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Exp # 07; Recrystallization Of Benzoic Acid
(A) PRE-LAB
:
The purpose of this experiment is to purify Benzoic acid through a process called recrystallization. The idea is that the impure organic compound (in this case, Benzoic acid+impurities) is to be heated in order for the different elements in the solution to break apart. After doing so we have to put the solution through a filtration process where it is also simultaneously cooled and the crystals are formed; thus the end product is the purified form of the organic compound-(Benzoic Acid).
BACKGROUND
:
Benzoic acid is a crystalline solid and it has a formula of (C7H6O2). It was first discovered in the 16th century by Alexius Pedemontanius. Benzoic Acid was a precursor in the preservation of fruits. It can be formed by oxidizing toluene and oxygen. Interestingly when fruits such as apples start to rot, they give off free benzoic acid. Since cats have a very low tolerance for benzoic acid, they can detect the rotting of food. Benzoic acid is used in the laboratory because it is cheap. It can also be purified through recrystallization process from water. It’s soubility is high in hot water and very low in cold water. Thus recrystallizing benzoic acid is quite easy because the crystals begin to form when it is cold because of its low solubility in low temperatures.
Since this experiment relies heavily on the solubility of a substance, we should start off by defining what solubility is: “the quantity of a particular substance that can dissolve in a particular solvent (yielding a saturated solution)”. We must take into account the solubility of Benzoic Acid in 25o C of water is 0.34 g/100ml. Increasing temperature increases the solubility of the substance. Hence we begin by heating up the impure form of the benzoic acid.
To understand the theory of recrystallization we must also understand saturation point. This is the “concentration in which no additional solute” (the benzoic acid) “will dissolve in the solvent” (water) at a certain temperature. Since Benzoic acid is a solid at room temperature we can bring the solution to its saturation point by heating the solution. This increases the solubility of the solution. When the solution is cooled the Benzoic acid will start to form into crystals, since lowering the temperature lowers the solubility of benzoic acid. We can put this through a filtering apparatus and catch the purified form of the benzoic acid on the filtering paper. If the experiment is done as accurately as can be we will have a purified crystal form of Benzoic acid on the filter paper. To make sure that it is accurate, the melting point temperature of the dried solute is obtained and compared with the literature melting point temperature of benzoic acid.
Recrystallization
(A mixture of (separated + dissolved (Solution
Impure Benzoic in solution) contains
Acid) impurites)
TABLE OF REACTANTS , PRODUCTS and SOLVENTS| REACTANT(Theoretical) | SOLVENT | PRODUCT(literature) | |
COMPOUND NAME
| C7H6O2 (Benzoic acid + Unspecified Impurities) | H20 | C7H6O2 Benzoic acid |
STRUCTURAL FORMULA
| | | |
MOLECULAR WEIGHT
| 122.12g/mol | 18.02g/mol | 122.12g/mol |
| m.p. or other | 122o C |
0o C
| 122o C |
| Density | 1.32g/cm3 | 1000kg/m3 | 1.32g/cm3 |
| Appearance | Colorless Crystalline Solid | Clear Liquid | Colorless Crystalline Solid |
| SAFETY, TOXICITY | Necessary for life | Toxic. |
http://en.wikipedia.org/wiki/Benzoic_acid
http://www.chem.siu.edu/chem341/c341lectures/c341recrystallization.pdf
CALCULATIONS
Weight
: 0.63g : (124-126)o C
% Yield C7H6O2
: 0.632g (100) = 62.33%
1.014g
%YIELD
|
Melting Point
| |
C7H6O2)
Benzoic Acid
(
|
62.33%
|
(124-126)o C
|
:
As we can see the percent yield for my results was not so satisfying. This experiment is very dependent on temperature since in this case (and in most other cases) temperature was in fact the determinant of solubility of benzoic acid. We had to follow the directions very carefully to get the perfect result-the ideal result. However I think when I was performing the experiment I might have lost a lot of the solutes in the process of filtration. Perhaps I filtered the solution too hot. Or maybe when I was pouring the solution into the filter I wasn’t careful enough.
Interestingly, however, the melting point range for my solute was pretty close to the literature melting point of benzoic acid. The more pure a solute is, the narrower the range of melting point. The temperature range for the melting point of my solute was only approximately 2o C.
:
I am not satisfied with my results. 62% yield is not too much. It’s like losing 48% (approximately) of the benzoic acid to the solution! On an exam it would be like getting a D, which is surely not a passing grade. Although my results were inaccurate in depicting the correct amount of benzoic acid to be obtained, I must say that the solute-however much I’ve retained- was both accurate and precise in comparison to the literature melting point of benzoic acid. So I must conclude that there are very little, if any, impurities in the benzoic acid that I have obtained, but I haven’t obtained all that I could. It’s the classic issue of quantity over quality.
CONCLUSION
DISCUSSION
RESULTS
Melting Point
: The melting point of the crystals should be around 122o C. We started off with about 1 gram of benzoic acid.
APPARATUS ASSEMBLY
: We will use the Erlenmeyer flask that allows us to stick rubber tubing coming out of it in order to proceed with filtration. When we are in the process of recrystallization we will use the rubber tubing (orange) and create a vacuum. On the top of the Erlenmeyer flask, a rubber stopper is to be placed on top of which a porcelain Buchner funnel. A filter paper will be placed and wetted on the Buchner funnel before we turn on the vacuum and start filtering the crystals. After the crystals have formed we will save then on a dish.
REFERENCES
CHEM 211 Lab Manual 2009-2010, Pierce College, pp 53-54MAIN REACTION, SIDE REACTIONS and MECHANISM
PURPOSE
this is good, keep up the standard!!!!
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