Freezing-Point Depression to Determine an Unknown Compound

Freezing-point depression is the phenomenon that is observed when the freezing point of a solution is lower than that of the pure solvent.

This phenomenon results from interactions between the solute and solvent molecules. The difference in freezing temperatures is directly proportional to the number of solute particles dissolved in the solvent.

The molar mass of a non-volatile solute can be calculated from the difference in freezing temperatures if the masses of the solvent and the solute in the solution are known.

This video will introduce the relationship between freezing-point depression and the molar mass of the solute, a procedure for determining molar mass of an unknown solute, and some real world applications of inducing and observing changes in freezing temperature.

Freezing point depression is a colligative property, meaning it is only affected by the ratio of solute to solvent particles, and not their identity.?

At the freezing point of a pure substance, the rates of melting and freezing are equal.

When a solution is cooled to the freezing point of its solvent, the solvent molecules begin to form a solid. It is less energetically favorable to form a mixed lattice of solvent and solute particles. The solute particles remain in the solution phase. Only solvent-solvent interactions contribute to lattice formation, so solvent-solute interactions reduce the rate of freezing compared to that of the pure solvent.

The temperature at which freezing begins is the freezing point of the solution. The solution continues cooling as it freezes, but this continued decrease in temperature reflects the increasing concentration of solute in the solution phase.

Eventually, the solution temperature is so low and so little solvent remains in the liquid phase that it becomes favorable for the solute particles to form a lattice. Once this point is reached, the temperature remains approximately constant until the mixture has frozen solid.

The molar mass of the solute, and therefore the identify of the solute, can be determined from the relationship between the freezing point of the pure solvent, the freezing point of the solution, and the molality of the solution. Molality, or m, is a measure of concentration in moles of the solute per kilogram of the solvent. This relationship depends on the the freezing point depression constant of the solvent and the number of solute particles produced per formula unit that dissolves.

Molality can be expressed in terms of molar mass, so the equation can be rearranged to solve for the molar mass of the solute. Plugging this into the freezing point equation allows the elucidation of the molar mass, once the temperature difference is known. Now that you understand the phenomenon of freezing point depression, let's go through a procedure for determining the molar mass of an unknown solute from freezing point temperatures. The solute is a non-ionic, non-volatile organic molecule that produces one particle per formula unit dissolved, and the solvent is cyclohexane.

To begin this experiment, connect the temperature probe to the computer for data collection. Insert the temperature probe and a stirrer into the sample container.

Set the length of data collection and the rate of sampling. Allow sufficient time in the data collection for the sample to freeze.

Set upper and lower limits of the temperature range to sample.

Add 12 mL of cyclohexane to a clean, dry test tube. Wipe the temperature probe with a Kimwipe. Insert the stopper assembly into the test tube such that the tip of the temperature probe is centered in the liquid and does not touch the sides or bottom.

In a beaker, prepare an ice water bath. Then, start the temperature data collection.

Place the test tube into the ice water bath, ensuring that the level of liquid in the test tube is below the surface. Continuously stir the liquid at a constant rate.

Once freezing begins, allow data collection to continue until the plot has leveled off at a constant temperature. This is the freezing point of pure cyclohexane. Remove the test tube from the ice water bath and allow it to warm to room temperature.

Once the cyclohexane has melted, accurately weigh the solid unknown material on weighing paper. Remove the stopper from the test tube and add the solid. Avoid allowing compound to adhere to the test tube.

Replace the stopper and stir the solution until the solid is completely dissolved. It is important that no solid crystals remain.

Set the parameters for data collection and prepare a fresh ice water bath. Start collection, place the test tube into the bath, and stir continuously at a constant rate. Once freezing begins, the freezing point continues to decrease due to the increasing solute concentration. Continue collecting data until the slope of this decrease is evident. When the experiment has finished, allow the solution of the unknown compound to warm to room temperature and then dispose of it according to the procedures for organic waste.

In this experiment, the unknown substance is known to be one of five possible compounds: biphenyl, bromochlorobenzene, naphthalene, anthracene, and dibromobenzene. The identity of the unknown can be determined by comparing its molar mass to these known substances.

The unknown solute produces one particle per formula unit dissolved. To calculate the molar mass of the unknown compound, the freezing point depression constant of cyclohexane, the mass of solute and solvent used, and the difference in freezing temperatures are all needed.

0.147 g of the unknown solute were used in this example. The freezing point depression constant of cyclohexane is 20.2 ?C-kg per mol of solute. The density and volume of cyclohexane are used to calculate the mass of the solvent.

The values of the freezing point of the pure solvent and the freezing point of the solution are determined from the plots.

If the compound is known to be one of a few possible compounds, as in this experiment, the molar mass can simply be compared to those compounds. Of the five options provided for this experiment, naphthalene is the closest match.

The phenomenon of freezing point depression has many applications both inside and outside the laboratory.

Calcium chloride is preferred to sodium chloride for treating icy roads because of the effects of freezing point depression. As calcium chloride releases one more particle than sodium chloride does, it depresses the freezing point of water further and thus melts ice at lower temperatures.

In this study, a melting experiment was conducted with two different iron-sulfur mixtures. The sample with the higher mass fraction of sulfur was completely liquid at the temperature of the experiment, whereas the sample with less sulfur was still partially solid. This demonstrates that with increased impurities, in this case sulfur, the observed melting point is lower than for the pure solid. Here, the melting point differences between the two samples lend insight into the formation of the Earth's core.

You've just watched JoVE's introduction to using freezing point depression to determine the identity of an unknown compound. You should now understand the phenomenon of freezing point depression, the relationship between freezing point depression and the molar mass of the solute, and why the phenomenon is useful to a variety of industries.

Thanks for watching!