Problem 14 - General Chemistry

Question

Why do molecular substances have relatively low melting points?

Step-by-Step Solution

Answer

Molecular substances have low melting points because they are held together by weak intermolecular forces, requiring less energy to overcome.

Step 1: Understanding Molecular Substances

Molecular substances are composed of distinct molecules held together by weak intermolecular forces, such as London dispersion forces, dipole-dipole interactions, or hydrogen bonds. These forces are much weaker compared to the intramolecular covalent bonds within each molecule.

Step 2: Analyzing Intermolecular Forces

The weak intermolecular forces in molecular substances mean that less energy is required to overcome these forces during phase changes, such as melting. Consequently, the energy needed to break these interactions is lower, resulting in lower melting points for these substances.

Step 3: Comparing to Other Types of Bonds

In comparison to ionic or covalent network solids which have strong ionic or covalent bonds respectively, molecular substances consist of weakly bonded molecules. This difference significantly impacts their melting points, making molecular substances melt at much lower temperatures.

Key Concepts

Intermolecular Forces

In molecular substances, intermolecular forces are the key players that influence their physical properties. These are weak forces of attraction between individual molecules. They include:

• London dispersion forces: These are temporary forces that result from momentary changes in electron density in a molecule. They are present in all molecules, whether polar or nonpolar, but are the only forces acting in noble gases and nonpolar compounds.
• Dipole-dipole interactions: Occur between polar molecules, where the positive end of one molecule is attracted to the negative end of another. These forces are stronger than London dispersion forces but weaker than hydrogen bonds.
• Hydrogen bonds: A special type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative atoms like nitrogen, oxygen, or fluorine. This bond is stronger than regular dipole-dipole forces.

Despite these differences, all these forces are still weaker than the covalent or ionic bonds found within molecules or crystalline solids. Because of this, molecular substances can break apart easily, leading to their characteristic low melting points.

Melting Points

Melting points are a crucial indicator of the strength of forces within and between molecules. In molecular substances, these points are typically low due to the relatively weak intermolecular forces holding molecules together. When a substance melts, energy is required to overcome these forces and allow molecules to slip past each other into a more fluid state.

In substances with strong internal structures, like ionic or covalent networks, high energy is needed to break apart the tightly held lattice of atoms or ions. Molecular substances, with their weak intermolecular bonds, require much less energy, translating into lower melting points. This makes them more likely to be in liquid form at room temperature, unlike their ionic or covalent lattice counterparts, which need higher temperatures to change phase.

Hydrogen Bonds

Hydrogen bonds are a fascinating and vital type of intermolecular force. These occur when a hydrogen atom, covalently bonded to a highly electronegative element such as nitrogen, oxygen, or fluorine, experiences an attraction to a lone pair of electrons on a neighboring electronegative atom. This creates a strong dipole-dipole interaction known as a hydrogen bond.

Hydrogen bonds are significant because they are considerably stronger than other intermolecular forces, like London dispersion or typical dipole-dipole interactions, but still not as strong as covalent or ionic bonds.

Their presence can raise the melting and boiling points of some substances. For example, water, which forms hydrogen bonds, has a relatively high melting point for a molecule of its size, due to the energy required to break these bonds. These bonds are crucial in biological processes and influence the properties of substances deeply, explaining why some molecular substances, like water and ammonia, have atypically higher boiling and melting points compared to others of a similar molecular weight.