What trends in boiling points are generally observed among homologous series?

What trends in boiling points are generally observed among homologous series?

• A. Boiling points decrease with increasing molecular weight
• B. Boiling points remain unchanged across the series
• C. Boiling points increase with increasing molecular weight
• D. Boiling points are unaffected by structural differences

Answer: (C)

Among homologous series, it is observed that boiling points generally increase with increasing molecular weight. This trend can be understood through the concepts of intermolecular forces and molecular structure. As molecular weight increases, the size of the molecule also increases, leading to greater surface area and more significant dispersion forces (also known as London dispersion forces). These forces arise from temporary dipoles that occur due to the movement of electrons within the molecule. Larger, heavier molecules typically have more electrons, which enhances their polarizability—the tendency of their electron cloud to distort in response to nearby electric fields. As a result, the increased molecular weight translates to stronger dispersion forces, which require more energy (in the form of heat) to overcome during the phase change from liquid to gas. Consequently, the boiling point increases. In contrast, the other options do not align with the observed trends in boiling points for homologous series. For example, the notion that boiling points remain unchanged or are unaffected by structural differences does not account for variations in intermolecular forces or the impact of branching and functional groups on boiling points. Understanding these trends is essential in organic chemistry when predicting the physical properties of compounds based on their molecular characteristics.

Among homologous series, it is observed that boiling points generally increase with increasing molecular weight. This trend can be understood through the concepts of intermolecular forces and molecular structure. As molecular weight increases, the size of the molecule also increases, leading to greater surface area and more significant dispersion forces (also known as London dispersion forces). These forces arise from temporary dipoles that occur due to the movement of electrons within the molecule.

Larger, heavier molecules typically have more electrons, which enhances their polarizability—the tendency of their electron cloud to distort in response to nearby electric fields. As a result, the increased molecular weight translates to stronger dispersion forces, which require more energy (in the form of heat) to overcome during the phase change from liquid to gas. Consequently, the boiling point increases.

In contrast, the other options do not align with the observed trends in boiling points for homologous series. For example, the notion that boiling points remain unchanged or are unaffected by structural differences does not account for variations in intermolecular forces or the impact of branching and functional groups on boiling points. Understanding these trends is essential in organic chemistry when predicting the physical properties of compounds based on their molecular characteristics.