Ammonia (\(\text{NH}_3\)) is a simple compound consisting of one nitrogen atom bonded to three hydrogen atoms. Considering its small size, one might expect it to have an extremely low boiling point, similar to other light gases. However, ammonia is an exception, possessing a surprisingly high boiling point compared to molecules of similar molecular weight. This anomalous behavior results from powerful, specific forces acting between its molecules.
What is the Boiling Point of Ammonia
The temperature at which liquid ammonia transitions into a gas is approximately \(-33.34^\circ\text{C}\) (or \(-28.0^\circ\text{F}\)) at standard atmospheric pressure. This value is considered high given its low molecular weight of about \(17.031 \text{ g/mol}\).
Methane (\(\text{CH}_4\)) is a molecule of similar mass, approximately \(16.04 \text{ g/mol}\), yet its boiling point is drastically lower, at about \(-161.5^\circ\text{C}\). The significant \(128^\circ\text{C}\) difference highlights that an unusually strong attractive force must be at work in liquid ammonia.
The Role of Intermolecular Forces
The boiling point of any substance measures the energy required to overcome the attractive forces between its molecules. These forces, known as intermolecular forces (IMFs), must be broken for molecules to enter the gaseous phase. For small molecules, the primary attractive force is typically the London Dispersion Force (LDF).
LDFs arise from temporary electron distributions and relate directly to a molecule’s size and number of electrons. Since ammonia is light and small, its LDFs should be weak, suggesting a very low boiling point. Ammonia is a polar molecule, experiencing dipole-dipole interactions where one end has a partial positive charge and the other a partial negative charge.
While dipole-dipole forces are stronger than LDFs, these interactions are generally insufficient to explain ammonia’s relatively high boiling point. Based on LDF and standard dipole-dipole forces alone, ammonia should boil at a much colder temperature. The explanation for this anomaly lies in a specific, powerful version of the dipole-dipole attraction.
Hydrogen Bonding: The Key Factor
The force responsible for ammonia’s high boiling point is hydrogen bonding, a specialized and powerful intermolecular attraction. This bond forms when a hydrogen atom bonded to a highly electronegative atom (such as nitrogen, oxygen, or fluorine) is strongly attracted to a neighboring electronegative atom. In ammonia, the highly electronegative nitrogen atom pulls electrons away from the hydrogen atoms, leaving them with a significant partial positive charge.
These partially positive hydrogen atoms are strongly attracted to the lone pair of electrons on the nitrogen atom of a nearby \(\text{NH}_3\) molecule. This continuous chain of attraction requires substantial energy to break, elevating the boiling point significantly. This interaction is demonstrated by comparing ammonia to phosphine (\(\text{PH}_3\)), which sits directly below nitrogen on the periodic table.
Phosphine has a molecular weight of nearly \(34 \text{ g/mol}\), almost double that of ammonia, suggesting stronger LDFs. However, phosphorus is not electronegative enough to form effective hydrogen bonds, resulting in phosphine’s much lower boiling point of \(-88^\circ\text{C}\). The fact that the smaller, lighter ammonia boils \(55^\circ\text{C}\) higher confirms that hydrogen bonding is the dominant factor.
Practical Applications of Ammonia’s Boiling Point
Ammonia’s boiling point and thermodynamic properties are responsible for one of its most important industrial uses: refrigeration. Ammonia is a highly effective refrigerant (R-717), especially in large-scale commercial and industrial cooling systems. Its boiling characteristics allow it to function efficiently in the vapor-compression refrigeration cycle.
Ammonia has a high latent heat of vaporization, meaning it absorbs significant heat energy as it transitions from a liquid to a gas. This property allows a small amount of liquid ammonia to achieve substantial cooling. Its boiling point of \(-33.34^\circ\text{C}\) is low enough for industrial cooling, yet high enough that the system can operate at lower pressures than required by refrigerants with extremely low boiling points. The ease of cycling between liquid and gas states makes it an energy-efficient cooling agent.