Crystallization Lab Report Text

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It is one of the fundamental procedures each chemist must master to become proficient in the laboratory. Crystallization is based on the principles of solubility: compounds solutes tend to be more soluble in hot liquids solvents than they are in cold liquids. If a saturated hot solution is allowed to cool, the solute is no longer soluble in the solvent and forms crystals of pure compound. Impurities are excluded from the growing crystals and the pure solid crystals can be separated from the dissolved impurities by filtration. This simplified scientific description of crystallization does not give a realistic picture of how the process is accomplished in the laboratory. Rather, successful crystallization relies on a blend of science and art its success depends more on experimentation, observation, imagination, and skill than on mathematical and physical predictions.

Understanding the process of crystallization in itself will not make a student a master crystallizer this understanding must be combined with laboratory practice to gain proficiency in this technique. To crystallize an impure, solid compound, add just enough hot solvent to it to completely dissolve it. The flask then contains a hot solution, in which solute molecules both the desired compound and impurities move freely among the hot solvent molecules. As the solution cools, the solvent can no longer hold all of the solute molecules, and they begin to leave the solution and form solid crystals. During this cooling, each solute molecule in turn approaches a growing crystal and rests on the crystal surface. If the geometry of the molecule fits that of the crystal, it will be more likely to remain on the crystal than it is to go back into the solution. Therefore, each growing crystal consists of only one type of molecule, the solute.

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After the solution has come to room temperature, it is carefully set in an ice bath to complete the crystallization process. The chilled solution is then filtered to isolate the pure crystals and the crystals are rinsed with chilled solvent. This first series of diagrams shows what happens if you let a crystallization proceed slowly. First by setting the flask at room temperature undisturbed until crystals form, and then carefully on ice. The red bar to the right of each image is a thermometer, to indicate the temperature.

If the solution is allowed to cool slowly, the impurities may attach briefly to the growing crystal lattice, but they soon leave as a compound with a more suitable geometry comes in to take their place. Suitable hexagons stay more readily in the growing lattice, and eventually pure crystals of orange hexagons are formed. The insoluble impurities are extracted from the mixture by dissolving the mixture in a hot solvent and filtered out. The soluble impurities are dissolved in the hot solvent and then remain in solution when the solvent is cooled, while the pure substance crystallizes. Therefore, the desired product should be as soluble as possible in hot solvent and as insoluble as possible in cold solvent.

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The selection of solvent is crucial as demonstrated by experiment 1 where the soluability of different polar and non polar substances is tested with polar and non polar solvents. The purpose of this report is to fully understand and master the ability to recrystalize an impure solid mixture, as it is an important tool that will be used in the lab many times. Drying the product transfer approximately 10 mg of each of the given solids antrhacene anthracene. Benzoic acid, and resocinol, into three clean microscale test tubes there will be nine total test tubes. Using a plastic pipette, drop.25 ml of one of the solvents water, toluene, ligroin into one of each of the test tubes containing each solid. Repeat this step for each solvent so that every possible combination of solvent to solid is made. Stir each solid solvent mixture with a clean 4mm glass stirring rod, breaking up any clumps.

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Observe each mixture and determine if the solid is readily soluble at room temperature. If the mixture is readily soluble at room temperature, add a few drops of de ionized water to see if a solid precipitates. If a solid does precipitate, heat the mixture in a sand bath and adjust the composition of the solvent pair to produce a hot solution saturated at the boiling point. Promptly remove the solution from heat and let stand undisturbed, observing the crystals that form. If the mixture is not readily soluble at room temperature, heat the mixture in a sand bath and observe if solution occurs. If the solid dissolves completely, it can be declared readily soluble in the hot solvent if some but not all dissolves, it is said to be moderately soluble, and further small amounts of solvent should be added via plastic pipette until solution is complete.

Once the mixture is dissolved in the hot solvent, cool the solution by either running the test tube under cool tap water, rubbing the walls of the test tube with the glass stirring rod to replace solids that may have sloshed up back to the main mixture body to ensure the correct solid to solvent concentration for crystallization. After crystals have formed, reheat the mixture to dissolve the solid, remove from heat, and let stand undisturbed for ultimate crystals to form. Observe all crystals noting crystal size, shape, and form eg needles, plates, and prisims.

Gently heat the water solid mixture in a sand bath until it comes to a boil the deeper the tube is in the sand, the hotter it will be. As soon as boiling begins, add water drop wise until all the solid just dissolves. Further cool the tube on ice and add a few drops of ice cold ethanol to the tube in order to remove water from the crystals. Once all the solvent is removed, using a stainless steel spatula, scrape the crystals onto a piece of filter paper. Fold the paper over the crystals and carefully squeeze out excess water before allowing the crystals to dry to constant weight. Using a 50 ml erlenmeyer flask, dissolve 15 g of raw sugar in a minimal amount of water by gently heating and stirring.

Allow the heated solution to cool slightly and add 250 mg of decolorizing charcoal norit pellets. Use a previously heated funnel and fluted filter paper to filter the hot solution. Repeat the same process for the remaining half of the solution, but use only 50 mg of the decolorizing charcoal, while boiling the solution for only 15 s after the addition of the charcoal.

The unknown used in this experiment is the product from the aldol rxn produced by the class in the first lab. Note that this procedure is slightly different from that described in the williamson lab manual. Weigh out 180 200 mg of the unknown substance, and place into a microscale test tube. Dissolve the unknown in a 90%/10% etoh/water mixture, by adding the solvent drop wise. Using a labquest and meltemp apparatus, record the melting temperature of the crystals. Source: williamson's macroscale and microscale organic experiments, 4th edition, lab 2, beginning on pg 56. Resorcinol: white, mid size grain powder this lab consisted of four separate experiments that were run, all involving techniques and procedures used in crystallization formation.

Crystalline organic substances are composed of molecules that are held together by van der waal's forces, which are relatively weak electrostatic forces between molecules. Due to the weak nature of these forces, most organic solids have a fairly low melting point, ranging from room temperature to 250°celsius williamson, k. Purification of a solid substance, in order to obtain crystals, involves dissolving the material in a hot solvent and then allowing the resulting solution to cool at room temperature. When the solute is dissolved in the hot solvent, the weak van der waal's forces holding all of the molecules together are broken and the solute molecules impurities and the desired compound precipitate. As the solution cools, the solvent is no longer able to hold all of the molecules and the solute leaves the solution to form crystals. Molecules with similar geometry will adhere to the crystal, while the impurities will remain in the solution.

Once crystals have formed, filtration is used to collect them and they are washed with a cold solvent to remove any impurities that may still be sticking to them. The crystals are then allowed to dry and the dry substance is weighed to determine the amount of pure product obtained from the initial impure substance. objectives: the purpose of the experiment involves using different solvents and 3 different solids to determine solubility of each substance and subsequently obtain a pure substance by recrystallization. The solution is then placed in a heat bath additional solvent is added if the solution is insoluble at room temperature. Because crystallization is based on solubility, different solvents would have different affects on the formation of crystals.

In this experiment, three different compounds are combined with three different solvents, for a total of nine different crystallization procedures. Ligroin is a volatile, highly flammable, liquid hydrocarbon mixture used chiefly as a nonpolar solvent procedure: experiment 1 solubility test

    transfer a small amount of solid into a reaction tube and add approximately 0.25 ml of selected solvent. Stir solution with a stirring rod to determine if solid is readily soluble at room temperature. If solvent does not dissolve solid, heat suspension and observe if solution occurs.

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