Cooling Behavior of a Solution

Description

This experiment is very similar to 066, Colligative Properties. Students use acetamide (CH₃CONH₂) as the solvent. Acetamide has the advantage over water of giving a much larger freezing point depression per mole of dissolved solute. Also, it is a polar solvent in which ionic substances dissolve and dissociate.

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Burns are possible from hot equipment. Fragile thermometers may break.

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Check equipment for temperature by placing hand near recently heated objects for a few seconds to detect heat. Do not touch if the object seems hot. Make the first touch of an object cautiously. Handle thermometers very gently.

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1. Obtain 10.0 g of acetamide in a clean, dry 18 x 150 mm test tube.
2. Fill a 400-mL beaker half full of water. Heat on a hot plate, or set the beaker on a wire gauze which has been mounted upon a ring and ring stand and heat the water with a burner. Insert the tube of acetamide gently into the hot water bath. Carefully place the stirring rod and the thermometer in the tube.
3. Heat the acetamide in the water bath to 90 °C. Careless work may lead to breaking an expensive thermometer and possibly causing a fire. If the fluid expands more than the thermometer can accommodate, it will break.
4. Remove the tube from the bath. Clamp the test tube in position.
5. Record time and temperature data every 0.5 minutes as the acetamide cools in air. Stir the liquid continuously by moving the stirring rod up and down. Record all temperatures to the nearest 0.1 °C.
6. Continue recording data for 2-4 minutes after freezing begins.
7. Weigh a 0.0050 mol sample of the assigned solute. Use a funnel made from a weighing paper to add this to the 10.0 grams of acetamide used above.
8. Return the tube to the water bath. Melt the mixture and heat the solution to about 85-90 °C. Stir continuously until all solute has dissolved.
9. Cool in air as above. Obtain time and temperature data at 0.5 minute intervals as the solution cools. Continue to stir the mixture and collect data until about 3 minutes after freezing begins.
10. To the mixture just cooled, add an additional 0.0050 mole of the same solute added before, and repeat the sequence.

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Name ___________________________ Class ________

Teacher__________________________

DoChem 067 Cooling Behavior of a Solution

Watch the movie and answer the following questions.

• Suppose, instead of removing the tube, you were simply to turn off the heat source to the bath. How would that change the cooling curve?
• Plot the time and temperature for the following data. Show temperature along the vertical axis and time along the horizontal axis.

Freezing point for acetamide = ___________________

1. Determine the change in freezing temperature, DTf, caused by adding 0.0050 mole of solute to acetamide.

   Use this class data.

   Acetamide + 0.0050 mole ...	FP, °C	ΔTf, °C
   urea	78.3	1.4
   NaI	75.0	4.7
   KI (determine from cooling curve)
   CaCl2	73.6	6.1

   Acetamide + 0.0100 mole...	FP, °C	ΔTf, °C
   urea	76.9	2.8
   NaI	69.7	10.0
   KI	70.9	8.8

2. Determine the additional change in freezing temperature, ΔTf, when the second amount of solute was added.
3. The temperature remained nearly constant during the solidification of acetamide. During the solidification of the solutions, however, the temperature drifted down. Account for this observation.
4. Use class data to compare average values of ΔTf when 0.0050 mole of each solute was added to 10.0 g of acetamide.

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Purposes

• To observe the behavior of a solution during cooling.
• To study the freezing point of acetamide solutions of different concentrations.
• To determine the freezing temperature depression constant of acetamide.

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(for 10 students; students work in pairs)

• 5 18 x 150 mm test tube
• 5 test tube clamp
• 5 Bunsen burner (or electric heater)
• 5 single-buret clamp
• 5 support stand
• 5 centigram balance
• 5 thermometer
• 5 wire stirrer
• 5 400-mL beaker
• 5 hot pad
• 5 spatula
• weighing paper
• clock with sweep second hand
• 100 g acetamide, CH₃CONH₂, reagent
• one or more of the following solutes:
  • 10 g urea
  • 15 g sodium iodide
  • 16 g potassium iodide
  • 11 g calcium chloride (anhydrous)

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• Sodium iodide and calcium chloride are hygroscopic. They should be dried in an oven and stored in a desiccator.
• Potassium iodide crystals should be powdered using a mortar and pestle. Large crystals dissolve more slowly than small crystals.
• Each student or group will add a solute assigned by the teacher in two separate 0.0050 mole increments. The following samples contain 0.0050 moles:

  urea	NH2CONH2	0.30 g
  sodium iodide	NaI	0.75 g
  potassium iodide	KI	0.88 g
  calcium chloride	CaC12	0.56 g

• The small amounts of solutes should be weighed using waxed or coated paper.
• All of the solute must be dissolved before the time and temperature data can be taken. Record data long enough to determine the freezing point of each solution. It is not necessary that all of the acetamide solidify.
• Labeled tubes containing the acetamide and acetamide solutions may be stored from year to year for reuse.
• Prepare a stirrer by rolling the end of a piece of heavy wire into a circular shape whose diameter is larger than that of the thermometer but smaller than that of the test tube. Bend the circle at a right angle to the rest of the wire, so that the circle becomes perpendicular to the wire.
• Rub the stirring rod along the inside of the test tube to pick up crystals, and then return the stirring rod to the liquid; this reduces the chance of supercooling. The small crystals act as nuclei for larger crystals to form. If supercooling does occur, have the students warm the solution just above the freezing temperature and then repeat the procedure.

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Teacher preparation: 30 minutes

Class Time: 40 minutes

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• Pour molten acetamide and acetamide solutions into a metal container (such as an empty peanut can). Allow to solidify. Place in a plastic bag and dispose of with ordinary solid wastes.
• Clean glassware with warm soapy water.
• Labeled tubes containing the acetamide and acetamide solutions may be stored from year to year for reuse. If done this way, students must receive information about masses from labels.

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Presentation Question:

• Suppose, instead of removing the tube, you were simply to turn off the heat source to the bath. How would that change the cooling curve?
  • The time axis becomes extended, but the principal features of the cooling curve remain unchanged. There is a long period where the temperature remains constant or falls slowly as the liquid solvent solidifies.

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• Assist the students in determining the freezing point of the solutions by showing them how to extrapolate the value of the freezing temperature from their graphs.
• The electrical conductivity of acetamide and its solutions may be demonstrated at a later time. This should assist the students in seeing that 0.0050 mole of KI or NaI lowers the freezing point of acetamide the same amount as 0.010 mole of urea. This leads to the conclusion that NaI and KI produce twice as many particles in solution as urea.
• Emphasize the fact that the solutes dissolved in the acetamide, they did not melt. The melting temperatures of the solutes should easily convince the students.

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Closure Questions:

Plot the time and temperature data from all three experiments. Show temperature along the vertical axis and time along the horizontal axis.

Sometimes freezing liquids tend to supercool or cool to a temperature below the freezing temperature before crystal formation begins. When crystallization begins, the temperature rises.

1. Determine the change in freezing temperature, DTf, caused by adding 0.0050 mole of solute to acetamide.
2. Determine the additional change in freezing temperature, ΔTf, when the second amount of solute was added.
3. The temperature remained nearly constant during the solidification of acetamide. During the solidification of the solutions, however, the temperature drifted down. Account for this observation.
4. Use class data to compare average values of ΔTf when 0.0050 mole of each solute was added to 10.0 g of acetamide.

Answers to Closure Questions:

See acetamide and acetamide solution graphs.

1. Change in freezing temperature caused by 0.0050 mole solute.

   Solute	ΔTf, °C
   urea	1.4
   NaI	4.7
   KI	4.4
   CaCl2	6.1

2. Change in freezing temperature caused by 0.0100 mole solute.

   Solute	ΔTf, °C
   urea	2.8
   NaI	10.0
   KI	8.8

3. The solid that forms is nearly pure solvent, acetamide. The freezing temperature lowers during the solidification of the acetamide solutions because the concentration of the solutes remaining in acetamide solution increases as some of the liquid acetamide freezes out and forms crystals.
4. The NaI and KI solutions produce changes in freezing point that are equal to approximately twice those for the same concentration of urea. NaI and KI are electrolytes and produce ions in solution. The students may not be able to discern the difference between the electrolytes and nonelectrolytes, unless the conductivity is demonstrated. The CaCl₂ produces a lowering about 1.5 times that of KI because it gives 3 particles per mole as compared to 2.

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Freezing point depression may be applied to the addition of antifreeze in automobile radiators, to the salting of ice in the production of ice cream, or to the addition of salt to icy roads.

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If thermistors capable of reading just above the melting temperature of acetamide are available, this experiment may be performed using the computer to take the temperature data directly and store it for graphical display.

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See Presentation? and Closure?.

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• freezing
• freezing temperature
• colligative property
• molality solvent
• solute

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