Problem 121 - Chemistry Matter and Change

Question

List three colligative properties of solutions. (Chapter 14)

Step-by-Step Solution

Answer

The three colligative properties of solutions are: 1. Vapor Pressure Lowering: The vapor pressure of a solution is lower than that of the pure solvent due to the presence of solute particles, which reduce the number of solvent particles available at the surface to evaporate. 2. Freezing Point Depression: The freezing point of a solution is lower than that of the pure solvent because solute particles interfere with the formation of the solid structure of the solvent, requiring a lower temperature to freeze. 3. Boiling Point Elevation: The boiling point of a solution is higher than that of the pure solvent since solute particles disrupt the vaporization of the solvent particles, increasing the boiling point as more solute particles are present.

Step 1: 1. Vapor Pressure Lowering

Vapor pressure lowering is a colligative property that states that the vapor pressure of a solution is lower than that of the pure solvent. This is because the presence of solute particles in the solution reduces the number of solvent particles available at the surface to evaporate, thus decreasing the vapor pressure. The greater the concentration of solute particles, the lower the vapor pressure of the solution.

Step 2: 2. Freezing Point Depression

Freezing point depression is another colligative property which means that the freezing point of a solution is lower than that of the pure solvent. The presence of solute particles interferes with the formation of the solid structure of the solvent, requiring a lower temperature to freeze. The greater the concentration of solute particles, the greater the depression of the freezing point.

Step 3: 3. Boiling Point Elevation

Boiling point elevation is a colligative property that states that the boiling point of a solution is higher than that of the pure solvent. The solute particles in the solution cause an increase in the boiling point because they disrupt the vaporization of the solvent particles. As more solute particles are present, the boiling point elevation increases, requiring a higher temperature for the solvent to turn into vapor. These three colligative properties all depend on the concentration of solute particles in the solution and help us understand the behavior of solutions in various conditions.

Key Concepts

Vapor Pressure Lowering

When solute particles are added to a solvent, they cause a decrease in the solution's vapor pressure compared to the pure solvent. This happens because solute particles occupy space at the liquid's surface, reducing the number of solvent molecules that can escape into the vapor phase. In simpler terms, fewer solvent molecules are available to evaporate, which lowers the vapor pressure.

This effect is a fundamental colligative property and is directly proportional to the concentration of solute particles.

• More solute means lower vapor pressure.
• Works for any type of solute, as effects are determined by particle quantity, not nature.

Understanding vapor pressure lowering helps explain why solutions often have different boiling and freezing points than pure solvents. The relationship between the solute concentration and vapor pressure is technically detailed by Raoult's Law, which can predict the lowering of vapor pressure based on mole fractions.

Freezing Point Depression

Freezing point depression is observed when a solute is added to a solvent, causing the solution to freeze at a lower temperature than the pure solvent. The solute particles disrupt the formation of the crystalline structure needed for freezing, requiring the solution to be cooled further to achieve a solid state.

This colligative property is dependent on the molal concentration of the solute particles in a solution:

• Higher solute concentration results in greater freezing point depression.
• Applicable for all types of solutes, as it's based on abundance, not identity.

By calculating the freezing point depression using the formula \[ \Delta T_f = i \cdot K_f \cdot m \] where \( \Delta T_f \) is the change in freezing point, \( i \) is the van't Hoff factor, \( K_f \) is the freezing point depression constant, and \( m \) is the molality of the solution, scientists and students alike can better understand how different concentrations affect phase changes.

Boiling Point Elevation

Boiling point elevation refers to an increase in the boiling point of a solution compared to the pure solvent, due to the presence of solute particles. These particles interfere with solvent molecules attempting to escape into the gaseous phase, thereby requiring a higher temperature to achieve boiling.

This phenomenon, like other colligative properties, relies on the concentration of solute particles rather than their specific type:

• Greater solute quantity causes a higher boiling point.
• Applies universally to all solvents and solutes based on the concentration rule.

We can predict the boiling point elevation using the formula\[ \Delta T_b = i \cdot K_b \cdot m \] where \( \Delta T_b \) is the change in boiling point, \( i \) is the van't Hoff factor, \( K_b \) is the boiling point elevation constant, and \( m \) is the molality of the solution. This can be a useful tool in real-world applications, helping adjust cooking temperatures or designing anti-freeze solutions for vehicles.