Freezing Point Depression - Chemistry LibreTexts
Freezing-point depression is the decrease of the freezing point of a solvent on addition of a The molar mass of a solute is determined by comparing mB with the amount of solute dissolved. on the solute concentration that can be estimated by a simple linear relationship with the cryoscopic constant ("Blagden's Law"). The freezing point of a solution is less than the freezing point of the pure solvent. This means that Kb is the molal freezing point depression constant, and m is the The following graph shows the normal freezing point for water (solvent) as a function of molality in several solutions containing sucrose (a non-volatile solute). Step 1: Calculate the freezing point depression of benzene. delta Tf = (Freezing point of pure molality = moles of solute / kg of solvent moles of naphthalene.
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.
Freezing-Point Depression to Determine an Unknown Compound | Protocol
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.
Boiling point elevation and freezing point depression
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: Such creatures have evolved means through which they can produce high concentration of various compounds such as sorbitol and glycerol. This elevated concentration of solute decreases the freezing point of the water inside them, preventing the organism from freezing solid even as the water around them freezes, or as the air around them becomes very cold.
Examples of organisms that produce antifreeze compounds include some species of arctic -living fish such as the rainbow smeltwhich produces glycerol and other molecules to survive in frozen-over estuaries during the winter months.
- Freezing Point Depression
- Freezing-point depression
- How does molality affect the freezing point?
In the case of the peeper frog, freezing temperatures trigger a large-scale breakdown of glycogen in the frog's liver and subsequent release of massive amounts of glucose into the blood. The degree of dissociation is measured by determining the van 't Hoff factor i by first determining mB and then comparing it to msolute.
In this case, the molar mass of the solute must be known. The molar mass of a solute is determined by comparing mB with the amount of solute dissolved. In this case, i must be known, and the procedure is primarily useful for organic compounds using a nonpolar solvent. Cryoscopy is no longer as common a measurement method as it once was, but it was included in textbooks at the turn of the 20th century.
Freezing-Point Depression to Determine an Unknown Compound
As an example, it was still taught as a useful analytic procedure in Cohen's Practical Organic Chemistry of in which the molar mass of naphthalene is determined using a Beckmann freezing apparatus. Freezing-point depression can also be used as a purity analysis tool when analysed by differential scanning calorimetry.
This is also the same principle acting in the melting-point depression observed when the melting point of an impure solid mixture is measured with a melting-point apparatussince melting and freezing points both refer to the liquid—solid phase transition albeit in different directions.
Consequently, solvents with higher chemical potentials will also have higher vapor pressures. Boiling point is reached when the chemical potential of the pure solvent, a liquid, reaches that of the chemical potential of pure vapor. Because of the decrease of the chemical potential of mixed solvents and solutes, we observe this intersection at higher temperatures.
In other words, the boiling point of the impure solvent will be at a higher temperature than that of the pure liquid solvent.
Freezing point is reached when the chemical potential of the pure liquid solvent reaches that of the pure solid solvent. Again, since we are dealing with mixtures with decreased chemical potential, we expect the freezing point to change. Unlike the boiling point, the chemical potential of the impure solvent requires a colder temperature for it to reach the chemical potential of the solid pure solvent.