12.10
Obniżenie punktu zamarzania i podwyższenie punktu wrzenia
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Q1: Why does adding a solute increase the boiling point of a solution?
Adding a non-volatile solute lowers the vapor pressure of the solvent. Since a solution requires higher temperature to reach atmospheric pressure, its boiling point increases compared to pure solvent. This phenomenon, called boiling point elevation, is a colligative property directly proportional to solute concentration.
Q2: How is boiling point elevation calculated for a solution?
Boiling point elevation (ΔTb) is calculated by multiplying the molality of solute by the molal boiling point elevation constant (Kb) specific to each solvent. For water, Kb equals 0.512 °C per molal, so a 2.00 molal solution elevates water's boiling point by 1.02 °C to 101.02 °C.
Q3: What causes freezing point depression in solutions?
Non-volatile solutes lower the vapor pressure of solutions, shifting the freezing curve to lower temperatures. This freezing point depression occurs because the solution's solid-liquid equilibrium point moves to a lower temperature. Like boiling point elevation, freezing point depression is a colligative property proportional to solute concentration.
Q4: How do you calculate the freezing point depression of a solution?
Freezing point depression (ΔTf) is calculated by multiplying molality by the molal freezing point depression constant (Kf) for the solvent. For water, Kf is 1.86 °C per molal, so a 0.5 molal glycol solution lowers water's freezing point by 0.93 °C to −0.93 °C.
Q5: Why is freezing point depression useful in practical applications?
Freezing point depression is exploited in de-icing applications using salt or calcium chloride on roads, and in antifreeze formulations with ethylene glycol for automobile radiators. Seawater freezes at lower temperatures than freshwater, allowing Arctic and Antarctic oceans to remain unfrozen below 0 °C.
Q6: How can you determine a solute's molar mass using freezing point depression?
Measure the freezing point of a solution with known solute mass and solvent mass. Calculate ΔTf, then determine molality using the freezing point depression constant. From molality and solvent mass, find moles of solute, then divide solute mass by moles to obtain molar mass.
Q7: What is the relationship between vapor pressure and boiling point elevation?
A solution's lower vapor pressure means it requires higher temperature to reach atmospheric pressure and boil. The phase diagram shows the vaporization curve of a solution lies below that of pure solvent, causing the boiling point to shift to a higher temperature at 1 atm pressure.