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Free Essay on Recrystalization
This laboratory introduced the process of recrystallizing crude acetanilide mixed with charcoal. Compounds can be purified by recrystalization because foreign molecules can seldom fir into the crystal lattice of another compound. The lab explored the method of isolating the impurity by breaking down the crystal structure by dissolution of the impure material in a suitable solvent and subsequent reformation of this crystal leaves the impurity in the solvent, and the solute remains relatively pure.
The technique of recrystallization involves the dissolution of the material to be crystallized in hot solvent followed by slow cooling of the solution. A moderate speed of crystal formation is desired.
There are seven steps involved in successful recrystallization. They are: Selection of an appropriate solvent, dissolution of the material in the selected solvent, filtration of the hot solution to remove solid impurities, formation of crystals as the solvent cools, separation of crystals from the supernatant solution, washing the crystals, and finally drying the crystals.
The experiments for this lab included finding the yield of the pure acetanilide and the melting point ranges for both crude acetanilide and purified acetanilide.
In this laboratory the results were not what were expected. This particular sample of crude acetanilide that was to be purified had overwhelming amounts of charcoal in it and this is what caused the percent yield to be so low. The procedure was performed correctly but there just wasn’t enough acetanilide in the sample to begin with to recover a decent amount.
The substantial amount of charcoal also accounts for the fact that the melting point of the crude sample was so high. The impurities of the charcoal are expected to raise the melting point somewhat, however in this case it raised the melting point so much that the sample still hadn’t melted at 130 C. The charcoal had completely overpowered the acetanilide.
There are also other areas in this experiment that could account for some error. Selection of an appropriate solvent is an important factor in this experiment and if done incorrectly could lead to some inaccurate results. The solvent should have the capacity to dissolve large amounts of the material being purified at its boiling point and a very small amount at room temperature. It should have a low enough boiling point so that it can be removed from the solid drying process and it should not react chemically with the solid. Water is the most commonly used solvent and was therefore used in this experiment because it fits all these requirements. Water is also the most polar solvent and therefore works well with other polar compounds.
The speed of crystallization can also be a cause of error. If the solution is cooled too fast then the crystals will be very small and might absorb impurities from the solution. If cooled too slowly the crystals will be very large and hard to dry and might contain impurities from the solvent.
At the end of the experiment when the melting point of the crystals was taken it came out to be between 105-107 C which is not too far off from the actual melting point range of 113-114 C. Since melting point is an index of purity, this experiment was successful at recovering a fairly pure compound from an impure mixture.