key term - Vapor Pressure Lowering

Definition

Vapor pressure lowering is a colligative property that describes the decrease in the vapor pressure of a solvent when a solute is added to it. This phenomenon occurs due to the reduction in the number of solvent molecules at the surface, which lowers the tendency of the solvent to evaporate.

5 Must Know Facts For Your Next Test

1. Vapor pressure lowering is a colligative property, meaning it depends on the concentration of the solute in the solution, not the identity of the solute.
2. The magnitude of the vapor pressure lowering is proportional to the molality of the solute, as described by Raoult's law.
3. Vapor pressure lowering is directly related to boiling point elevation, as the decrease in vapor pressure increases the boiling point of the solution.
4. Vapor pressure lowering is an important consideration in the design of distillation and evaporation processes, as it can affect the efficiency of these operations.
5. The presence of a solute in a solution can also affect the freezing point of the solution, leading to freezing point depression, another colligative property.

Review Questions

• Explain how the addition of a solute to a solvent affects the vapor pressure of the solution, and describe the relationship between vapor pressure lowering and Raoult's law.
  • The addition of a solute to a solvent decreases the vapor pressure of the solution. This is because the solute particles occupy space in the solution, reducing the number of solvent molecules at the surface that can evaporate. The magnitude of the vapor pressure lowering is directly proportional to the mole fraction of the solute, as described by Raoult's law. Specifically, Raoult's law states that the vapor pressure of a solution is equal to the vapor pressure of the pure solvent multiplied by the mole fraction of the solvent. As the mole fraction of the solvent decreases with the addition of a solute, the vapor pressure of the solution decreases accordingly.
• Discuss the relationship between vapor pressure lowering and boiling point elevation, and explain how these colligative properties are connected.
  • Vapor pressure lowering and boiling point elevation are closely related colligative properties. The addition of a solute to a solvent not only decreases the vapor pressure of the solution, but it also increases the boiling point of the solution. This is because the decrease in vapor pressure makes it more difficult for the solution to reach the vapor pressure of the pure solvent, which is the condition for boiling. As a result, the solution must be heated to a higher temperature to reach the same vapor pressure as the pure solvent, leading to an increase in the boiling point. The magnitude of the boiling point elevation is directly related to the magnitude of the vapor pressure lowering, with a greater decrease in vapor pressure resulting in a larger increase in boiling point.
• Analyze the importance of understanding vapor pressure lowering in the context of various chemical and industrial processes, such as distillation and evaporation.
  • Vapor pressure lowering is a crucial concept in the design and optimization of various chemical and industrial processes, such as distillation and evaporation. In distillation, the goal is to separate the components of a mixture based on their differences in volatility. The presence of a solute in the mixture can lower the vapor pressure of the solvent, making it more difficult to achieve the desired separation. Understanding vapor pressure lowering allows engineers to adjust the operating conditions, such as temperature and pressure, to compensate for the solute's effect and improve the efficiency of the distillation process. Similarly, in evaporation processes, where the goal is to remove a solvent from a solution, the presence of a solute can significantly impact the rate of evaporation and the energy required to drive the process. By accounting for vapor pressure lowering, engineers can design more effective and energy-efficient evaporation systems, leading to cost savings and improved process performance.

Related terms

Raoult's Law:

Raoult's law states that the vapor pressure of a solution is proportional to the mole fraction of the solvent, which is the basis for understanding vapor pressure lowering.

Molality:

Molality is the measure of the concentration of a solution, expressed as the number of moles of solute per kilogram of solvent, and is used in the calculation of vapor pressure lowering.

Boiling Point Elevation:

Boiling point elevation is another colligative property that is related to vapor pressure lowering, as the addition of a solute to a solvent increases the boiling point of the solution.