher boiling point in water compared to ammonia.

Key concepts

Molecular Structure and Polarity

Understanding the molecular structure and its associated polarity is pivotal when analyzing substances like water (H₂O) and ammonia (NH₃). Molecules are composed of atoms linked by chemical bonds, which can lead to uneven distribution of electrical charge if the atoms have different electronegativities. For example, the oxygen atom in water is more electronegative than the hydrogen atoms, creating a polar molecule with a partial negative charge on the oxygen side and partial positive charges on the hydrogen sides.

Similarly, in ammonia, the nitrogen atom is more electronegative than hydrogen atoms. However, the polarity of ammonia is less than that of water due to nitrogen's lower electronegativity than oxygen. This distinction in polarity affects the molecule's behavior and interaction with other molecules. Remember, the shape or geometry of the molecule also affects polarity. Water's bent shape and ammonia's trigonal pyramidal structure are crucial in understanding their differing polarities and subsequent intermolecular interactions.

Hydrogen Bonding

Hydrogen bonding is an essential intermolecular force that plays a significant role in determining the physical properties of compounds like water and ammonia. It occurs when a hydrogen atom, covalently bonded to a highly electronegative atom (such as oxygen in water or nitrogen in ammonia), experiences an attraction to another electronegative atom with a lone pair of electrons.

The strength of these hydrogen bonds is directly related to the electronegativity of the atoms involved; thus, in the case of water, the hydrogen bonds are particularly strong due to oxygen's high electronegativity. In contrast, the hydrogen bonds in ammonia are somewhat weaker due to the lower electronegativity of nitrogen. These varying strengths of hydrogen bonds can dramatically influence properties such as boiling points, which is why water has a much higher boiling point than ammonia despite their similar molar masses.

Dipole-Dipole Interactions

Dipole-dipole interactions occur between polar molecules, where the positive end of one molecule is attracted to the negative end of another. Both water and ammonia exhibit these forces due to their molecular polarity. A molecule like water, with its strong dipole moment caused by the large difference in electronegativity between oxygen and hydrogen, will experience stronger dipole-dipole interactions than ammonia. This is because the nitrogen to hydrogen electronegativity difference is smaller, making its dipole moment and consequently, its dipole-dipole interactions, weaker. These forces, while not as strong as hydrogen bonds, are nonetheless significant contributors to the overall intermolecular forces holding a liquid together.

Boiling Point Comparison

The boiling point of a substance is an indicator of the energy required to overcome its intermolecular forces and turn it from a liquid into a gas. For polar molecules like water and ammonia, both hydrogen bonding and dipole-dipole interactions are influential, but hydrogen bonding is usually the deciding factor. By comparing the boiling points — water at 100°C and ammonia at -33°C — we can deduce that the intermolecular forces in water are significantly stronger than in ammonia.

This difference is mainly attributed to the stronger hydrogen bonds in water due to the higher electronegativity of oxygen. A greater amount of heat is needed to break these interactions in water, thereby leading to a higher boiling point. This demonstrates how the type and strength of intermolecular forces, influenced by molecular structure and polarity, are critical factors in determining the thermal properties of a substance.