Problem 36 - Chemistry The Molecular Science

Question

Phosphine, \(\mathrm{PH}_{3}\), is a hydride of phosphorus as ammonia, \(\mathrm{NH}_{3}\), is of nitrogen. Explain why phosphine boils at a much lower temperature than \(\mathrm{NH}_{3}\) in spite of having a molar mass twice that of ammonia.

Step-by-Step Solution

Answer

Phosphine boils at a lower temperature due to weaker van der Waals forces compared to ammonia's hydrogen bonds.

Step 1: Understanding Boiling Point Factors

The boiling point of a substance is influenced by the strength of its intermolecular forces. Stronger forces require more energy (heat) to break, leading to a higher boiling point.

Step 2: Compare the Intermolecular Forces

Ammonia (\(\mathrm{NH}_3\)) has hydrogen bonds due to the electronegative nitrogen and the presence of hydrogen atoms. Phosphine (\(\mathrm{PH}_3\)), however, primarily exhibits weaker van der Waals forces due to the lower electronegativity of phosphorus.

Step 3: Analyze Effect of Molecular Mass

While higher molecular mass often suggests stronger dispersion forces, it is not as significant as strong hydrogen bonding when determining boiling point. Thus, the lower mass of ammonia does not compensate for the strong hydrogen bonds present.

Step 4: Conclude Based on Intermolecular Forces

Since hydrogen bonds in ammonia are much stronger than the van der Waals interactions in phosphine, \(\mathrm{NH}_3\) has a higher boiling point despite \(\mathrm{PH}_3\) having a greater molar mass.

Key Concepts

Intermolecular Forces

Intermolecular forces are attractions between molecules, which play a crucial role in determining the physical properties of substances, like the boiling point. When a substance boils, the intermolecular forces must be overcome to change from a liquid to a gas.
There are several types of intermolecular forces, including:

• Hydrogen bonding
• Van der Waals forces
• Dipole-dipole interactions

Stronger intermolecular forces lead to higher boiling points, as more energy is required to separate the molecules. In the case of phosphine (\(\mathrm{PH}_3\)) and ammonia (\(\mathrm{NH}_3\)), their different intermolecular forces explain the variations in their boiling points.

Hydrogen Bonding

Hydrogen bonding is a special type of intermolecular force. It occurs when a hydrogen atom is bonded to a highly electronegative atom, such as nitrogen, oxygen, or fluorine. This creates a strong dipole, allowing the hydrogen to bond with an electronegative atom in a nearby molecule.
Ammonia (\(\mathrm{NH}_3\)) forms hydrogen bonds because the nitrogen atom is very electronegative, which makes the hydrogen atoms slightly positive. This interaction leads to a much higher boiling point for ammonia since these bonds are quite strong.
In comparison, phosphine (\(\mathrm{PH}_3\)) lacks this capability because phosphorus is not as electronegative as nitrogen, preventing the formation of hydrogen bonds. This results in weaker intermolecular attractions in phosphine.

Van der Waals Forces

Van der Waals forces are the weakest type of intermolecular attractions. They arise from temporary dipoles that occur when electron clouds move around an atom or molecule. These forces include:

• London dispersion forces
• Dipole-dipole interactions

In molecules like phosphine (\(\mathrm{PH}_3\)), van der Waals forces are the primary form of intermolecular interaction. These forces are generally weaker than hydrogen bonds, leading to lower boiling points.
Despite phosphine having a greater molar mass than ammonia, the presence of only van der Waals forces means it has a much lower boiling point compared to ammonia, which benefits from stronger hydrogen bonds.