Lab Conclusion: Recrystallization & Melting Point Experiment – Brandon Skenandore
Total word count: 474
Time spent researching and writing: 2 hrs
The purpose of this laboratory experiment was to recrystallize impure benzoic acid and biphenyl products from the solvent lab the previous week. Also, the melting point of biphenyl was found in its pure and impure forms and compared to the melting point of the original mixture. First of all, the compounds from last week were remeasured to account for any loss in mass. Biphenyl’s mass was 1.013 g. Ethyl 4-aminobenzoate’s mass was 1.199 g. The mass of benzoic acid was .843 g. The total additive weight for these three compounds was 3.055 g, compared to 3.001 g originally measured. This is pretty close. The difference could be because there still was some moisture in the compounds that made the compounds weigh more. The original mixture measured also may not have contained the same amount of each compound, since so little a sample was used (…)
Maybe the original mixture was not mixed well enough, or the ethyl 4-aminobenzoate clung closely to the water molecules. Also, the volume of ethyl 4-aminobenzoate was twice that of the benzoic acid, which means that the benzoic acid is denser.
Once the two products were dissolved and started cooling, crystallization immediately took place, since the solution was fully saturated. The crystals of both benzoic acid and biphenyl looked like little shards of glass, which shows that these are indeed pure (or at least more pure that before) substances. However, there was a high decrease in yield during recrystallization, probably for at least four reasons. The first reason was that some of the compound stuck to the glass of the flask when pouring it into a Buchner vacuum funnel. The second reason was that some of the dried compound stuck to the actual funnel paper when transferring into a container. Also, some compound was lost during the transfer from the funnel to the container, as some was spilled on the counter. Finally, the fourth reason is the impurities of the compound stayed dissolved in the solvent when drying the crystals.
According to this data, the compounds were indeed separated last week, since the observed melting point of pure biphenyl was 63-65 ºC compared to the impure biphenyl of 58-62 ºC. The melting point of the original mixture was as low as 52-58 ºC. The separation of the biphenyl, therefore, was successful as the melting point of the impure compound was raised as well as narrowed to a difference of only 4 ºC. From there, the compound was purified and once again the melting point was raised and narrowed to a difference of only 2 ºC. However, this is lower than the literature value of 68.93 ºC. However, this literature value is at a higher atmospheric pressure since it is at sea level. This experiment was done at a higher altitude and there was therefore less atmospheric pressure, lowering the boiling point.