6.6
Dépression du point de congélation et élévation du point d’ébullition
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Q1: What causes freezing point depression when a solute is added to a solvent?
Adding a non-volatile solute reduces the chemical potential of the liquid solvent without affecting the solid solvent's chemical potential. This shifts the intersection point on a chemical potential versus temperature plot to a lower temperature, lowering the freezing point. The effect depends on the solute's molality and the solvent's cryoscopic constant.
Q2: How is freezing point depression calculated?
Freezing point depression is calculated as the product of the solute's molality—the number of moles of solute per kilogram of solvent—and the solvent's cryoscopic constant. The cryoscopic constant is specific to each solvent and can be determined experimentally or derived theoretically from thermodynamic properties.
Q3: Why does adding salt to water raise its boiling point?
A non-volatile solute reduces the chemical potential of the liquid solvent but does not affect the vapor's chemical potential. This shifts the intersection point on the chemical potential plot to the right, requiring a higher temperature for the liquid to reach the chemical potential needed for vaporization, thereby raising the boiling point.
Q4: What is the formula for boiling point elevation?
Boiling point elevation equals the solute's molality multiplied by the solvent's ebullioscopic constant, also called the boiling point constant. This constant is unique to each solvent and can be determined experimentally or derived theoretically from the heat of vaporization for that solvent.
Q5: How does chemical potential explain why salt lowers the freezing point of water on roads?
Salt reduces the chemical potential of liquid water without affecting solid ice's chemical potential. This lowers the temperature at which liquid and solid water have equal chemical potential, depressing the freezing point. As a result, ice melts at temperatures below 0°C, slowing ice formation on roads.
Q6: Why do solutes affect liquid phase chemical potential but not solid or gas phase chemical potential?
Solutes disrupt intermolecular interactions in the liquid phase, altering its chemical potential significantly. However, solutes have negligible effects on solid and gas phase chemical potentials under comparable conditions because they do not substantially interact with the ordered solid structure or dispersed gas molecules.
Q7: What role does molality play in determining colligative property effects?
Molality—moles of solute per kilogram of solvent—directly determines the magnitude of freezing point depression and boiling point elevation. Both effects are proportional to molality multiplied by their respective solvent constants. This relationship holds for dilute solutions where colligative properties depend only on solute concentration, not solute identity.