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Recrystallization of Benzoic Acid Outline

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Purification – physical separation of impurities and by-products from an impure sample pure substance. *Distillation- for liquid compounds,
*Recrystallization- solid compounds

– precipitation of a solid cpd. from a saturated solution in the form of crystals through cooling or evaporation -7 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
– 2 Kinds of Impurities: 1) more soluble
2) less soluble than the main component
– 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 0oC = 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 – Depends on: 1) solubility of the compound

2) differences in solubility of the desired solutes & impurities dissolved in the solvent – Common Solvents Used:
Solvent| Formula| Polarity| Boiling Pt. (oC)|
Water| H2O| Very Polar| 100|
Ethanol| CH3CH2OH| Polar| 78|
Methanol| CH3OH| Polar| 65|
dichloromethane| CH2Cl2| Slightly Polar| 40|
Diethyl ether| (CH3CH2)2O| Slightly Polar| 35|

IV. Experimental
0.100 g of powdered impure benzoic acid, 1 small piece of boiling chip, and 2 mL of distilled water were added respectively into a vial. While swirling the mixture constantly, it was then heated to boiling until the benzoic acid had dissolved completely in the water.

The solution was then cooled by adding a few drops of distilled water. A pinch of activated carbon was added after. Then, the mixture was heated to boiling for a second time for a few minutes until the volume of the mixture had decreased by approximately 1 mL.

The hot mixture was then immediately poured into a 5mL disposable syringe (without the needle) with a cotton plug inside at the bottom of the syringe tube to act as a filter. The piston of the syringe was then pressed to force the liquid through the filter, collecting the filtrate into another vial.

The hot saturated solution or filtrate was then cooled to room temperature before sealing the vial and slow-cooling it in an ice bath. The crystals yielded were collected in a pre-weighed filter paper on a 50 mL beaker (as an improvised filter paper support). Small increments of ice-cold water were added to rinse the vial to get the remaining crystals.

The filter paper containing the collected crystals was then carefully taken out from the filter system, then was folded with the crystals inside, and then pressed in between folded tissue paper to squeeze out excess water. After allowing the filter paper and crystals to dry up completely, the filter paper with the crystals was weighed. The weight of the filter paper alone was subtracted from the obtained weight to get the weight of the yielded crystals. Percent recovery was computed for by dividing the weight of the yielded crystals by the weight of the impure benzoic acid.

V. 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%.

VI. Discussion
The chemical equation for the reaction of benzoic acid with water is as follows: C6H5COOH(aq) + H2O(l) C6H5COO-(aq) + H3O+(aq).
It is a reversible reaction allows the dissolved benzoic acid to recrystallize or go back to its solid form as a purer compound. Water= appropriate or ideal solvent for benzoic acid due to huge solubility difference in water. * insoluble @ low temp 2.1g of benzoic acid dissolves in 1L of water at 10oC * soluble @ high temp 68g of benzoic acid dissolves in 1L of water at 95oC 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. Boiling chip- induces boiling

– added to the mixture while @ ROOM temp– NOT boiling temp– to prevent spilling or boiling over Activated charcoal- used for decolorization
– made up of finely separated C atoms w/ great surface area – adsorb the impurities from sol’n (aka it causes impurities to attach & be trapped onto the charcoal atom’s surface). – C atoms too BIG to pass thru filter paper minimizes impurities! *NOTE: Too much activated C loss of the pure substance. Filter syringe- used to increase purity to filter out activated charcoal & other impurities on the cotton plug Sol’n 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 because the added mass of water while weighing affects or increases the % recovery. Formula for percent recovery was used:

% recovery= weight recovered benzoic acid x 100 weight impure benzoic acid

VII. Answers to Questions
1. List the properties of an ideal solvent to perform the purification of an organic compound by the crystallization technique. * dissolve the solute @ high temperatures or boiling pt
* does NOT dissolve solute @ low or room temp.
* either dissolves impurities (@ room temp) or NOT dissolve them at all (@ high temp) so that impurities WON’T crystallize with solute & can be thus removed by filtration. * Inert= should NOT react with solute or there will be a lesser yield of pure crystals due to molecular interaction * volatile so it can be easily removed from the solute crystals.

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. STUDENT A

@80oC 10 g =2.75 g s
x mL 100mL
x= 363.636 mL H2O
@10oC y grams = 0.21 g d
363.636 mL 100mL
y= 0.7636g benzoic acid
9.24g benzoic acid
9.24g benzoic acid

10 g- 0.7636g=

@95oC 10 g = 6.80 g d
x mL 100mL
x= 147.058 mL H2O
@18oC y grams = 0.27 g d
147.058mL 100mL
9.60g benzoic acid acid
9.60g benzoic acid acid
y= 0.397 g benzoic acid

10 g- 0.397g=

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? Separating Y

1. Dissolve the mixture in 100g H2O and boil to 100⁰C. Everything should dissolve except for Y. 2. Cool mixture to room temperature
3. Filter. This will separate impurity Y from X and impurity Z. Separating Z
1. Add H2O up to 100mL to the filtrate.
2. Heat to boiling (100oC)
3. Slowly cool to room temp
4. Filter the mixture. The filter paper residue= pure X crystals. *Formation of Z is insignificant because it is only 1g and has a different crystal structure than X, excluding Z from X’s crystallization.

90% or 9g of X will be obtained
90% or 9g of X will be obtained
b. How much pure X could be obtained after one recrystallization from water? 10g of X – 1g X dissolved x 100% =
10 g of X *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? * A = 1000g H2O to dissolve 10g X @ room temp.
A = 1000g H2O to dissolve 10g X @ room temp.
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. 10g of X = 1g of X
A g H2O 100g H2O
* B= 900g H2O to dissolve 9g Z @ room temp.
B= 900g H2O to dissolve 9g Z @ room temp.
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. 9g of Z = 1g of Z
B g H2O 100g H2O
* C= 9g of X dissolved in
900g H2O @ room temp.

C= 9g of X dissolved in
900g H2O @ room temp.

Slowly cool the solution to form X crystals.
C g of X = 1g of X
900g H2O 100g H2O

1 g of X crystallized
1 g of X crystallized

10g X- 9g X dissolved=

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. VIII. Conclusion

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

IX. References

“Crystallization.” University of Colorado. University of Colorado, n.d. Web. 4 Feb. 2013 <http://orgchem.colorado.edu/CCCE/frame/images/handbook.pdf>. “Recrystallization.” Hartnell College. Hartnell College, n.d. Web. 17 Dec.
2012. <http://www. hartnell.edu/faculty/shovde/chem12a/lablecturehandouts/recrystallizationlecture.htm>. “Recrystallization: a purification technique for solids.” German Jordanian University. German Jordanian University, n.d. Web. 17 Dec. 2012. <www.gju.edu.jo/admin/s32files/3-RECR.doc>.

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