Freezing-Point Depression to Determine an Unknown Compound | Protocol
Pure, crystalline solids have a characteristic melting point, the temperature at which In theory, the melting point of a solid should be the same as the freezing . The compound cyclohexane has a melting point (or freezing point) of about 6 °C. A series . such as calcium chloride (CaCl2), are often utilized for this purpose. Learning Objectives. Describe For any pure substance, the temperature at which melting occurs—known as the melting point—is a characteristic of that substance. What is the energy change when g of C 6H 6 freeze at °C? . The relationship between the ΔH sub and the other enthalpy changes is as follows.
Cool to well below 0oC, being careful to avoid disturbing it. Trigger freezing by stirring, or seeding with a small ice fragment. Normal freezing To freeze a liquid you must cool it down, but what happens to the temperature when the liquid solidifies? After the previous experiment, students may predict that it rises.
Experiment Place a thermometer in a test tube of cyclohexane so that its scale remains visible from 0oC upwards. Clamp the tube in a beaker of ice and water. Stir gently, recording the temperature at known intervals until the temperature drops to near zero.
Plot temperature against time. The temperature will stay constant while the liquid solidifies. What prevents the temperature dropping further until the liquid is completely frozen? When a liquid solidifies, bonds form between the particles in this case, cyclohexane molecules.
Bond formation releases energy, counteracting the energy removed by the iced water. So, the temperature stays constant. Once all the liquid has frozen, no more energy is released, so continued cooling with iced water causes the temperature to start falling again.
Difference Between Melting and Freezing Point
Students can reverse the experiment starting with frozen cyclohexane and warming it in a beaker of hot water, recording temperatures until it returns to about 15oC.
Other low melting point compounds, such as ice, octadecanoic stearic acid or ethanamide acetamidecan replace cyclohexane to show that constant temperature during melting and freezing is a general principle, not unique to cyclohexane. Boiling water by cooling it This is a simple demonstration, but even post students often struggle to explain the results. Royal Society of Chemistry Fit a ml round-bottom flask with a 2-hole bung carrying a oC thermometer, a short glass tube, and a short length of rubber tubing with a sealing clip.
The thermometer scale from about 20oC upwards must be visible above the bung, so a long thermometer may be better. In front of your students, half fill the flask with tap water. Clamp the flask and insert the bung, ensuring that the clip is open. Heat the flask until the water boils and steam issues from the rubber tube.
Continue boiling for at least 30 seconds while a student confirms that the thermometer reads about oC as expected. Stop heating and immediately close the clip. Invert the flask, so the thermometer bulb is in the hot water. Read the temperature; a little below oC, so the water has stopped boiling. Place the clamp stand and inverted flask in a sink or trough.
Ask a student to pour a little cold water over the flask and read the resulting temperature. The temperature drops as expected, but the water unexpectedly boils again for a while.
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Pouring over further portions of cold water causes repeated re-boiling while the temperature gradually drops to around 30oC, or even lower.
The initial boiling produces steam, flushing the air from the flask, which is then sealed. Cooling with cold water condenses the steam, lowering the pressure inside the flask. At lower pressures, liquids boil at lower temperatures. This re-boiling produces more steam, increasing the pressure, and thus raising the boiling point above the current water temperature, so boiling ceases. This is how early steam engines worked.
Steam pushes a piston upwards. Water sprayed into the cylinder condenses the steam, creating a vacuum. The reduced pressure below the piston allows the atmospheric pressure above it to push it back down.
Once freezing begins, as long as liquid and solid are both present, the temperature remains constant until the entire mass has solidified. Allow the computer to continue recording the temperature until the plot has leveled off at a constant temperature. Note that once the cyclohexane has frozen solid, the temperature starts to decrease again. When a sufficient number of data points have been collected, stop the data collection.
Melting Point, Freezing Point, Boiling Point
Remove the test tube from the ice-water bath and let it warm up to room temperature. Adjust the y-axis limits so the plot fills the page. Title the graph, and then print it.
Preparing a Solution of the Unknown Compound Accurately weigh 0. Check to be sure the cyclohexane contained in the test tube has melted. Remove the stopper from the test tube and carefully add the unknown solid to the cyclohexane, avoiding the loss of any compound adhering to the sides of the test tube or stopper. Replace the stopper and re-weigh the paper to account for any crystals that remain on it. Stir the solution in order to completely dissolve the solid.
It is important that no crystals remain. Make a new ice-water bath. Measuring the Freezing Point of the Unknown Compound Prepare the computer to collect a second set of data. Move the test tube that contains the solution into the ice-water bath. Immediately begin stirring the solution continuously and at a constant rate. Collect the data for — s in order to clearly see the change in slope that occurs as the solution freezes. Stop the data collection. Save the data, adjust the limits of the y-axis, title the graph, and print it.
Do not throw any cyclohexane or unknown compound down the sink. Pour the liquid mixture into the "Laboratory Waste" jar.
Rinse the test tube and temperature probe with acetone to remove the last traces of any crystals, pouring the rinses in the waste jar.
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.
Latent Heat and Freezing and Boiling Points
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.