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.

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.

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.

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.

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.

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.

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.

12.10

Freezing Point Depression and Boiling Point Elevation

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The temperature at which the vapor pressure of a liquid equals the atmospheric pressure is known as its boiling point.

Since adding a non-volatile solute lowers the vapor pressure of a solvent, a solution requires a higher temperature to increase its vapor pressure to a point that equals the atmospheric pressure. Thus, the boiling point of a solution is greater than that of a pure solvent.

These changes in vaporization can be examined over a range of temperatures and pressure using a phase diagram.

A solution has a lower vapor pressure than the pure solvent at all temperatures. So, the vaporization curve of the solution would lie below that of the solvent.

At 1 atm, the curve corresponds to a temperature higher than the boiling point of the pure solvent.

The increase in the boiling point of the solution compared to that of the pure solvent is known as boiling point elevation.

The boiling point of a solution is a colligative property. The temperature increase, or ΔT_b, is directly proportional to the concentration of solute and can be calculated by multiplying the molality of the solute and the molal boiling point elevation constant.

The boiling point elevation constant has the units °C per molality, and is different for each solvent. For water, the constant is 0.512 °C per molal.

So, a 2.00 molal aqueous solution will elevate the boiling point of water by 1.02 °C to 101.02 °C.

The addition of a non-volatile solute also lowers the freezing point of the solution compared to that of a pure solvent.

At the triple point, the vapor pressures of the solid, liquid, and gaseous states are equal.

Because a non-volatile solute lowers the vapor pressure of the solution, the entire freezing curve, which extends upward from the triple point, shifts such that the solution freezes at a lower temperature.

This decrease in the freezing temperature of a solution compared to that of a pure solvent is known as freezing point depression.

Like the boiling point, the freezing point of a solution is also a colligative property.

The temperature decrease or ΔT_f is directly proportional to the concentration of solute and can be calculated by multiplying the molality of the solute and the molal freezing point depression constant.

The freezing point depression constant also depends on the solvent and has the units °C/m.

For water, the freezing point depression constant is 1.86 °C per molal.

Thus, a 0.5 molal glycol solution will lower the freezing point of water by 0.93 °C to −0.93 °C.