Question

In terms of intermolecular forces, why does the boiling point of a particular type of compound (for example, an alkane) increase as the molecular weight increases?

Short answer

The boiling point of a compound increases as the molecular weight increases because larger molecules with higher molecular weights have stronger intermolecular forces. As a result, it requires more energy to overcome these forces, which is manifested as a higher boiling point.

Step-by-step solution

Understand Molecular Weight Effect

Molecular weight is the sum of the atomic weights of the atoms in a molecule. Larger molecules tend to have higher molecular weights. In the case of alkanes (hydrocarbons), as their molecular weight increases, so does the size of the molecule.

Understand Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion which act between neighboring particles like atoms, ions, or molecules. They can include forces such as Van der Waals forces (which involve induced dipole-induced dipole attractions). The strength of these forces often grows with the size of the molecules involved

Link Molecular Weight and Intermolecular Forces with Boiling Point

It takes a certain amount of energy to overcome the intermolecular forces to convert a substance from a liquid to a gas (i.e., to reach boiling point). Larger molecules with more atoms, and hence higher molecular weights, have stronger intermolecular forces (more specifically, higher Van der Waals forces). Therefore, compounds with higher molecular weights need more energy to overcome these forces and transition into a gaseous state, resulting in a higher boiling point.

Key concepts

Molecular Weight

Molecular weight refers to the sum of the atomic weights of all the atoms within a molecule. It is an essential factor in determining the physical properties of substances. As molecules increase in size, so does their molecular weight. For example, in alkanes, which are hydrocarbons consisting of carbon and hydrogen only, molecular weight is directly related to the number of carbon atoms present.

Understanding molecular weight is crucial because:

• It helps predict how molecules will behave under different conditions.
• Larger molecules often take on different physical states due to their increased mass.
• Molecular weight affects the energy required for phase transitions, such as converting from a liquid to a gas.

The impact of molecular weight spans various scientific fields and is a key factor in determining many fundamental properties of materials.

Van der Waals Forces

Van der Waals forces are a type of intermolecular force that includes attractions like London dispersion forces. These forces occur due to temporary dipoles that result when electron clouds within molecules become unevenly distributed.

Important characteristics of Van der Waals forces include:

• These forces are generally weaker than other types of intermolecular forces such as hydrogen bonds, but they play a significant role in non-polar molecules like hydrocarbons.
• Van der Waals forces become more significant as molecular size (and thus molecular weight) increases, because larger molecules have more electrons and a greater surface area for interactions.
• They contribute substantially to the overall boiling point of a compound, as more energy is needed to overcome these forces for larger molecules.

Understanding these forces helps explain why molecules behave differently as their size and mass increase.

Boiling Point

The boiling point of a substance is the temperature at which it changes from a liquid to a gas. This physical property is significantly influenced by the strength of intermolecular forces keeping the molecules in a liquid state.

Factors affecting boiling point include:

• The molecular weight of the substance: As molecular weight increases, so does the boiling point generally, due to stronger Van der Waals forces.
• The type of intermolecular forces present: Stronger forces within a liquid require more energy to overcome, leading to a higher boiling point.
• Structural factors: The shape of the molecule can also influence boiling point.

In summary, the molecular interactions and size play a crucial role in determining a compound’s boiling point, with heavier and larger molecules resulting in higher boiling points.