Question

Which is expected to have the higher boiling point, \(\mathrm{C}_{8} \mathrm{H}_{18}\) or \(\mathrm{C}_{4} \mathrm{H}_{10}\) ? Explain your choice.

Short answer

has a longer carbon chain and a higher molecular mass than which generally leads to stronger London dispersion forces and a higher boiling point.

Step-by-step solution

Understand Molecular Size

Consider the molecular size of each compound. Larger molecules typically have stronger intermolecular forces, particularly London dispersion forces, due to a greater number of electrons, which helps to create temporary dipoles. carbon chain length results in stronger dispersion forces.

Compare Molecular Masses

Compare the molar masses of ceturates with higher molecular masses tend to have higher boiling points because they have stronger intermolecular forces.

Predict Boiling Point Based on Intermolecular Forces

Since has a longer carbon chain and a higher molecular mass than is expected to have the higher boiling point.

Key concepts

Intermolecular Forces

When comparing boiling points of different substances, one of the essential factors to consider is the intermolecular forces at play. These forces are the attractions that occur between molecules, and they play a significant role in determining a substance's physical properties, such as boiling point. There are several types of intermolecular forces, but the main ones include hydrogen bonding, dipole-dipole interactions, and London dispersion forces. The strength and type of these forces contribute to how much energy is required to change a substance's state, with stronger forces necessitating more energy. Thus, a higher boiling point indicates stronger intermolecular forces within a compound.

London Dispersion Forces

London dispersion forces are a type of intermolecular force that are present in all molecular substances. These are the weakest forces and result from the temporary dipoles that occur due to the movement of electrons within molecules. The strength of London dispersion forces is influenced by the number of electrons in a molecule—the more electrons, the stronger these forces can become. Importantly, these forces become significantly stronger with increased molecular size because larger molecules have a greater electron cloud that can easily distort and induce temporary dipoles in neighboring molecules.

Molecular Size

Molecular size, or the volume occupied by a molecule, can have a direct influence on the boiling point through its effect on London dispersion forces. As molecules become larger, they have a greater surface area in contact with neighboring molecules. It allows for more significant interactions between them, intensifying the London dispersion forces. Consequently, as the molecular size increases, so does the energy required to overcome these intermolecular attractions. Therefore, substances with larger molecules generally have higher boiling points than smaller molecules, assuming that the type of intermolecular forces remains constant.

Molar Mass

Molar mass, the mass of one mole of a substance, is intrinsically tied to both molecular size and the strength of dispersion forces. In general, when comparing molecules with similar structures and types of intermolecular forces, the molecule with the higher molar mass will have more electrons and a larger electron cloud. This translates to stronger London dispersion forces and thus a higher boiling point. However, it is important to recognize that molar mass is just one factor. The type of intermolecular forces present and the molecular structure can also greatly influence the boiling point, but among homologous series of compounds, molar mass is a reliable indicator for boiling point trends.