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fei swinton

on 8 February 2013

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Transcript of Recrystallization

Purification of Benzoic Acid by Recrystallization INTRODUCTION I n t r o d u c t i o n Purification physical separation of impurities and by-products from an impure sample pure substance. Distillation for liquid compounds Recrystallization for solid compounds Recrystallization precipitation of a solid compound from a saturated solution in the form of crystals through cooling or evaporation STEPS : 1) selecting the appropriate solvent 2) dissolving the solute 3) decolorizing the solution 4) removing suspended solids 5) crystallizing the solute 6) collecting and washing the crystals 7) drying the product impure solid first dissolved in an appropriate solvent when heated appropriate/ideal solvent dissolves component to be purified when hot (boiling point) does NOT dissolve component to be purified when cold= room temp.or 0 degree C either dissolves impurities when cold OR does NOT dissolve impurities when hot - components will then crystallize in its purer form as the solution is cooled. Less soluble component will crystallize first As the crystals are formed, the correct molecules that will fit in the crystal lattice are selected while the wrong molecules are ignored, resulting in a pure solid Depending on ....... 1) the solubility of the compound 2) the differences in solubility of the desired solutes & impurities dissolved in the solvent P R O C E D U R E 1. Weigh 100mg of the ground sample (impure Benzoic Acid) 2. Add a small piece of boiling chip, and 2mL of distilled water into the vial with benzoic acid 3. With constant swirling, heat the mixture to boiling until the sample dissolves completely, adding more solvent if necessary. 4. Cool the solution by adding a few more drops of distilled water. Add a pinch of activated carbon. Heat for a few minutes until a change is observed in the mixture and until its volume goes down to about 1mL. 7. Collect the crystals in a previously weighed filter paper that is set on an improvised filter paper support. Rinse the vial with small increments of ice-cold water to collect the remaining crystals in it. 5. Pour the hot mixture quickly into a filter syringe collecting the filtrate into a vial. Force the liquid through the filter by using pressure from the piston of the syringe. 6. Let the hot saturated solution cool to room temperature, then cool in an ice bath. 8. Open the filter paper, fold it with the crystals inside and squeeze out the water by inserting it between folded tissue paper and pressing on it. 9. Let the filter paper and the crystals dry completely, then weigh. RESULTS From the 0.100 g of the impure sample of benzoic acid, 0.030 g of pure benzoic crystals was yielded, having a percent recovery of 30%. D I S C U S S I O N The chemical equation for the reaction of benzoic acid with water is as follows: It is a reversible reaction allows the dissolved benzoic acid to recrystallize or go back to its solid formas a purer compound. Water
= appropriate or ideal solvent for acid due to huge solubility difference in water. The mixture constantly swirled as the mixture was being heated increases rate of dissolution and entropy of the system increasing the interaction between water and benzoic acid molecules. The complete dissolution of benzoic acid led to a clear solution. Adding the boiling chip while at room temperature. it prevents the solution from boiling over It induces boiling of the mixture, which prevents it from spilling over Activated charcoal- used for decolorization made up of finely separated C atoms w/ great surface area adsorb the impurities from solution C atoms too BIG to pass thru filter paper minimizes impurities thus increasing purity Too much charcoal may cause loss of pure substance Filter syringe used to increase purity to filter out activated charcoal & other impurities on the cotton plug Solution was filtered rapidly to have as little residue of benzoic acid crystals as possible yielded from crystallization already taking place as the temp.& solubility of the benzoic acid was decreasing. Slow cooling in an ice bath to help crystals arrange in a finer undistorted molecular geometry, excluding impurities from the crystal formation. In case crystallization doesn’t take place while cooling use SEED CRYSTAL Its similar structure to the pure crystals will serve as “source code” only molecules that will fit perfectly in the lattice will crystallize, leaving out impurities with a different structure. Complete drying of the filter paper and crystals minimize inaccuracy in results Formula for percent recovery was used: A N S W E R T O G U I D E Q U E S T I O N S 1. List the properties of an ideal solvent to perform the purification of an organic compound by the crystallization technique. • dissolve the solute at high temperatures or boiling point • does NOT dissolve solute at low or room temperature • either dissolves impurities (at room temp) or NOT dissolve them at all (at high temp) •should NOT react with solute • volatile 2. In purifying by crystallization from a solvent, is it advisable to cool the solution slowly or rapidly? Explain. COOL SLOWLY so that crystals will form in its most correct& undistorted molecular geometry, leaving impurities out from the crystal formation. Rapid cooling will result a greater yield in the end (because impurities will have been included), thus leading to inaccurate results. 3. What advantages does water have as a crystallization solvent? •non-flammable •safe and non-toxic •used for a wide range of temperatures •high heat capacity and can dissolve different substances •polar, thus, it can dissolve substances, especially polar ones, easily. 4. Solubility of benzoic acid in water: 0.21g/ 100mL water at 10oC; 0.27g/ 100mL at 18oC; 2.75g/ 100mL at 80oC; 6.80g/ 100mL at 95oC. Two students crystallized 10 g samples of benzoic acid from water, the first dissolving benzoic acid at 80oC and filtering at 10oC, the second dissolving the acid at 95oC and filtering at 18oC. Calculate the quantity of water each student was required to use and the maximum recovery of benzoic acid possible in each case. 5. A Solid (X) is soluble in water to the extent of 1g per 100g of water at room temperature and 10g per 100g of water at the boiling point.
a. How would you purify X from a mixture of 10g of X with 0.1g of impurity Y, which is completely insoluble in water, and 1g of impurity Z, having the same solubility characteristics in water as X? b. How much pure X could be obtained after one recrystallization from water? Solubilities of different solutes are independent of each other, thus, 1 g of solutes X and Z would dissolve at room temperature. c. How much pure X could be obtained after one recrystallization from a mixture of 10g of X with 9g of Z? • X and Z must be dissolved first. Solubilities of X and Z are independent of each other. Solid X,being the larger amount, is used as basis. •Water must then be boiled until 900g of water is left to ensure impurity Z won’t dissolve and crystallize with X under room temperature. •Slowly cool the solution to form X crystals. d. Based on the result obtained, what is suggested about the use of crystallization as a purification technique? Using crystallization to separate and purify solid substances is not so efficient because of its heavy dependence on the substance’s solubility, making it difficult to separate substances of the same solubility and of significant amounts. C O N C L U S I O N Recrystallization, although a common purification technique for its simplicity and usefulness, is not such an efficient method because it CAN’T: •purify non-solids •separate compounds whose components (the main component and impurity) have the same solubility due to recrystallization’s heavy dependence on solubility, especially when both components exist in significant amounts •completely purify the end-product F I N
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