Lab Session 4- Freezing point depression

Objective: To investigate the relationship between the molality and the freezing point of a solution.



Hypothesis:

In order for a liquid to freeze, the formation of regular structures inside it is essential. As a solution is the mixture of a solvent and a different solute it involves two or more compounds, with different particle structures. Hence, as the concentration (molality = mole of solute/ kg of solvent) increases, there would be a higher amount of the uneven mixture and therefore it would be more difficult for the solution to form regular structures. All in all, as stated by the formula which determines the change in freezing point, we could sum it up by saying that the freezing point of the new solution will decrease. The adding of more solute, making a greater molality, will cause a Freezing point depression. 



Table of results

Mass of sugar in solution (g)	Molality (mol/kg)	Attempt 1 - Freezing point (oC)	Attempt 2 - Freezing point (oC)	Average freezing point (oC)	Change in freezing point compared to pure water (oC)
0	0	0	0.6	-0.3	0.3
0.5	0.2	0.6	0.8	- 0.7	0.7
1.0	0.6	0.3	0.1	-0.2	0.2
1.5	0.8	1.7	1.7	-1.7	1.7
2.0	1.2	2.0	2.2	-2.1	2.1
2.5	1.4	2.9	2.5	-2.7	2.7

Graph

Conclusion

We are able to conclude from this graph that our hypothesis was correct. Regarding the line of best fit, it shows a inversely proportional relationship beween molality and freezing point. This means that as the molality of a solution increases, its freezing point decreases and freezing point depression occurs. This happens due to the fact that if there are more molecules of solute dissolved in a solvent, the structure of the solution is not be nice and fixed, and consequently it is more difficult to turn from a liquid to a solid. Therefore, it would need a lower temperature to freeze.

Evaluation of results

Even though most of the results are coherent, we are able to see an incoherece in the table of results that the 0,6 molality solution has a freezing point of -0,2. This makes no sense concerning the fact that the 0 molality solution has a freezing point of -0,3 and the 0,2 molality solution of -0,7. Therefore, the 0,6 molality solution should have had a lower freezing point than the one of 0,2 molality, as the more solute the solution has, the bigger the freezing point it. This might have been caused by an error in the procedure. Consequently, we have erased this abnormality when drawing the graph.

Evaluation of method

-       A major problem with the method was the determination of the freezing point. As we had to determine it ourselves, we never knew the exact point at which the solution had formed an regular ice structure. Hence, it could be stressed that the measuring requirements of the method were not accurate enough. Instead, in order to dela with this problem a machine could be used to determine the excat point at which the solution freezes so that there is no possible deviation in the results accuracy.

-       Moreover, it was also notified the difficulty of cooling down the solution to the exact temperature at which it was supposed to freeze. All in all, freezing a test tube filled with water by using a bucket full of ice is not an accurate method of calculating the new freezing point of the solution. As time passed by, the ice started to melt and although salt was added inside the external mixture, temperature kept decreasing due to room temperature. If I was to perform the experiment again I would use a motorized refrigerator which would measure constantly temperatures every second and device which could determine the exact formation of regular structures inside the mixture.

-       Finally, when the last test tubes with the highest concentrations were introduced inside the beaker full of ice, we must highlight that temperature was almost bellow -3ºC. Regarding Rault’s Law, a solution freezes when it structure becomes regular. This is a progressive reaction, which in turn means that it needs to be triggered by a factor which can start the reaction. Considering that temperatures inside the beaker were already lower than their actual freezing points, we found out that after 5 minutes inside the ice mixture, the solution didn’t form crystals. This was due to the fact that temperature was always constant, stopping crystal formation. To prevent this problem I would consider using a different recipient to cool down each beaker and hence create a temperature depression that can trigger crystal formation.

References

-Cssac.unc.edu,. (2015). Raoult's Law; boiling point elevation; freezing point depression — CSSAC. Retrieved 17 February 2015, from https://cssac.unc.edu/programs/learning-center/Resources/Study/Guides/Chemistry%20102/Solutions

-Chem.purdue.edu,. (2015). Freezing Point Depression. Retrieved 17 February 2015, from http://www.chem.purdue.edu/gchelp/solutions/freeze.html

-Kenkel, J. (2011). Basic chemistry concepts and exercises. Boca Raton: CRC Press.