chem31.1expt1

Experiment #1:Purification of Benzoic Acid by Recrystallization

John Edward O. Tanchuco & Carolyn Marie D. Legaspi

Section AB2April 29, 2009

Experiment Objectives

• To purify and separate benzoic acid crystals from an impure sample using crystallization.

• To perform the proper laboratory techniques in recrystallization.

• To understand the concepts and computations involving purification and recrystallization.

Introduction

• Purification involves the physical

separation of contaminants from a sample to produce the desired pure compound.

aims to remove by-products and impurities from an impure sample.

for liquids, we usually employ distillation but for solids, we use recrystallization.

Introduction

• Recrystallization a common purification

method for organic compounds esp. solids

the dissolution of the solid with an appropriate solvent at a high temperature & its recrystallization at a low temperature.

two types of impurities: (1) more soluble than main component (2) less soluble than main component

Recrystallization Process

• An impure solid compound is dissolved in a solvent and crystallizes as the solution cools.

• Compounds, which are less soluble, will crystallize first.

• As crystals form, it selects the correct molecules, that fit in the crystal lattice & ignore the wrong molecules, resulting in a pure solid.

Recrystallization Process

• The crystallization process heavily depends on the differences in the solubilities of the desired solutes and the impurities dissolved in the solvent.

Recrystallization Process

• Limitations the process cannot

separate pure substances and impurities with the same solubilities in a given solvent.

the process is not perfect; it cannot assure a perfectly pure solute, but it does increase its purity.

the process is limited to solid solutes.

Recrystallization Solvent

• In the ideal setting… the solvent would

completely dissolve the compound to be purified at high temperature, usually the boiling point of the solvent, and the compound would be completely insoluble in the solvent at room temperature or at 0oC.

and vice versa for the impurity

Recrystallization Solvent

• In the real world…In the real world, this will

never happen and recrystallization is a technique that has to be practiced and perfected.

Regardless of crystallization method, the purity of the solid can be verified by taking the melting point. 

Recrystallization Solvent

solvent formula polarity boiling point (0C)

water H2O very polar 100ethanol CH3CH2O

Hpolar 78

methanol CH3OH polar 65dichloromethane

CH2Cl2 slightly polar

40

diethyl ether

(CH3CH2)2

Oslightly polar

35

Common Recrystallization Solvents

* Note: Properties of a good recrystallization solvent will be elaborated later.

Recrystallization vs. Precipitation

Recrystallization

Precipitation

Rate slow fast

Formation of crystals

selective random

Shape of crystals

pure regular crystals

amorphous solid

Amount of impurities

negligible significant

Experimental

Step 1: Mix boiling chip, 100 mg impure benzoic acid, & 2 ml distilled water. Dissolve and heat while constantly swirling.

benzoic acid solution

Erlenmeyerflask

hot plate

Discussion

• Water is an ideal solvent for benzoic acid. at 10⁰C, 2.1 g of benzoic acid

dissolves in 1000 ml of water. but at 95⁰C, 68g benzoic acid is

soluble per 1000 ml of water. this implies that at different

temperatures, benzoic acid has an huge solubility difference in water.

Discussion

• Constant swirling at a high temperature. swirling speeds up the

dissolution of benzoic acid in water

agitation increases the entropy of the system, thus increasing the interaction between benzoic acid and water molecules.

the complete dissolution of benzoic acid results to a clear solution.

Discussion

• Adding the boiling chip while at room temperature. adding the boiling chip

at room temperature prevents boiling over.

this means that the solution will not spill out, since the boiling chip induces boiling of the mixture.

Experimental

Step 2: Cool the solution. Add activated charcoal. Add a few drops of water. Heat again until observable change is seen.

Discussion

• Decolorizing the solution with activated charcoal. activated charcoal are carbon

atoms that are finely separated. these can adsorb impurities (stick

to the surface of the substance) from the solution but are quite large to pass through the filter paper.

this results to minimization of impurities, and increased purity.

WARNING: too much activated carbon could cause the loss of the pure substance.

Experimental

Step 3: Pour the hot solution in the filter syringe and force the liquid through the syringe.

vacuum(suction)

filtrate

benzoic acid

Buchnerfunnel

Discussion

• First filtration of the solution activated charcoal used,

as well as other impurities, would be separated from the solution and left in the cotton plug.

this lessens the impurities in the crystallization process, and increases the purity of the yielded substance.

Discussion

• Filtering the solution rapidly. as filtration is taking place so is

the crystallization process. the decrease in temperature

causes a decrease in the solubility of the benzoic acid crystals.

some of the pure crystals would be separated from the filtrate and would be left as residue.

a lesser yield would result if the solution was not poured rapidly.

Experimental

Step 4: Let the mixture cool in the ice bath.

Discussion

• Slow cooling in ice bath. slow cooling makes the

crystals arrange finely, thus ensuring correct molecular arrangements/geometry.

this helps the crystals form in an undistorted manner and exclude the impurities in crystal formation.

Experimental

Step 5: Collect the crystals on a filter paper. Rinse vial with ice –cold water to collect the remaining crystals in it. Use a seed crystal if necessary.

Discussion

• Using a seed crystal. in cases, that crystallization

while cooling does not take place, a seed crystal is employed.

the seed crystal has the same structure as the pure crystal to be recovered.

the seed crystal serves as a “source code” where the desired solid in the solution begins crystallization.

Discussion

• Using a seed crystal. since the lattice is a

perfect fit, the other dissolved crystals would crystallize out as well.

impurities would remain dissolved in solution since its structure differs from the seed crystal and cannot fit in the lattice.

Experimental

Step 6: Squeeze excess water from the filter paper. Dry it completely & weigh the filter paper.

Discussion

• The filter paper and crystals must be completely dried. the added mass of water

while weighing produces an inaccuracy in the desired data (% recovery) due to the solvent molecules.

Results

• % recovery of benzoic acid crystals

Weight of impure sample = 100 mg

Weight of filter paper = 400 mg

Weight of filter paper and benzoic acid = 450 mg

Weight of pure crystals = 50mg

% recovery: 50mg/100mg x 100% = 50%

Discussion

• Not all groups had the same yield even if everyone used 100 mg of the same impure benzoic acid sample.

• Even if the same bottle was the source of the benzoic acid, the amount of impurities from each group may have varied.

• In conclusion, the quantity of impurities and crystals recovered in each group varied.

DiscussionPossible error source Resulting yield

Incomplete dissolution of impure sample

Less yield

Excessive activated chracoal

Less yield

Slow filtration of first filtrate

Less yield

Reduced forcing of liquid from the syringe

Less yield

Rapid cooling Greater yield

Spilling of filtrate Less yield

Incompletely dried filter paper

Greater yield

Weighing filter paper while hot

Less yield

Comparative Group ResultsGroup #

Percent Recovery

Error Source

1 50% Spilling

2

3

4 38% Spilling

5

6

7 80% Minimal spilling

8

9

Conclusion

• Recrystallization is a laboratory technique used in purification of solids. It is not a perfect process and does not produce an accurate yield.

• A large amount of impurities with the same solubility as the pure substance is also present, making effective separation difficult.

Guide Questions

Question#1:

• List the properties that an ideal solvent should have to perform the purification of organic compound by recrystallization technique.

Answer

• In the ideal setting… the solvent would

completely dissolve the compound to be purified at high temperature, usually the boiling point of the solvent, and the compound would be completely insoluble in the solvent at room temperature or at 0oC.

and vice versa for the impurity

Answer

• A suitable solvent should be:1. Soluble at the boiling

point of the solvent and slightly soluble at room temperature. (if soluble, hard to separate pure crystals because the molecules interact)

2. The impurities should either dissolve at room temperature or insoluble in the hot solvent so that impurities can be removed by filtration.

Answer

• A suitable solvent should be:3. The solvent should not react

since their interaction would result to a lesser yield of pure crystals.

4. The solvent should be volatile enough to be easily removed from the solute/s. This allows rapid drying of the solid compound after it has isolated from the solution.

Guide Questions

• Question#2:

What advantages does water have as a crystallization solvent?

Answer

exists as a liquid and can be used for a wide temperature range (1˚C -100˚C)

non-flammable and non-volatile

polar which can easily dissolve molecules esp. polar substances

high heat capacity and can dissolve different substances

safe and non-toxic compared to other organic solvents

Guide Questions

• Question#3:

Two students crystallized 10g samples of benzoic acid from water, the first dissolving benzoic acid at 80˚C and filtering at 10˚C , the second dissolving at 95˚C and filtering at 18˚C .

Calculate the quantity of water each student was required to use and the maximum recovery of benzoic acid possible in each case.

Answer

1st Student:

@ 80˚C10 g = 2.75 g X1 ml 100 ml

X1 = 363.6364 ml

@ 10˚C

Y1g = 0.21g363.63 ml 100mlY1 = 0.7636 g

10 g – 0.7636 g = 9.24 g

2nd Student:

@ 95˚C10 g = 6.80 g X2 ml 100 ml

X2 = 147.0588 ml

@ 18˚C

Y2g = 0.27g147.06 ml 100mlY2 = 0.3971 g

10 g – 0.3971 g = 9.60 g

Guide Questions

Question # 4:

A Solid (X) is soluble in water to the extent of 1 g per 100 g of water at room temperature and 10 g per 100 g of water at the boiling point.

Guide Questions

a) How would you purify X from a mixture of 10 g of X with 0.1 g impurity Y, which is completely insoluble in water and 1 g impurity Z having the same solubility characteristics in water as X?

Answer

Dissolve the mixture in 100 g water and heat until boiling (100⁰C). Cool the mixture to room temperature. Then filter to separate impurity Y from the mixture.

Add water to the filtrate up to 100ml. Heat the mixture again until boiling until all crystals dissolve. Slowly cool the mixture to room temperature. Filter the mixture.

Answer

The residue left on the filter paper is the purified X crystals.

The formation of impurity Z is insignificant since it is minimal in quantity (1g) and has a different lattice structure from the solid X structure thus excluding Z in crystal formation.

Guide Questionb) How much pure X could be

obtained after one recrystallization from water?

ANSWER:10g – 1g x 100% = 90% 10 gSince the solubilities of different

solutes are independent of each other, at room temperature, it would be assumed that 1 g of solutes X and Z would dissolve. So, 9 g of pure X solute will crystallize.

Guide Question

c) How much pure X could be obtained after one recrystallization from a mixture of 10g of X with 9 g of Z?

Answer

To retrieve pure solid X crystals from the mixture, all X and Z must be dissolved. The solubilities of X and Z are independent of each other and to dissolve all crystals, solid X is used as basis for it is present in a larger amount.

10g = 1g X = 1000 g water

X g 100g

Answer

To ensure that Z doesn’t crystallize back, the final amount of water must ensure that solute Z crystals remain dissolved in water under room temperature. This can be done by heating the solution until 900 g water is left. 9g = 1g Y= 900 g

water Y g 100g

Answer

The solution is slowly cooled until crystals are formed. Z g = 1g Z= 9 g

solid X 900g 100g

In 900 g water, 9 g of solutes X and Z are dissolved which means only 1 g of pure solid X can crystallize from the solution.

d) Based on the results obtained, what is suggested about the use of crystallization as a purification technique?

ANSWER:The crystallization technique is very

inaccurate because it heavily relies on the differences in solubility. Therefore, it is difficult to separate substances with the same solubilities especially when present in the same mixture with significant amounts.

THANK YOU!!!Group 1 Section AB2