Freezing Point Depression Determination
Freezing Point Depression Determination
This paper is a full determination for certain chemicals and their boiling points. It lists some already but given the atomic numbers of any material this project includes a conversion and calculation chart to find the freezing point of most any material. GOOD LUCK!
In this lab we determined the freezing point, and Kf, of pure 2,4,dichloralbenzne as well as a 2,4,dichloralbenzne/biphenyl solution. We used this information to determine the molar mass of an unknown (#24) by the 3rd step in the experiment which was a 2,4,dichloralbenzne/unknown solution. All of the above we charted the time temperature for the later calculations.
This experiment shows how one determines the temperature-composition diagram for a two-component system. The procedure will consist of obtaining cooling curves for the pure substances and a number of their mixtures. A cooling curve is constructed by melting a sample, then allowing it to cool, measuring the temperature at regular intervals. When only melt is present, there is a constant cooling rate. As the solid begins to form, the system remains at a constant temperature until the melt is completely converted to solid. The eutectic composition is that at which two solids crystallize out in a ratio equal to that of the melt, and the cooling curve obtained would have the same characteristics as that of a pure substance. The eutectic temperature is the melting point of such a mixture.. The addition of impurity to each of the pure components decreases the freezing point so that two curves are obtained which intersect at the eutectic point.
The freezing point of a solvent depends upon the concentration of the dissolved solute and the nature of the solvent. If the dissolved solute is a nonelectrolyte, then the decrease in the freezing point, DELTA T, is proportional to the molality, m,( moles of solute per kg of solvent) of a dilute solution according to the equation:
DELTA T = Kfm
Where Kf is the molar freezing-point depression constant unique for each solvent.
Super cooling often occurs in this system. The cooling curve will show a momentary dip below the mixture’s freezing point, and then will revert back up to the freezing point. This is usually helpful in determining the freezing point, provided the super cooling is not too pronounced. Excess super cooling may be avoided by seeding the melt with very small crystals of the solid.
B. Purpose of the investigation.
This experiment as stated above shows how one determines the temperature-composition diagram for a two-component system. The freezing point of a solvent depends upon the concentration of the dissolved solute and the nature of the solvent. If you follow the procedure stated below you will learn about freezing point depression as well as understand why we use certain solutions for different applications like antifreeze and break fluid, and it’s not just limited to cars.
First thing you will need to do is Preview the procedure and understand what is going on so you know all the precautions, how to obtain chemicals execute the experiment clean up and make the calculations. In the first step you will need to gather all the supplies you need.
Needed interments and chemicals(Gather the following ):
1. A cooled down hot plate
2. A support ring
3. 600 ml beaker filled of tap water and 2 or 3 boiling chips
4. 16×150-mm test tube
5. A bent wire or glass rod for stirring
6. 2 clamps
8. cork to hold thermometer
9. chemicals stated below as needed
B. Outline of procedure
Steps to execute procedure
1. RE-READ ALL SAFETY INRUCTIONS (BELOW)
2. Gather supplies
3. Clean all glassware and thermometer and wipe clean with chem. wipes
4. Set up apparatus like to the right
5. Measure the test tube to the nearest .01 gram
6. Add 5 to 6 grams of 2,4,dichloralbenzne
7. than re-measure the test tube
8. record that on your data sheet
9. use the stirrer to push all of the solid down into the tube so it can melt
10. put apparatus back together
11. You will use all of the 2,4,dichloralbenzne from Part I for Part II.
12. Allow the contents of the test tube to solidify and the test tube to cool to room temperature. Stand the test tube in a beaker of cool tap water to speed the cooling. Dry the outside wall of the test tube before you begin Part II.
13. Weigh the dry test tube from Part I to the nearest 0.01g. Record this mass on Data Sheet 2. Transfer 0.9-1.3 g of biphenyl to the test tube and weigh the tube again.
14. Record this mass on Data Sheet 2.
15. heat the content to 65 degrees c
16. Remove the heat source and let the beaker slowly cool. Stir the liquid in the test tube as it cools.
17. Record the temperature to the nearest 0.1 °C, at 1-min intervals. Record the temperature-time readings on Data Sheet 2
18. Note the temperature at which the first crystals appear and the maximum temperature reached after crystallization begins.
19. Continue observing and recording your measurements until the 2,4,dichloralbenzne/biphenyl temperature begins to decrease again.
20. Using a test tube holder, pick up the hot test tube. Inside a fume hood, pour the test tube contents into the container labeled “Discarded Solutions”.
21. Clean the solid from the test tube, thermometer, and stirrer with acetone. Discard the acetone washings inside the fume hood into the “Discarded Solutions” container.
22. Thoroughly dry the test tube for use in Part III.
23. Obtain an approximately 1 -g sample of an unknown from your laboratory instructor.
24. Record the identification code of your unknown on. Data Sheet 3.
25. Prepare a solution of 2,4,dichloralbenzne and your unknown, using the following procedure
26. Measure the mass of the clean, dry test tube used in the experimental apparatus, to the nearest 0.01 g.
27. Add 5-6 g of 2,4,dichloralbenzne to the test tube and measure the mass to Nearest 0.01 g. Record this mass on Data Sheet 3.
28. Transfer 0.9-1.3 g of your unknown compound to the test tube and weigh the test tube again. Record this.
29. heat the content to 65 degrees c
30. Remove the heat source and let the beaker slowly cool. Stir the liquid in the test tube as it cools.
31. Record the temperature to the nearest 0.1 °C, at 1-min intervals. Record the temperature-time readings on Data Sheet 3
32. Note the temperature at which the first crystals appear and the maximum temperature reached after crystallization begins.
33. Continue observing and recording your measurements until the 2,4,dichloralbenzne/unknown temperature begins to decrease again.
34. Using a test tube holder, pick up the hot test tube.
35. Inside a fume hood, pour the test tube contents into the container labeled “Discarded Solutions”.
36. Clean the solid from the test tube, thermometer, and stirrer with acetone. Discard the acetone washings inside the fume hood into the “Discarded Solutions” container.
1. avoid inhaling fumes
2. don’t touch hot test tubes
3. only use acetone inside a fume hood
4. wash you hands quickly if chemical contact is made with skin
5. biphenyl is flammable and suspected carcinogen they are irritants and some are toxic
6. wear safety goggles
7. be careful with thermometer don’t break it when setting up
8. when you leave lab make sure to wash you hands well
III Results-Data and Observations
Colligative properties of solutions depend upon the concentration of solute particles equal concentrations of ions or molecules cause equal freezing point depressions, regardless of what the solute is. And my experimental results agree with this prediction
Since colligative properties depend upon the number of particles in solution, a one mole solution of an electrolyte, which dissociates in solvent, lowers the freezing point more than a one mole solution of a non-electrolyte. This difference is believed to be due to the interionic attractions that prevent the ions from behaving as totally independent particles. The activity or effective concentration of the ions is less than would be indicated by the actual concentration. Some of the ions may exist as solvated units called an ion pairs. The more dilute the solution of an electrolyte, the more widely separated the ions, the less the interionic attractions, and the closer the effective concentration of the ions approaches the actual concentration.