Recrystallization was attempted in this experiment to purify the Benzoic Acid. Contaminated benzoic acid crystals were collected and used to find the melting point to determine the purity of Benzoic acid. Recrystallization is a way chemists are able to purify any contaminated solid samples. Melting range for Benzoic acid was estimated to be between 122?C-128?C. The known melting point of Benzoic acid recovered is 121-122?c, based on the experimental melting point that was recorded suggests the recovered Benzoic acid is 42% pure substance. Based on all the information gathered from the IR spectrum, it was able to identify a carbonyl group at 1700 cm-1. . At 1600cm-1 we identified a C=C bond at that frequency, which in the known IR it is shown in the same position.
The objective of the experiment was to recrystallize a sample of Impure Benzoic acid and determine the % recovery. While using melting point and IR spectroscopy to determine the purity.
Recrystallization is the process when a solute dissolves into a hot solvent and then forms a solid again by crystallizing in a cooled solvent. Pure compounds contain strong molecular bonds that allow them to form crystals where as impure substances boast weaker bonds which do not allow them to form crystals. This allows the impure substances to separate from the crystallized Benzoic acid. The crystal substance is dissolved in a hot solvent which will increase the solubility of the solution and eliminate any impure substances. Then the solution is cooled in an ice-water bath which will decrease the solubility of the solution causing the pure substance to recrystallize. The slow cooling of the substance promotes formation of pure crystals, because the molecules of the impurities do not fit well with the newly forming crystals. Recrystallization is the primary method for purifying solid organic compounds.Pure sugar and salt are obtained through recrystallization in every day life. This is done by dissolving sugar in a water and heating the solution. After which it is made to cool slowly at room temperature. As it cools, it begins to form crystals of pure sugar. The sugar is now pure because it doesn’t trap the impurities while recrystallizing .
Materials ; Methods:
In this experiment we gathered two 250 ml Erlenmeyer Flasks. Four beakers were gathered one 600ml, two 300 ml, and one 50 ml. Other materials gathered for the experiment included: One hot plate, ring stand, one vacuum pump, two Buchner funnels, 2 circle cut coffee filters, two Buchner funnel filters, two stir rods, 4 boiling chips, 2 small trays, measuring balance, mp capillaries, mp apparatus, distilled water, mortar and pestle, and an IR spectrophotometer. During this Experiment the Chemicals used were Benzoic acid, DI water, and Activated Charcoal.
To start the experiment, 2g of impure benzoic acid are measured out. After 45 ml of DI water was measured out. The benzoic acid was placed into a 125 ml erlenmeyer flask along with the 45 ml of water. Then placed 5 boiling chips inside to keep from boiling over and placed the erlenmeyer flask on a hot plate where we would wait for it to boil. There was also placed a 50 ml beaker filled with distilled water on the hot plate. Reason being that the use this water to wet the surface of the Buchner funnel to help guide the liquid through the paper. Then a set up of the vacuum filtration that would help purify the benzoic acid. A Buchner funnel was placed onto a separate empty 125ml Erlenmeyer flask where we would quickly pour the boiling benzoic acid into to capture contaminates. We would then throw gathered contaminates and reboil already filtered benzoic acid. Once crystals were dissolved we then let it cool to room temperature and place the erlenmeyer flask containing benzoic acid into an ice bath. This was done to help crystalize the benzoic acid. We then placed 5 ml of water into flask containing benzoic acid to help unstick the crystal from the flask to pour out and gather the purified crystals. We then gathered them onto a transfer plate to dry. Once dry we would use the melting point apparatus to determine the melting point. From this we would be able to determine the purity of the benzoic acid. A IR Spectrophotometer was used to gather an image to further compare with the known IR of pure benzoic acid.
Throughout the experiment, we made observations from the start. During the start of the distillation we placed the activated carbon causing the already impure benzoic acid to turn dark black. We did not see any changes to the color while the mixture was being boiled. When pouring mixture through vacuum filtration we did notice that the filter paper was not cut properly causing some activated carbon to escape through to the mixture.Another observation was made when scooping out crystals we did notice that there was still activated carbon remaining and also that the crystals were not properly dry. Because the crystals were not properly dry we believe that is why our melting point was not accurate.
The original mass of gathered of the impure Benzoic acid was 2.02g. The recovered benzoic acid had a mass of .864g. The calculated percent recovery was 42%. The equation used was calculated in the steps below:
% Percent Recovery = Mass of recovered / Mass of impure Material x 100%
% Recovery = (.864g / 2.02g) x 100
% Recovery = 42%
Melting Point results were collected through two separate trials:
Based on the data seen from the known IR. We were able to identify frequencies that were in also seen in the Experimental IR.
Typically, at lower temperatures, the solubility of materials in solvents are much lower than higher temperatures, so if we want to rinse and avoid dissolution of material, we must have it at ice cold temperatures. There could be supersaturation, and will not let 100% of crystals form, therefore, lower yield was expected. What we were able to gather from filtering out the contaminates and completing the recrystallization process was that it resulted in giving us a 42% recovery of benzoic acid. The melting point of benzoic acid is 121?C -123?C. The results we got from the melting point apparatus was 122?C – 128?C meaning that the goal of the experiment was within range and we were able to gather enough benzoic acid to classify it as so. The % recovery gathered was 42%. We feel that it was less then 50% recovery, because we failed to separate the remains of the activated carbon that was placed and the start of the experiment.