Question

Define the term boiling point. How does the boiling point of a liquid change as intermolecular forces become stronger? Using kinetic-molecular theory (KMT), explain how differences in intermolecular forces affect boiling point.

Short answer

The boiling point is the temperature at which liquid turns to gas. Higher intermolecular forces increase boiling point because they require more energy to overcome. KMT explains this by linking temperature to kinetic energy needed to overcome the forces.

Step-by-step solution

Define Boiling Point

The boiling point of a substance is the temperature at which its vapor pressure equals the external pressure surrounding the liquid. At this temperature, the liquid changes to gas.

Impact of Intermolecular Forces on Boiling Point

The boiling point of a liquid increases as the strength of the intermolecular forces increases. This is because stronger intermolecular forces require more energy (higher temperature) to overcome and allow the liquid to transform into gas.

Explanation Using Kinetic-Molecular Theory (KMT)

According to the kinetic-molecular theory, particles in a liquid gain kinetic energy as temperature increases. If intermolecular forces are strong, more kinetic energy (higher temperature) is needed to overcome these forces and reach the boiling point. Therefore, a liquid with stronger intermolecular forces will have a higher boiling point.

Key concepts

Intermolecular Forces

Intermolecular forces are the attractions between molecules that hold them together. These are different from the bonds within molecules, as they occur between separate molecules. Common types include:

• London dispersion forces: Weakest type, present in all molecules.
• Dipole-dipole attractions: Occur between polar molecules.
• Hydrogen bonds: Strongest type, found in molecules where hydrogen is bonded to nitrogen, oxygen, or fluorine.

The strength of these forces plays a major role in determining the properties of a substance, including its boiling point
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Stronger intermolecular forces mean that more energy is needed to separate the molecules. This translates to a higher boiling point because higher temperatures are required to provide enough energy to overcome these forces.

Kinetic-Molecular Theory

The kinetic-molecular theory (KMT) explains the behavior of gases in terms of particles in constant, random motion. Here are the key points:

• Particles are in continuous random motion.
• The average kinetic energy of particles is proportional to the temperature.
• There are no intermolecular forces in an ideal gas; however, real gases do have these forces.
• Collisions between particles and with the walls are elastic, meaning no energy is lost.

When applied to liquids, KMT helps explain boiling points. As temperature increases, particles gain kinetic energy. For a liquid to boil, its particles must have enough energy to overcome intermolecular forces and escape into the gas phase.

Therefore, stronger intermolecular forces mean that particles need more kinetic energy (i.e., a higher temperature) to boil. This results in a higher boiling point for substances with stronger intermolecular forces.

Vapor Pressure

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid phase. In simpler terms, it's the tendency of particles to escape from the liquid and become gas. Here are some important points:

• Vapor pressure increases with temperature, as more particles have enough energy to escape the liquid.
• Substances with high vapor pressures are said to be volatile and evaporate easily.
• When vapor pressure equals atmospheric pressure, the liquid boils.

The relationship between vapor pressure and boiling point is crucial. A higher vapor pressure at a given temperature means the liquid is closer to boiling. Conversely, stronger intermolecular forces result in a lower vapor pressure because fewer particles have the needed energy.

This means a higher temperature is needed to equalize vapor pressure with atmospheric pressure, leading to a higher boiling point.