Question

What is boiling point? How does it depend on intermolecular forces?

Short answer

Boiling point is the temperature at which a liquid turns into vapor. It depends on the strength of intermolecular forces; stronger forces lead to higher boiling points.

Step-by-step solution

- Define Boiling Point

Boiling point is the temperature at which a liquid's vapor pressure equals the external pressure surrounding the liquid, causing it to turn into vapor.

- Understand Intermolecular Forces

Intermolecular forces are the forces of attraction or repulsion which act between neighboring particles (atoms, molecules, or ions). Common types include hydrogen bonds, dipole-dipole interactions, and London dispersion forces.

- Relationship Between Boiling Point and Intermolecular Forces

The boiling point of a substance is influenced by the strength of its intermolecular forces. Stronger intermolecular forces require more energy (i.e., higher temperature) to break, thus increasing the boiling point. Conversely, weaker intermolecular forces require less energy to break, resulting in a lower boiling point.

Key concepts

intermolecular forces

Intermolecular forces are the attractive or repulsive interactions between neighboring particles such as atoms, molecules, or ions. They govern many physical properties of substances, including their boiling points.

• Types of Intermolecular Forces: There are several types of intermolecular forces, including hydrogen bonds, dipole-dipole interactions, and London dispersion forces.
• Strength: The strength of these forces varies. For instance, hydrogen bonds are generally stronger compared to London dispersion forces.
• Role in Boiling Point: Stronger intermolecular forces mean higher boiling points because more energy is needed to break these forces and convert the liquid into vapor.

Understanding intermolecular forces helps in predicting and explaining the boiling points of different substances.

hydrogen bonds

One of the strongest types of intermolecular forces is the hydrogen bond.

• Formation: Hydrogen bonds form when a hydrogen atom is covalently bonded to a highly electronegative atom like oxygen, nitrogen, or fluorine and is also attracted to another electronegative atom nearby.
• Impact on Boiling Point: Substances with hydrogen bonds tend to have higher boiling points because a significant amount of energy is required to break these bonds during the phase change from liquid to vapor.

For example, water has a high boiling point relative to its molecular weight due to the presence of hydrogen bonds.

vapor pressure

Vapor pressure is the pressure exerted by the vapor when a liquid evaporates.

• Equilibrium Condition: When a liquid's vapor pressure equals the external pressure, it begins to boil.
• Relation to Boiling Point: At the boiling point, the vapor pressure of a liquid matches the surrounding atmospheric pressure, allowing bubbles to form within the liquid.
• Influence of Intermolecular Forces: Liquids with strong intermolecular forces have lower vapor pressures at a given temperature, necessitating a higher temperature (higher boiling point) to reach equilibrium vapor pressure.

Understanding vapor pressure is vital for understanding why substances boil at different temperatures under different pressures.

temperature

Temperature is a measure of the average kinetic energy of particles in a substance. It significantly affects boiling points and phase changes.

• Definition: Boiling point is the temperature at which a liquid’s vapor pressure equals the external pressure.
• Energy Absorption: As temperature increases, the kinetic energy of the liquid's particles increases, weakening the intermolecular forces and facilitating the phase transition to vapor.
• Practical Example: At higher altitudes where atmospheric pressure is lower, water boils at a temperature lower than 100°C because it needs less energy to match the reduced pressure.

Thus, temperature and intermolecular forces are closely intertwined in determining the boiling point of a substance.