OBJECTIVE
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To purify benzoic acid by recrystallization
To determine the melting point of pure benzoic acid
ABTRACT
Recrystallization was attempted in this experiment to purify the organic compound, benzoic
acid. Benzoic acid crystals were collected and used to find the melting point to determine the
purity of benzoic acid. Experimental melting range for benzoic acid was found to be 120 C121 C. But based on the results‚ the melting point that has obtained was 122.1°C.The
percentage difference shown was 10% exceed. This finding suggests that the benzoic acid
that was recovered is not really pure because of the experimental melting point that was
recorded suggests that the recovered benzoic acid contain some impurities. Percent recovery
of benzoic acid was found to be 35.45% that is lower than the actual one which should be
79%.The percentage difference may due to several factors that caused the loss of product.
INTRODUCTION
Recrystallization is a common method used to purify a solid. As the name implies a solid
sample with suspect purity is dissolved into an appropriate solvent. Liquids are customarily
purified by distillation, while solids purified by recrystallization; sometimes called simply
“crystallization”. When heat is involved a solvent is chosen in which the solute has a low
solubility at room temperature, but a higher solubility at an elevated temperature. If the are
any impurities that have the same solubility as the main component, then a different solvent
needs to be chosen.
When organic substances are synthesized in the laboratory or isolated from plants, they will
obviously contain impurities. Several technique for purifying these compounds have been
developed. The most basic of these technique for the purification of organic solids is
recrystallization, which relies on the different solubilities of solutes in a solvent. Compounds,
which are less soluble, will crystallize first. The crystallization process itself helps in the
purification because as the crystals form, they select the correct molecules, which fit into the
crystal lattice and ignore the wrong molecules.
Organic compounds that are solids at room temperature are usually purified by
recrystallization. The general technique involves dissolving the materials to be recrystallized
in hot solvent and cooling the solution slowly. The solid that crystallized out from the
solution is very pure material. During the recrystallization process, solid impurities that do
not dissolve in hot solution are normally eliminated through filtration. The dissolved
impurities remain in cold solution while the pure compound recrystallized out of the solution.
MATERIALS
Chemicals :
Benzoic acid
Distilled water
Apparatus :
Conical flasks
Hirsh / Buchner funnel
Watch glass
Filter funnel
Capillary tubes
Beaker
FLOWCHART
Part A
Weight about 1.0 g
benzoic acid into 100
ml volumetric flask.
Add 15 ml water and
anti-bumping
granules
(3-5 pieces )
Heat the mixture on
a hot plate until the
solvent boils.
Filter the hot mixture of
benzoic acid through a
fluted filter into conical
flask.
Prepare the fluted
filter paper and put it
in the funnel. Heat 20
ml water.
Add 2-3 ml water if
required and continue
boiling until all
benzoic acid has
dissolved.
Add boiling water if
crystallization occurs on
filter paper. Add until all
the crystals are dissolved.
Wash
crystals with
cold water
and dry.
Place the filter paper
with crystals in a watch
glass and leave for 4
hour to air dry.
After filtration, boil
filtrate to produce
more concentrated
solution.
Filter the
crystals through
a Buchner
funnel.
Cool the solution to
room temperature.
After compound has
recrystallized out,
place flask into ice
bath for a few
minutes .
Concentrate the
solution by
heating on the hot
plate and cool if
no
recrystallization
occurs.
Part B
After the crystal dry completely, weight the pure benzoic acid recovered , calculated the %
recovered.
Break the crystal into powder, fill the powder in 3 capillary tubes.
Read the melting point using BUCHI B540
Compare the result with the actual melting point of benzoic acid
RESULTS
1) Data on the impure Benzoic Acid
a) Mass of benzoic acid + weighing paper
: 1.8632
b) Mass of weighing paper
: 0.8500
c) Mass of impure benzoic acid
: 1.0132
2 ) Data for recrystallized benzoic acid
a) Mass of recrystallized benzoic acid +
weighing paper
: 1.2101
b) Mass of weighing paper
: 0.8500
c) Mass of recrystallized benzoic acid
: 0.3601
d) Calculation of percentage recovery
: 1.0132 × 100 %
0.3601
( show calculation )
e) Melting point of recrystallized benzoic acid
35.54 %
: 122.1
3 ) Melting point of recrystallized benzoic acid ( Theory ) :
The melting of the crude benzoic acid was 118-122 C , while the melting point of the
recrystallized benzoic acid was 114-122 C. When recrystallized using just water as the
solvent the benzoic acid appear as a white, powdery solid weighing 0.87 g. The percent
recovery was 79%.
DISCUSSION
The percentage recovery of benzoic acid is only 35.54% but the actual one should be
79%.The percentage difference may due to several factors that caused the loss of products.
One of the factor is that the volume of water added to the solution is too much which making
the solution not saturated enough to produce maximum yield of benzoic acid after cooling.
Normally‚ larger volume of water used will tend to the products to dissolve more easily. The
benzoic acid crystallized on the filter paper during the hot filtration. The additional hot water
need to be added to dissolve the benzoic acid crystals on the filter paper which causes the
solution to be more dilute so that the products is lost in the solution.
In this experiment‚ the benzoic acid is dissolved in hot water while only a little amount of
benzoic acid is able to dissolve in cold water. The benzoic acid cannot dissolve well in cold
solution because of its hydrophobic benzene ring. However the carboxyl group –COOH that
attached to the benzene ring allows some of the benzoic acid soluble in water. In hot solution‚
the increase in temperature causes the water molecules to has more kinetic energy and move
faster. As a result‚ it allows the water molecules to penetrate through the benzoic acid solid
and hence solubilisation of benzoic acid occurs. In addition‚ other impurities present in
mixture cannot dissolve in water. So‚ water is a good solvent to be chosen in the experiment.
The bumping granules were added in the experiment. The bumping granules are small
insoluble and porous stones made of calcium carbonate or silicon carbide. There are a lot of
pores inside the bumping granules in which it provides nucleation site to trap air and creates
space to allow the bubble of solvent to form. When the bumping granules are heated‚ it will
release tiny bubbles which can prevent bumping and boiling over of the mixture so that the
loss of solution can be avoided even it is boiled. The adding of bumping granules must be
added before boiling of the solution instead of after boiling. This is because adding of
bumping granules to a solution near its boiling point will induce flash boiling point as well.
The bumping granules are not soluble in the solvent and hence they can be filtered out by
using filter paper but they are not reusable.
During the hot filtration most of the residue was removed and stay on the filter paper while
the benzoic acid solution pass through the filter paper and goes into the conical flask. During
cooling process‚ the hot solution was allowed to cool slowly to the room temperature and
then only immersed in an ice-bath. The solution should be protected from the contaminants
by covering with a piece of filter paper. Fast cooling always produces relatively small
crystals because the particles do not have sufficient time to arrange themselves in proper
conformation so it is not advisable to cool down the hot solution immediately in ice-bath. The
small size of crystal form may trap impurities easily. Oppositely‚ slow cooling allowed the
molecules to interact and arrange themselves properly and hence they form larger size of
crystals. But large particles may causes some solvent being trapped inside the crystals.
During cool filtration the water soluble impurities that might dissolve in water which was
filtered out through the suction filtration. However‚‚ some of the impurities might be trapped
on the surface of the benzoic acid crystals so small volume of ice cold water should be used
to wash the benzoic acid crystals dissolve the particular impurities. The crystal was dried in
the oven at 100°C.A fresh piece of filter paper can be used to place under the filter paper with
benzoic acid crystals.
The purity of a crystal can be determined by its melting point. A narrow range of melting
point indicates high purity of the sample otherwise broad range of melting point indicates the
presence of impurities in the crystal. The melting point of the recovered benzoic acid
obtained experimentally is 122.1°C but the actual one should be 122°C. The percentage
difference of the melting point recovered by benzoic acid is about 10%.The percentage
difference is little bit high because the presence of impurities in the crystal. Maybe the
filtration that has conducted was not in the proper way causes the impure substance flow into
the solution that has separated in the conical flask. Sometimes the impure substance maybe in
the form of tiny particle and cannot observed with the naked eyes causes the error in this
experiment will happen.
CONCLUSION
Theoretically the melting point of recrystallized benzoic acid was 114 oC – 122oC . Organic
compounds in the form of solids are purified by recrystallization. In this experiment ,
benzoic acid was used for a recrystallization process. The melting point of the recrystallized
benzoic acid obtained experimentally is 122.1oC . The result shows that the benzoic acid was
not really pure due to some possible sources of error done during this experiment. Thus , the
percentage recovery of the purified benzoic acid for this experiment is just 35.54%. The
result really show a very large gap between the value from the theory. Therefore , some
possible sources of error had been identified such as the loss of products , this experiment
does not follow the steps as the crystal was break into the powder first before weighing it
resulting the fine powder fall or flown by the air . Moreover, fast cooling of the solution
generally produced more crystals of relatively small dimensions. Besides, the compound is
slightly contaminated with impurities which included the water molecules that trapped inside
the benzoic acid crystals.
RECOMMENDATIONS
In order to obtain accurate results, there are a few recommendations that should be taken
during conducting the experiment. The solution should be protected from the contaminants
by covering with a piece of filter paper. Next, the filtration process should be done properly
as the filter paper must be fold in the right way to avoid the solid impure mixed with the
solution. Thus , the cooling process should be done slowly allowed the molecules to interact
and arrange themselves properly. Moreover , if the filter paper was torn on the corner to snug
fit in the funnel, the torn corner side must be weighed with the benzoic acid to avoid any
errors. Once the suction filtration process is completed, the collected crystals should be
washed with a little more ice-cold solvent to remove final soluble impurities which would
otherwise be left on the surface of the crystals.
TUTORIALS
1. What is the purpose of the recrystallisation process ?
Recrystallization, also called fractional crystallization, is a process that involves dissolving an
impure compound in a solvent at a high temperature to create a concentrated solution and
then cooling the solution slowly. The drop in temperature decreases the solubility of both the
chemical compound and the impurities it contains. The pure portion of the compound
crystallizes, leaving behind the impurities in the solution. Filtering out the pure crystals
separates them from any impurities, and repeating the recrystallization process creates an
even purer compound.
For the recrystallization process to be successful, there must be only a small
quantity of
impurities in the chemical compound, and they must be soluble. It is possible to predict the
outcome of the process by using a solubility curve, which is a graphical representation of how
the solubility of a substance in a solvent varies with temperature. For the best outcome, the
solubility curve should rise rapidly as the temperature rises.
Performed correctly, the recrystallization procedure is a highly effective way to obtain a pure
sample of a compound. However, it can take a long time because the slower the rate of
cooling, the larger the pure crystals, which makes filtering easier.
2. Explain why anti-bumping are added before any solution is heated ?
Because a heated solution is likely to boil, probably sooner than later. For a liquid to boil
smoothly, small bubbles have to nucleate on the bottom and rise to the top. The boiling stone
(aka anti-bumping granule) allows the small bubbles to form.
3. Why is suction filtration favoured over gravitational filtration when separating pure
crystals from its supernatant liquid after the recrystallisation ?
By comparison, suction filtration has more efficiency than gravitational filtration. More
unneeded water can be removed through suction filtration. The time taken of pure crystals to
dry is shortened.
4. In general, water is not a good solvent for the recrystallization. Explain this statement.
Water is known as a universal solvent and can dissolve almost anything especially salts or
anything else with a hydrogen bond like sugar. Since water dissolves so many things, it
would be a poor choice to use in recrystallisation. The crystals would not form as well to be
filtered out because the water would keep them in solution. For recrystallisation we need to
find a solvent that has very little attraction for the crystals and one that would allow them to
come out of solution.
5. Provide the probable reasons of low or high melting point of your experimental benzoic
acid when compared to theoretical result.
The melting point of the recovered benzoic acid obtained experimentally is 122.1°C but the
actual one should be 122°C. The percentage difference of the melting point recovered by
benzoic acid is about 10%.The percentage difference is little bit high because the presence of
impurities in the crystal. Maybe the filtration that has conducted was not in the proper way
causes the impure substance flow into the solution that has separated in the conical flask.
Sometimes the impure substance maybe in the form of tiny particle and cannot observed with
the naked eyes causes the error in this experiment will happen.
6. List the usages of benzoic acid in industry.
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Food preservative
Medicinal
Feedstock
Biology and health effects
7. State the class/family of benzoic acid in organic chemistry.
Benzoic acid, C6H5COOH, is a colourless crystalline solid and a simple aromatic
carboxylic acid
8. Draw the structure of benzoic acid.
9. How is benzoic acid been synthesized ?
Benzyl chloride
a) Write the chemical equation of above process.
C6H5CH2CI + 2 KOH + 2 O – C6H5COOH + KCI +H2O
b) Name the starting materials, process name, catalyst being used and side product.
Starting materials : benzyl chloride, potassium permanganate, sulphur dioxide
Process name : Oxidation of benzyl alcohol
Catalyst : Ru, Pd, Cu
Side product : Benzene
REFERENCES
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Lab Manual
Yeap Tok Kheng,Chemistry SPM Highlights,Pearson Malaysia Sdn.Bhd,2014,
p341-390