12.8
Abaissement de la pression de vapeur saturante
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Q1: Why does adding a solute lower the vapor pressure of a solution?
When a nonvolatile solute dissolves in a volatile solvent, solute particles occupy the liquid surface, reducing the number of solvent particles available to vaporize. Since the condensation rate remains unchanged, dynamic equilibrium is reestablished at a lower vapor pressure with fewer solvent molecules in the gaseous state.
Q2: What is Raoult's law and how does it apply to vapor pressure?
Raoult's law states that the partial pressure of a solution equals the mole fraction of the solvent multiplied by the vapor pressure of the pure solvent. For a solution with 3.5 mol water and 1.5 mol glycerol at 25°C, the mole fraction is 0.70, yielding a vapor pressure of 16.7 torr compared to pure water's 23.8 torr.
Q3: How is vapor pressure lowering calculated from the mole fraction of solute?
Vapor pressure lowering (ΔP) is directly proportional to the mole fraction of the solute. Using the equation derived from Raoult's law, if the solute mole fraction is 0.3 and pure water's vapor pressure is 23.8 torr, the pressure lowering equals 7.14 torr, demonstrating this colligative property.
Q4: What distinguishes colligative properties from other solution properties?
Colligative properties depend only on the concentration or number of solute particles, not their type. Vapor pressure lowering is a colligative property, meaning any nonvolatile solute at the same concentration produces identical pressure reduction, unlike properties such as pH that depend on solute identity.
Q5: What role does dynamic equilibrium play in vapor pressure lowering?
In a pure solvent, dynamic equilibrium occurs when vaporization and condensation rates are equal. In a solution, the reduced solvent surface area decreases vaporization rate. Condensation rate remains constant, so equilibrium reestablishes at lower vapor pressure with fewer solvent molecules in the vapor phase.
Q6: Why must a solute be nonvolatile for vapor pressure lowering to occur?
A nonvolatile solute has negligible vapor pressure and cannot escape into the gas phase. This ensures that only solvent molecules contribute to the solution's vapor pressure. If the solute were volatile, it would add its own partial pressure, complicating the relationship and preventing the simple lowering effect observed with nonvolatile solutes.
Q7: How does entropy explain vapor pressure lowering in solutions?
The more dispersed nature of matter in a solution, compared to separate solvent and solute phases, stabilizes solvent molecules and hinders their vaporization. This entropy effect results in lower vapor pressure and correspondingly higher boiling point, reflecting the thermodynamic stabilization of the solution state.