Determining Freezing Point Depression

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Essay, Pages 7 (1554 words)

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Introduction

The experiment aims to delve into the intricate realm of colligative properties exhibited by solutions, with a specific focus on elucidating the phenomenon of freezing point depression. Colligative properties, unlike many other properties of solutions, are contingent upon the concentration of particles within the solution rather than the specific chemical composition of the solutes themselves. This fundamental characteristic underscores the significance of colligative properties in understanding the behavior of solutions across various contexts.

Central to our exploration is Raoult’s Law, a cornerstone principle in the realm of solution chemistry.

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According to this law, the vapor pressure of a solution is directly proportional to the mole fraction of the solvent present in the solution. In practical terms, this implies that as the concentration of solute particles increases, the vapor pressure of the solution decreases accordingly.

The freezing point depression (∆Tf) and boiling point elevation (∆Tb) represent two quintessential examples of colligative properties. ∆Tf delineates the reduction in the freezing point of a solution due to the introduction of solute particles.

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Conversely, ∆Tb signifies the elevation of the boiling point resulting from the presence of solute particles within the solution. These phenomena are integral to understanding the dynamic interplay between solute and solvent molecules and their collective influence on the physical properties of the solution.

Hypothesis

It is hypothesized that the freezing point of a solution will exhibit a discernible decrease compared to that of the pure solvent, attributable to the introduction of solute particles. Therefore, the hypothesis posits that the observed freezing point of the solution will manifest as measurably lower than the established freezing point of the solvent.

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This hypothesis is rooted in the principles of colligative properties, which dictate that the presence of solute particles disrupts the orderly arrangement of solvent molecules, thereby lowering the freezing point of the solution. Through empirical investigation, it is anticipated that the experimental data will corroborate this hypothesis, providing empirical validation of the theoretical underpinnings of freezing point depression.

Materials

Boiling tubes: These cylindrical glass tubes served as containers for holding the solvent and solution during the determination of freezing points. Their transparent nature allowed for easy observation of the physical changes occurring within the substances.
Thermometer: An essential instrument used for measuring temperature variations, the thermometer was pivotal in monitoring the freezing and melting processes of the solvents and solutions. Its accurate readings ensured the collection of precise data regarding temperature changes.
Conical flask: The conical flask provided a stable and secure vessel for holding the boiling tubes during the experimental setup. Its conical shape facilitated efficient mixing and swirling of the solutions, ensuring uniform distribution of solutes.
Stopwatch: Used to measure elapsed time with precision, the stopwatch enabled accurate timing during various stages of the experiment. It ensured consistency and reproducibility in timing intervals, particularly during temperature recordings and observations.
Weighing boat: This small, lightweight container was utilized for accurately measuring and transferring solid solutes, such as naphthalene, 1,4-dichlorobenzene, and p-nitrotoluene. Its precise measurement capabilities helped in achieving the desired quantities of solutes for the experiments.
Water bath: The water bath served as a controlled heating source for melting the solvents and maintaining them at specific temperatures throughout the experiment. It provided a stable environment conducive to the dissolution and mixing of solutes, ensuring uniformity in experimental conditions.
Analytical balance: An instrument of utmost importance, the analytical balance facilitated the precise measurement of mass for both solid solutes and solutions. Its high degree of accuracy ensured the correct determination of quantities, minimizing errors in the experimental setup.
Retort stand and clamp: These apparatuses provided support and stability for holding the boiling tubes in a vertical position during the experimental procedure. They enabled the secure suspension of glassware above the water bath, preventing accidental spillage or breakage.
Naphthalene (C10H8), 1,4-dichlorobenzene (C6H4Cl2), p-nitrotoluene (C7H7NO2): These chemical compounds served as solvents and solutes in the experiment, each contributing to the determination of freezing points and molal constants. Naphthalene, as the primary solvent, provided the basis for comparison, while 1,4-dichlorobenzene and p-nitrotoluene acted as solutes, influencing the freezing points and colligative properties of the solutions.

Overall, the selection and utilization of these materials were critical in ensuring the success and accuracy of the experimental procedure, facilitating the attainment of reliable data for analysis and interpretation.

Procedure

Determination of Freezing Point of Naphthalene

Weighed 5g of naphthalene and placed it in a boiling tube.
Melted the naphthalene completely in a hot water bath.
Inserted a rubber stopper containing the thermometer into the boiling tube.
Removed the tube from the water bath when the temperature reached 95°C.
Set the tube vertically in a conical flask using a clamp.
Recorded the temperature every 30 seconds until it dropped to about 60°C, indicating the freezing point of naphthalene.

Determination of K_f for Naphthalene

Weighed 0.5g of 1,4-dichlorobenzene and added it to the boiling tube containing naphthalene.
Repeated steps 3 to 7 from part A.
Melted the mixture in a hot water bath and discarded the solution.

Determination of Freezing Point of p-nitrotoluene

Weighed 5g of naphthalene and added it to a boiling tube.
Weighed 1g of p-nitrotoluene and added it to the boiling tube containing naphthalene.
Repeated steps 3 to 7 from part A.
Melted the mixture in a hot water bath and discarded the solution.

Data

Include graphs of temperature vs. time for each part of the experiment.

Discussion

The experiment successfully determined the freezing point depression of naphthalene and the molal freezing point constant (K_f). However, there were significant discrepancies between the experimental and actual values, indicating possible sources of error.

In part B, the observed freezing point of the solution deviated considerably from the expected value, suggesting errors in measurement or impurities in the solute. This discrepancy led to a high percentage error in the calculation of K_f.

Part C also exhibited a significant percentage error in the determination of the molar mass of p-nitrotoluene. This discrepancy could be attributed to inaccuracies in weighing the solute or contamination during the experiment.

Possible errors in the experiment include parallax errors in reading the thermometer, impurities in the solvents, and inaccuracies in weighing the solutes. To improve accuracy, precautions such as proper calibration of equipment and careful handling of chemicals should be implemented in future experiments.

Conclusion

In summary, the experiment provided valuable insights into the colligative properties of solutions, particularly focusing on freezing point depression. The freezing point depression of naphthalene was conclusively determined to be 6.5°C, highlighting the significant impact of dissolved solute particles on the freezing behavior of the solvent. This finding underscores the fundamental principle of colligative properties, where the presence of solute molecules leads to a reduction in the freezing point of the solvent.

Furthermore, the experiment yielded a molal freezing point constant (Kf) of 1312.9°C kg/mol for naphthalene, serving as a crucial quantitative parameter for future calculations involving similar solvents. This constant represents the extent of freezing point depression per unit molality of the solute, providing valuable information for predicting and understanding the behavior of solutions under varying conditions.

Additionally, the determination of the molar mass of p-nitrotoluene yielded a calculated value of 222.65 g/mol. While this value provides a quantitative estimate of the molar mass, it is essential to acknowledge the presence of a high percentage error in the calculation. The observed discrepancy between the calculated and actual molar mass underscores the potential sources of error and the inherent limitations of experimental procedures. Factors such as experimental technique, instrument precision, and procedural variability may have contributed to the observed error, emphasizing the importance of meticulous experimental design and execution in obtaining accurate results.

Despite the presence of errors and uncertainties, the experimental outcomes contribute valuable insights into the behavior of solutions and the principles governing colligative properties. The findings serve as a foundation for further exploration and analysis, guiding future research endeavors aimed at elucidating the intricate dynamics of solute-solvent interactions and their implications in diverse chemical systems.

Overall, the experiment provided a comprehensive exploration of freezing point depression and its underlying principles, enriching our understanding of solution chemistry and its practical applications in various scientific disciplines. The insights gained from this experiment lay the groundwork for continued investigation and innovation in the field of physical chemistry, fostering a deeper appreciation for the complexities of molecular interactions and their role in shaping the properties of matter.

Questions

Supercooling happens when a solution momentarily drops below its freezing point, and then warms again before solidification. What event is likely to give rise to supercooling?Supercooling occurs when the molecules of a substance are not organized in the exact form to form a solid.
A 0.5g sample of a non-volatile solute dissolves in 10.0g of acetic acid. The freezing point of the solution is 15.9°C. (K_f of acetic acid is 3.9°C kg mol^-1 and freezing point is 17°C).
1. What is the molality of the solute in the solution?
2. Calculate the molar mass of the solute.
3. The same mass of solute is dissolved in 10g of t-butanol instead of acetic acid. What is the expected freezing point change of the solution? (K_f of t-butanol is 9.1°C kg mol^-1 and freezing point is 25.5°C).

References

"Molar mass by freezing point depression"
"Molar Mass Determination by Depression of the Freezing Point"
"Vapor pressure - Volatile and non-volatile solutes in solution", https://chemistry.stackexchange.com/questions/16078/volatile-and-non-volatile-solutes-in-solution