6.6
Freezing Point Depression and Boiling Point Elevation
When a non-volatile solute is added to a pure solvent, it results in the lowering of the freezing point of the solvent. This phenomenon is called free…
When a non-volatile solute is added, it lowers a solvent’s freezing point, known as the freezing point depression. This effect is calculated as the product of the solute’s molality and the solvent’s cryoscopic constant.
The chemical potential, μ versus temperature plot shows that the μ of the solvent in both its solid and liquid states decreases with temperature.
The intersection point, where the μ of the liquid equals that of the solid, represents the freezing point of the pure solvent.
When a solute is added, the μ of the liquid solvent decreases, while the μ of the solid solvent remains unchanged, shifting the intersection to a lower temperature.
Similarly, adding a non-volatile solute to a pure solvent elevates the solution's boiling point, which can be expressed as the product of the molality of the solute and the boiling point constant of the solvent.
This change is due to the solute reducing the solvent's μ value without affecting the vapor, shifting the intersection point to the right and raising the boiling point.
Overall, adding a solute always changes the chemical potential of the liquid phase, while the chemical potentials of solids and gases remain unchanged.
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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.