A student performs a crystallization on an impure sample of biphenyl. The sample weighs 0.5 g and contains about 5% impurity. Based on his knowledge of solubility the student decides to use benzene as the solvent. After crystallization, the crystals are dried and the final weight is found to be 0.02 g. Assume that all the steps in the crystallization are performed correctly, there are no spills, and the student lost very littly solid on any glassware or in any of the transfers. Why is the recovery so low?
thanks so much in advance!
How soluble is biphenyl is benzene? Perhaps it is just too soluble, and/or there is not enough difference in solubility at low temp and high temp.
I had this same question in my last lab. The reason this occurs is due to the solubility of biphenyl and benzene. There is not much difference between room temp and boiling so not much is going to crystallize out. Benzene is too good of a solvent for biphenyl and shouldn't be used for re-crystallization.
The low recovery of the final product in this crystallization process can be attributed to several factors:
1. Insufficient solubility: The solubility of the impure biphenyl in benzene may be significantly higher than that of the pure biphenyl. This means that some of the biphenyl may have remained in solution, along with the impurities, when the crystallization was performed. Therefore, when the crystals are collected and dried, they only represent a fraction of the total amount of biphenyl in the impure sample.
2. Loss during transfers: Despite the student's efforts to minimize loss during transfers, it is still likely that some of the biphenyl was lost during the various steps of the crystallization process. This could include loss during the filtration or transfer of the crystals, as well as losses due to handling or accidental spills.
3. Incomplete separation: Even though the crystallization process is designed to separate the desired compound from impurities, it is possible that complete separation was not achieved. Since the impure sample contains about 5% impurity, it is likely that some impurities remained in the final product even after the crystallization process.
It is worth noting that obtaining a 100% recovery is often challenging in any purification process, particularly for impure samples. The final recovery depends on various factors, including the solubility characteristics of the compounds involved, the efficiency of the purification method, and the skill of the experimenter in minimizing losses.
The low recovery in this experiment can be attributed to a few factors. Let's break it down step by step:
1. Initial sample weight: The student started with a sample of biphenyl that weighed 0.5 g. However, this sample also contained approximately 5% impurities. Since the impurities are not removed in the crystallization process, they will contribute to the overall weight loss.
2. Solubility in benzene: The student chose benzene as the solvent for crystallization based on its known solubility properties. Biphenyl is soluble in benzene, but the impurities might have different solubility properties. If the impurities are more soluble in benzene compared to biphenyl, they will dissolve along with biphenyl during the crystallization process, leading to loss during subsequent steps.
3. Recrystallization process: During the recrystallization process, the sample is dissolved in the solvent (benzene), and then the solution is cooled down to promote crystal formation. The crystals are subsequently collected and allowed to dry. In each step, there is a potential for loss due to imperfect techniques (e.g., incomplete transfer, spills, etc.).
4. Final weight: After drying, the student found the final weight of the crystals to be 0.02 g. This indicates that a significant portion of the initial material was lost during the crystallization process, possibly due to the reasons mentioned above.
To summarize, the low recovery is primarily due to the presence of impurities in the initial sample, the solubility of biphenyl and impurities in benzene, and the potential for loss during the recrystallization and drying steps.