Lecture Notes 13 + Experiment 13 : DETERMINATION OF FREEZING POINT PDF

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Lecture Notes 13 + Experiment 13: DETERMINATION OF FREEZING POINT...

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Experiment 13 Determination of the Freezing Point Constant of a Solvent OBJECTIVES: Collect freezing point data for a pure solvent and then a solution. Construct cooling curves from each set of data collected. Determine freezing point of each system examined from analysis of its cooling curve. Determine Kf for the solvent cyclohexane from its Tf

INTRODUCTION: The Freezing Point1 of a solution is one of four physical properties of that solution, which are collectively called colligative properties. These physical properties of solutions depend upon the number but not the kind of solute particles in a given amount of solvent. The four colligative properties of a solution affect the solvent by: (1) lowering its vapor pressure, (2) raising its boiling point, (3) lowering its freezing point, (4) generating an osmotic pressure. For example, antifreeze (ethylene glycol) added to an auto cooling system prevents freeze-up in the winter by inhibiting the freezing process of the water, requiring us to cool it to a lower temperature.2 The ethylene glycol solution freezes below the normal freezing point of water (0C). The extent to which the freezing point drops is directly proportional to the concentration of ethylene glycol in the water. Note that the same antifreeze in your car radiation system that protects it from freezing in the winter, protects it from boiling over in the summer, since the boiling point is raised.

1

The normal freezing point of a substance is the temperature at which a phase transition occurs between the solid and liquid state when the material is heated or cooled at 1 atm pressure. 2 Remember that as we cool the liquid, we are slowing down the molecules, making it progressively easier for the attractive intermolecular forces between solvent molecules to lock together into a solid state.

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If a solution is dilute3 enough and if the solute is a nonvolatile4 substance which is also a nonelectolyte, the freezing point depression, Tf, and the boiling point elevation, Tb, are both directly proportional to the molality, m, of the solute in the solution. The proportionality can be expressed in the following equations: Freezing Point Depression (1) Tf = Kf m Boiling Point Elevation

(2) Tb = Kb m

Molality of a Solution

(3)

m = moles solute kg solvent

Equations similar to those above permit us to calculate the vapor pressure and osmotic pressure of a solution also, however, it is outside the scope of this lab to discuss those. You will, however, find all four colligative properties discussed in your textbook. In the preceding equations, Kf is the molal freezing point constant for the solvent and Kb is the molal boiling point constant for the solvent. It is in this constant that the freezing characteristics of different solvents can be distinguished from one another. Some typical constants are given below: Kf and Kb values for Several Solvents Solvent Water Benzene Acetic Acid Nitrobenzene

F.P., C 0.00 5.45 16.6 5.73

Kf, C/m 1.86 5.01 3.57 7.00

B.P., C 100.0 80.2 118.3 210.9

K b, C/m 0.512 2.53 3.07 5.24

Referring to this table, your can see that if you have enough nonvolatile, nonelectrolytic solute in nitrobenzene to make a 1 m solution, it should freeze at - 1.27 C (7.00 degrees lower than pure nitrobenzene) and it would boil at 216.1 C (5.24 degrees higher than pure nitrobenzene). In this experiment we will use the colligative property of freezing point depression to determine the freezing point depression constant for a solvent using two different solutes. Keep in mind that the solutes must be nonvolatile in the temperature range of investigation and that they must also be nonelectrolytes. A three-step process is generally employed. Step 1.

3

First a solution is prepared using the unknown solute and Tf is determined experimentally.

The concentration of solute particles must be low enough that there is almost no chance they will bump into one another. The colligative property equations fall into error when we get enough solute that solute-solute interactions in solution become significant. 4 The solute cannot have much of a vapor pressure. We must assume all the vapor in equilibrium with the solution comes from the solvent in order to use the colligative property equations. In chapter 12 we have another equation called Raoult’s Law, which is more general. It will let us calculate vapor pressure for a solution containing a volatile solute.

Determination of Freezing Point Constant

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Tf = Tsolvent - Tsolution Step 2.

Secondly, molality is calculated using equation (3) m = moles solute kg solvent

Step 3.

Thirdly, we solve for the freezing point depression constant of the solvent Kf,cyclohexane = Tf / m

EXPERIMENTAL DESIGN: A. Freezing point of Cyclohexane. Cyclohexane is the pure solvent we will use. You will determine its freezing point experimentally. Tf,solvent B. Freezing point of a Naphthalene-Cyclohexane Solution. We put a known solute, naphthalene, into the cyclohexane and freeze it. Using this freezing point along with that of the pure solvent above we subtract to get Tf for the solution. We then use this to calculate the Freezing Point Constant, Kf, for cyclohexane. Kf,cyclohexane = Tf / m Note that in this part we will be able to calculate the molality, m, of the solution because the solute is known. The molecular formula for naphthalene is C10H8. C. Freezing point of a Naphthalene-Cyclohexane Solution (second time). Again we put a known solute, naphthalene, into the cyclohexane and freeze it. Using this freezing point along with that of the pure solvent above we subtract to get Tf for the solution. We then use this to calculate the Freezing Point Constant, Kf, for cyclohexane. Kf,cyclohexane = Tf / m Note that in this part we will be able to calculate the molality, m, of the solution because the solute is known. The molecular formula for naphthalene is C10H8. COOLING CURVE: You will use a cooling curve to determine the freezing point of the solvent cyclohexane (A), the naphthalene-cyclohexane known solution (B) and a second naphthalene-cyclohexane solution (C). In each case, the cooling curve starts by putting the appropriate system in a large test-tube and cooling it down from room temperature until it is completely frozen. You will stir constantly to ensure even cooling and use a computer to record the temperature at regular time

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Experiment # 13

intervals. This will give you a table of time vs. temperature data which you will be plotted to determine the freezing points. Time (sec) 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Temperature (C) 25.0 20.3 15.2 10.4 5.5 3.2 4.3 4.2 4.1 4.0 3.9 3.5 3.2 2.7 2.2 1.7

All cooling curves can be approximated by three straight line segments: A The liquid cooling B The freezing of the liquid C The solid cooling Note that if the liquid being cooled is impure--as is the one illustrated above--the B segment will not be perfectly horizontal, but rather will tilt downward a bit. When the cooling curve is done for the pure cyclohexane solvent, you should see the B segment be perfectly level. Note that we can see what is called supercooling at the 50 second point in the data. You will not necessarily get this in your data, but it is not uncommon. It represents the liquid dropping to a temperature lower than the freezing point before the crystals of solid begin to form, then the temperature jumps back up to normal and continues in an expected manner. Note that the freezing point is the temperature at which the A and B segments intersect. By extrapolating the behavior of the solution before and after freezing, into the region of phase transition, we are able to get a better picture of the transition. During the transition period, there are significant fluctuations in behavior within different regions of the sample and our data is more prone to be in error.

Determination of Freezing Point Constant

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PROCEDURES: A. Determination of the Freezing Point of Cyclohexane 1. Obtain a 200-mm test-tube and place it in an empty beaker. Weigh empty test-tube + beaker and record mass. 2. Add approximately 20 mL of cyclohexane to the test-tube. Reweigh the beaker + test-tube + cyclohexane and record mass.

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Caution: Cyclohexane is flammable. Be sure there are no open flames in the lab.

3. Prepare the computer for data collection by attaching a Vernier Temperature probe to CH I (note a DIN adapter cable is required) of the Vernier LabPro interface device and connect the LabPro to an IBook Laptop computer via a USB port. AC Power should be supplied to both the LabPro interface and the IBook computer. 4. Turn on the Laptop computer and open the "Freezing Point" shortcut from the IBook Dock Tool panel at the bottom of the computer screen. This will open a program called Logger Pro 3 that has been set up with the experiment parameters to take data for this experiment. 5. Prepare the apparatus setup as shown. You will need an ice water bath--Add some salt so it will stabilize at a temperature a well below 0 C, stir in enough salt until T = -6C. You will need to put the stopper in carefully so that the stirrer fits around the thermometer.

*

Do not immerse the test-tube containing the cyclohexane into the ice-bath until you are ready to begin taking data.

Figure 1

6. Remove the test tube from the ringstand and immerse it in the ice bath. Stir the test tube with the temperature probe in it at a continuous even rate. You should see an initial steep drop (segment A of the curve) as the liquid cools. You should see a slowing down, or possibly some supercooling, as the crystals begin to form and you move into a solid-

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Experiment # 13

liquid equilibrium system. This should level off to an almost constant temperature for a while, until all the solid freezes. You will detect first a slush and then only solid. After the sample is completely solidified, you may see an increase in the cooling rate as you now see the pure solid cooling. 7. You should collect data for 3 minutes. It is o.k. if you don’t get out of region B, because to determine the freezing point you need only fit the A and B straight lines and look for their intercept. 8. After 3 minutes, terminate the measurements by selecting Stop to end the data collection run. Raise the test-tube from the ice bath and set it in a beaker of warm water to melt. 9. You will want to get your final freezing point temperatures from specific regions of the data.While holding down the mouse button, use the mouse to scroll across a portion of the curve from left to right, selecting only the points from one part of the curve at a time. 10. Then Choose Analyze Data from the Analyze menu. To determine the freezing temperature of the pure solvent, you need to determine the mean (or average) temperature in the Time portion of graph with nearly constant temperature. Select Analyze Figure 2 Select Statistics 11. To store a set of data and collect the data for another experiment. Select Store latest data under the data menu. This allows both data sets to be printed on the same graph. B. Molal Freezing Point Constant of Cyclohexane plus Naphthalene. 1. Weigh out about 0.15 - 0.20 grams of naphthalene, C10H8, and record mass. Use weighing paper. Carefully remove stopper and temperature probe from the top of the cyclohexane and add the naphthalene. Replace the stopper assembly for another freezing point run.

*

Be sure no crystals adhere to the wall of the test tube.

2. Repeat steps A.4 through A.8 for the naphthalene-cyclohexane solution. 3. Unlike pure Cyclohexane, cooling a mixture results in a slow gradual drop in temperature during the time period when freezing takes place. Select one region at a time and get a best-fit line using the linear fit function under the Analyze menu. Then when you have lines for the two regions use the interpolate function under the Analyze menu to determine the intersection of those two best-fit lines.

Freezing Point

Time

Figure 3

Determination of Freezing Point Constant

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4. You can discard the entire sample and clean and dry your test-tube after this part. Use waste containers provided for this purpose (ask your instructor). You will re-use this same apparatus for a fresh sample in part C. 5. Choose Print Graph from the File menu and print a graph showing both Data. C. Molal Freezing Point Constant of Cyclohexane plus Naphthalene (second time). 1. Measure out about 20 mL of cyclohexane and place in your large test-tube. Weigh test-tube + beaker + cyclohexane and record mass. 2. Weigh out about 0.15 - 0.20 grams of Naphthalene and record mass. Use weighing paper. Carefully remove stopper and thermometer from the top of the cyclohexane and add the naphthalene. Replace the stopper assembly for another freezing point run. 3. Repeat steps A.4 through A.8 for the second naphthalene - cyclohexane solution. 4. Discard the solution into the waste container provided for this purpose (ask your instructor). 5. Print out a plot of the cooling curve data for the naphthalene -cyclohexane solution. 6. Clean-up and disassemble apparatus.

*

You should probably take a look at your data and see if you can get reasonable freezing points for each of your three trials before putting things away. You may wish to do another run on some of these, since the process doesn’t take too long.

*

Note you must also completely finish today’s lab activity and turn it in, since this is the last scheduled lab meeting.

Alternatively, .15 to .20g of naphthalene can be added to the solution from Part Band refrozen. You lab instructor will tell you what to do for Part C.

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PRELAB QUESTIONS

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

You should prepare in advance (prior to coming to lab) to answer questions based on this lab. You will be quizzed on concepts taken from this lab similar to those listed below. Further reference materials may be found in your textbook.

You should be able to answer any question (similar to the following) about the concepts listed below: 1)

From the data below, compute the freezing point depression constant Mass of first sample of cyclohexane 11.323 g Freezing point of cyclohexane

5.90 oC

Mass of naphthalene 0.268 g Freezing point of naphthalene-cyclohexane solution 2.29 oC Ans.: Kf,cyclohexane = Tf / m Tf = (2.29 oC - 5.90 oC) = 3.61 oC and m = moles solute = 0.268 g C10H8 (1 mol C10H8/ 128.12 g C10H8) = 0.185 m kg solvent 0.011323 kg Kf,cyclohexane = Tf / m = 3.61 oC / 0.185 m = 19.5 oC / m

2) You should be able to answer questions from the procedure such as: a) What is a colligative property? b) What precautions are taken to ensure that the correct freezing points of pure cyclohexane and of the solutions are measured? c) How do you determine the freezing point of a solution that does not have a well-defined transition in the cooling curve? d) How will the solution be stirred with the temperature probe inserted into the test tube? e) What temperature should the ice-bath be stabilized at for this experiment?

Determination of Freezing Point Constant

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DATA & CALCULATIONS

Your Name: __________________________

Date: _________________

Lab Partner: __________________________

Check all entries for consistent application of the rules for significant digits and use correct units. A. Determination of the Freezing Point of Cyclohexane. 1.

Mass of test tube, beaker and cyclohexane

________________

2.

Mass of test tube and beaker

________________

3.

Mass of cyclohexane

________________

4.

Freezing point of cyclohexane

________________

B. Molal Freezing Point Constant of Cyclohexane + Naphthalene. 5.

Mass of weighing paper and naphthalene

________________

6.

Mass of weighing paper

________________

7.

Mass of naphthalene

________________

8.

Mass of cyclohexane

________________

9.

Freezing point of naphthalene-cyclohexane solution

________________

10. Molality of cyclohexane solution

________________

11.

________________

Freezing point depression

12. Freezing point depression constant, Kf

________________

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Experiment # 13

C. Molal Freezing Point Constant of Cyclohexane + Naphthalene 2nd time. 13. Mass of test tube, beaker and cyclohexane

________________

14. Mass of test tube and beaker

________________

15. Mass of cyclohexane

________________

16. Mass of weighing paper and naphthalene

________________

17. Mass of weighing paper

________________

18. Mass of naphthalene

________________

19. Freezing point of naphthalene -cyclohexane solution

________________

20. Freezing point depression

________________

21. Molality of naphthalene - cyclohexane solution

________________

22. Freezing point depression constant, Kf

________________

23. Average Freezing point depression constant, Kf (Part B and C)

________________

Be Sure to attach the graphs of the cooling curves which you used to determine your Freezing Points Sample Calculations:

Determination of Freezing Point Constant

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FINAL QUESTIONS

1) What are the three important sources of error in this experiment? How would these errors affect the value of the freezing point depression constant, Kf?

2) What will be the effect on the freezing point depression constant, Kf, for each of the following? Explain each answer. (Hint think how these may affect your measured values)

a) The freezing point is recorded with a large amount of frozen solid (for Part B only).

b) A small amount of cyclohexane was spilled unknowingly after weighing (Part B only).

c) Not all the naphthalene dissolved in the cyclohexane (Part B only)....