Why does a larger, higher molecular weight hydrocarbon generally have a higher melting point?

A larger, higher molecular weight hydrocarbon generally exhibits a higher melting point primarily due to the increased number of intermolecular forces that are present in the substance. As the molecular weight of a hydrocarbon increases, the number of atoms and bonds within the molecule also increases, leading to a greater potential for interactions between the molecules, such as van der Waals forces.

These interactions become more significant with larger hydrocarbons, as they have more electrons and a larger electron cloud, which can lead to stronger dispersion forces. As a result, more thermal energy (or heat) is required to overcome these intermolecular forces in order to transition from a solid to a liquid state, which manifests as an increase in the melting point.

The connection between molecular weight, the number of interacting groups, and the melting point is a key principle in understanding the physical properties of organic compounds. Therefore, when considering the melting point of hydrocarbons, the relationship between molecular size, intermolecular forces, and the energy needed to break those forces is pivotal in explaining the observed trends in melting points.